case study related to glass evidence

Crime Scene Management within Forensic science pp 107–127 Cite as

Paint, Soil, and Glass Evidences: A Silent Witnesses

• Shikha Choudhary 3 &
• Aadya Ramesh 3  
• First Online: 03 January 2022

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“Every contact leaves a trace” as stated by Locard’s principle of exchange implies trace evidence like paint, soil, and glass due to their ubiquitous nature. These evidences can be encountered in a small scale at a crime scene but act as a crucial silent witness for forensic investigation. Trace evidence like these can easily be used to link the crime scene to the victim or suspect. The significance of the trace depends largely upon the quality of sample collection. This chapter compromises the detailed study of physical as well as chemical properties of such trace evidence. In addition to that, forensic examination techniques with advanced elemental analysis for identification are also incorporated in this chapter. The examination facilitates the process of solving cases of road accident, home burglary, sexual assault, etc.

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Department of Forensic Sciences, Lovely Professional University, Phagwara, Punjab, India

Shikha Choudhary & Aadya Ramesh

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Neeta Raj Sharma

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Choudhary, S., Ramesh, A. (2021). Paint, Soil, and Glass Evidences: A Silent Witnesses. In: Singh, J., Sharma, N.R. (eds) Crime Scene Management within Forensic science. Springer, Singapore. https://doi.org/10.1007/978-981-16-4091-9_5

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https://www.nist.gov/programs-projects/glass-evidence-analysis

Glass Evidence Analysis

T he goal of this focus area is to improve the field of glass evidence analysis by developing new matrix-matched glass standards and by evaluating more objective approaches to evidence interpretation, such as the likelihood ratio. The latter will be accomplished through the development of glass databases that may be used to assign a significance to an association or exclusion in forensic casework. The projects focus on the two elemental analysis techniques X-Ray Fluorescence (XRF) and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) that are commonly used by forensic glass analysts.

Description

Top view of broken laminated windshield glass.

Quantitative XRF Glass Database

Recent research efforts in the forensic trace field have focused on the development of databases to assign a significance to forensic evidence (i.e., to report the strength of an association or exclusion). In the forensic analysis of glass using µ-XRF, samples are typically compared using element intensity ratios. However, because of differences in instrumentation, the development of a database compiled with element ratios collected by different laboratories is not currently feasible. The collection of  quantitative  XRF data will address this limitation. Thus, this project focuses on the development of a quantitative XRF glass database that can be used by forensic practitioners to assign a significance to forensic glass evidence.

Glass Standards

The forensic community uses several glass standards and reference materials from NIST and abroad that are currently low in stock.  Additionally, the community relies on published consensus values or on certified values that were collected more than 20 years ago and at sampling scales that are not relevant to the forensic community.  This project aims to address these gaps by (1) reanalyzing elemental concentration (at trace and bulk sampling scales) for several commonly used glass SRMs with modern instrumentation and methods ( i.e. , LA-ICP-MS), and (2) collaborating with Dr. Almirall’s group at Florida International University to aid in the design and assessment of three new reference glasses.  

Interpretation of Glass Evidence

Although the forensic community has reached a consensus on the analysis of glass evidence using LA-ICP-MS, there is currently no standardized method for the interpretation of glass evidence. Recent efforts in the community have focused on the use of databases to provide a significance to glass evidence using more objective approaches (e.g., likelihood ratio). This collaborative project, involves an inter-laboratory study, designed as a mock forensic case, that aims to evaluate the performance of the likelihood ratio for the interpretation of glass evidence.

Publications

"Evaluation of the Performance of Modern X-Ray Fluorescence Spectrometry Systems for the Forensic Analysis of Glass", Ruthmara Corzo, Troy Ernst, Joseph Insana, Claudia Martinez-Lopez, Jodi Webb, Emily Haase, Peter Weis, Tatiana Trejos, Forensic Chemistry , 31 (2022) 100447; https://doi.org/10.1016/j.forc.2022.100447 .

"An interlaboratory study to evaluate the forensic analysis and interpretation of glass evidence", Katelyn Lambert, Shirly Montero, Anuradha Akmeemana, Ruthmara Corzo, Gwyneth Gordon, Emily Haase, Ping Jiang, Oriana Ovide, Katrin Prasch, Kahlee Redman, Thomas Scholz, Tatiana Trejos, Jodi Webb, Peter Weis, Wim Wiarda, Sharon Wilczek, Huifang Xie, Peter Zoon, Jose Almirall, Forensic Chemistry , 27 (2022) 100378; https://doi.org/10.1016/j.forc.2021.100378 .

"An interlaboratory study evaluating the interpretation of forensic glass evidence using refractive index measurements and elemental composition", Ruthmara Corzo, Tricia Hoffman, Troy Ernst, Tatiana Trejos, Ted Berman, Sally Coulson, Peter Weis, Aleksandra Stryjnik, Hendrik Dorn, Edward “Chip” Pollock, Michael Scott Workman, Patrick Jones, Brendan Nytes, Thomas Scholz, Huifang Xie, Katherine Igowsky, Randall Nelson, Kris Gates, Jhanis Gonzalez, Lisa-Mareen Voss, Jose Almirall, Forensic Chemistry , 22 (2021) 100307; https://doi.org/10.1016/j.forc.2021.100307 .

• Original Article
• Open access
• Published: 17 May 2019

Evidential significance of multiple fracture patterns on the glass in forensic ballistics

• Neelesh Tiwari 1 ,
• Abhimanyu Harshey 1 ,
• Tanurup Das 1 ,
• Sughosh Abhyankar 1 ,
• Vijay Kumar Yadav 1 ,
• Kriti Nigam 1 ,
• Vijay Raj Anand 2 &
• Ankit Srivastava   ORCID: orcid.org/0000-0002-6888-4434 1  

Egyptian Journal of Forensic Sciences volume  9 , Article number:  22 ( 2019 ) Cite this article

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In the current scenario, glass plays a vital contribution in our standard everyday life. Fractured and fragmented glasses are most commonly encountered at the crime scene. During the commission of the crime, glass breaks and shatters into fragments or into minute particles that serve as trace evidence and may provide a significant lead for the probe. The fracture of glass could be the consequence of an intentional act or unintentional act. For the forensic purpose, study of glass fractures, i.e., fractography has paramount importance as it can provide noteworthy data and encourages an assortment of assessments to investigate the case especially in the shooting cases where a firearm is used. Multiple fractures on a glass pane also have enough potential to establish an investigative link. Presently, air guns are being used in many of the cases due to their simple accessibility. There are a number of cases that have been reported that involved the use of air guns.

This study was conducted to evaluate the pattern of multiple fractures on the glass by the air rifle. For this purpose, glass panes of various thicknesses were test fired perpendicularly at a fixed range with an air rifle having round nose pellet. Measurements and analysis were made on some fixed parameter. Data was analyzed graphically and statistically to check the consistency in the trends.

Consistency in the trends was observed, which is analyzed by the test for the goodness of fit. The findings of this work have provided some significant different perspectives that may effectively contribute to the criminal investigation.

Glass is a supercooled mixture of the metallic silicates. Generally, glass is referred to as transparent, semitransparent, non-crystalline, and amorphous solid having number of physical properties, such as durability, rigidity, refractive index, and density (Copley 2001 ). These physical properties, especially refractive index, provide means of investigation for the forensic purpose (Jauhari et al. 1974 ; Stoney and Thornton 1985 ; Gogotsi and Mudrik 2010 ). Glass or glass fragments serve as physical evidence of a great value for the investigation of many offenses such as the burglary, arson, hit and run cases, shooting, and assault. A large variety of physical evidences is included under the term “Trace Evidence”. Locard’s Principle of Exchange is the fundamental concept in the production of physical and trace evidence (Caddy 2001 ; Robertson and Roux 2010 ; Mistek et al. 2018 ).

The analysis of glass fracture has been a subject matter of interest to the forensic community for a long time as it enables the investigator to establish lots of considerations such as cause of fracture, direction of impact, and angle of impact. The term “Fractography” represents the study of surface fracture to identify the mechanism of such fracture (Caddy 2001 ; Harshey et al. 2017 ).

Many eminent researchers gave their efforts to explain the mechanism of the fracture. Griffith echoed that fracture starts from the previous flaws known as Griffith flaw’s (Griffith 1920 ). Law of Griffith was further extended as dynamic fracture mechanism by Mott ( 1964 ). Theoretically, the strength of any material is defined in terms of inter-atomic bond. Tensile stress depends upon various factors such as condition and size of surface, duration of load, and environmental conditions (Overend et al. 2007 ). As the projectile hits the glass pane, cracks are created due to the transfer of projectile’s energy. Cracks propagates through the path of least resistance. The energy shock waves cause specific damage to the glass, originated from the point of impact (Grady 2010 ). In the context of forensic glass fracture analysis, the aspect of energy loss was experimentally demonstrated by Waghmare et al. ( 2003 ).

When any projectile, e.g., bullet or stone, hits the glass surface, stretching and compression occur. As the limit of tensile stress is crossed, it results in the breaking of the glass. The impact of the bullet or stone causes two special types of fractures, namely termed as radial and concentric (also known as spiral) fractures as demonstrated in Fig.  1 . As the high velocity projectile penetrates the glass pane, there is a formation of a crater-shaped hole that is termed as cone fracture. The appearance of the cone may be observed at wide exit side of the hole (Mcjijnkins and Thornton 1973 ). In this context, the shape of the chip pattern (also called mist zone) around the hole is indicative of the direction of impact (Harshey et al. 2017 ). Hackle marks, stress marks, mirror zone, etc., are some other phenomena that may be observed in the fractured glass pane (Caddy 2001 ). These characters often provide a significant help to the investigator in the investigation of variety of offenses.

Radial and concentric fractures

In this study, the soda-lime glass was taken. Soda-lime glass majorly contributes in the manufacture of windowpanes (flat glass), containers, etc. Sand (SiO 2 , 63–74%), soda ash (Na 2 CO 3 , 12–16%), and limestone (CaO, 7–14%) contributes as raw material in the manufacturing of soda lime glass (Curran et al. 2000 ).

No chemical reaction is used in the air gun and the mechanism of air guns involves the mechanically pressurized air or other means. When the trigger is released, compressed air is let off into the barrel, pushing along a specially made slug or pellet in front of it. On the basis of mechanism, there are three types of air gun, namely spring piston air gun, pneumatic air gun, and compressed gas (CO 2 ) air gun (Vanzi 2005 ; Abhyankar et al. 2018 ). They have been utilized as a part of chasing, sporting, and fighting. These are easily available in an open market with fewer rules and regulations and also considered relatively safe. Although these have been found to have the potential to cause fatal and grievous injuries such as bone micro-fracture (Kieser et al. 2013 ), and ocular injuries (Schein et al. 1994 ) and even may cause death (Lal and Subrahmanyam 1972 ). It is evident from the literature that air guns (both rifle and pistol) are being used in illicit activities for a long period and in current time also (Burnt and Packey 1979 ; Harshey et al. 2017 ; Morgan et al. 2019 ). Various researchers studied the different aspects of glass and air gun that may be of forensic interest such as wounding potential and behavior of air gun pellet in ballistic gel (DiMaio et al. 1982 ; Wightman et al. 2010 ; Hallikeri et al. 2012 ; Stankov et al. 2013 ; Ansari and Chakrabarti 2017 ; Vedrtnam and Pawar 2017 ; Hsiao and Meng 2018 ). In firing incidences, glass may also serve as an intermediate object. Therefore, in the light of above facts, this study was performed with the aim to analyze the multiple fractures on a glass pane made by air rifle.

Materials and methodology

In this work, 3-, 4-, and 5-mm-thick window panes of 1 ft 2 were chosen. Three- and 5-mm-thick glasses were transparent while a 4-mm-thick glass category involved both privacy and transparent panes. These panes were fixed in a metal frame with the help of clay. The frame was placed perpendicularly to the muzzle of the rifle at a fixed distance of 12 ft. A total of 100 samples were test fired with .177″ (4.5 mm) caliber Air Rifle. Two fires, one after another, were made on the same side of the glass pane at an average distance of 2.5–3.0″ between both points of impact in order to analyze the multiple fractures. Table  1 summarizes the details about the used air rifle and round nose pellet.

Fractured panes were measured with the help of a vernier caliper. Figure  2 shows the hole diameter, thickness, and the diameter of the mist zone that served as parameters for the sample analysis. Mean value and the standard deviation of the measurements for the above said parameters were calculated. Statistical operation, i.e., “chi-square test (test of goodness of fit),” was also applied in this series of analysis to check whether the data is consistent or not. Level of significance ( α ) is the probability of type one error (rejection of null hypothesis when it is true). The null hypothesis (H 0 ) is accepted when the calculated value of chi-square is less than the tabulated value at a particular degree of freedom (for “ n ” sample, degree of freedom is “ n  − 1”).

Representation of the parameters: the hole diameter ( X ), diameter of mist zone ( Z ), thickness of mist zone ( Y )

An analysis of the multiple fracture pattern was performed on the basis of measurements made on the fractured pane. In most of the cases, 3-mm thick glass panes broke down on the course of second shot. Table  2 shows the value of mean and standard deviation of the measurements and the description of the chi-square test for the first and second target (T1 & T2) of 3-mm transparent, 4-mm transparent and privacy, and 5-mm transparent glass panes. The trends in T1 and T2 were studied comparatively. In order to analyze the consistency, chi-square test was applied on the hole diameter of T1 and T2 of all the glass panes. Null hypothesis was rejected (as the calculated value was greater than the tabulated value) only in the case of T2 of 3 mm. A consistency in the hole diameter was observed that has also been statistically proved.

On the visual examination of the fracture glass panes, it was revealed that 3-mm fractured panes possess circular-shaped mist zone. Mist zone of 4-mm (both transparent and privacy) glass panes was found to be elliptical or triangular with rounded edges while mist zone of 5-mm-thick glass panes was found to be irregular.

This study was performed to analyze the multiple glass fracture. From the measurement, it was noted that the hole diameter values lies between 4.1 and 6.7 mm. Previously, Harshey et al. ( 2017 ) analyzed the pattern of glass fractures that were made by the air rifle on the glasses of different thickness. In this approach, a special aspect of glass coating with Sun Control Film (SCF) was also taken under consideration. It was found that the hole diameter ranges between 4.77 and 7.5 mm. SCF does not contribute any notable difference in fracture pattern. It is notable that this presented study gave results in a similar manner (reproducibility of the results) as that of pronounced by Harshey et al. ( 2017 ) and Abhyankar et al. ( 2018 ).

Another extensive approach is also reported by Waghmare et al. ( 2016 ). In this study, glasses were fractured by improvised pistol and several regular firearms manufactured in the Indian Ordinance Factory, India. Concerning the analysis of the data, the hole diameter is somewhat doubled to bullet caliber, except in case of a 9-mm pistol. Except in the case of improvised pistol, the values of hole diameter were in between 11.04 and 14.63 mm. Improvised pistol does not show any consistency as its manufacturing is not standardized. The above-discussed studies show various trends of hole diameter made by different firearms. The type of weapon (i.e., whether standard firearm or air gun) could be determined on the basis of hole diameter by which glass was fractured. The pattern of glass fracture also differs in case of different firearms, i.e., the patterns made by air rifle are different from that of caused by regular firearms. Similarity in the patterns may be observed on general visual examination.

Apart from it, this study also provides a means to establish the sequence of shot. In this study, many panes were obtained in which radial cracks of T1 and T2 did not meet each other (Fig.  3 ). In such condition, the sequence of shot may not be established by the rule of thumb. Results of this study showed that it was found that the hole diameter of T1 for 4-mm transparent and 5-mm transparent is greater than that of T2 while the hole diameter of T2 is greater for 4-mm privacy pane. This may be indicative for the establishment of the sequence of hole.

Multiple fracture on the glass pane

This study was performed with the aim to analyze the multiple fracture pattern made by air rifle of .177″ caliber. In this study, regularity was observed in the measurements made on fixed parameters. Trends and observations may lead to the determination of the kind of weapon that may play a key role in forensic investigation. It can easily distinguish between the fracture pattern made by regular firearm and the air rifle. The hole diameter value lies in between 4.1 mm and 6.7 mm. This study also provides another mean to estimate the sequence of hole. This aspect may be explored further. Consistency in the features, trends, and characteristics is found to be decreasing with the increasing thickness of the glass panes. The findings of this work may efficaciously help in the forensic investigation in the course of the scene of crime investigation and to interpret the fractured glass panes.

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Neelesh Tiwari, Abhimanyu Harshey, Tanurup Das, Sughosh Abhyankar, Vijay Kumar Yadav, Kriti Nigam & Ankit Srivastava

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Tiwari, N., Harshey, A., Das, T. et al. Evidential significance of multiple fracture patterns on the glass in forensic ballistics. Egypt J Forensic Sci 9 , 22 (2019). https://doi.org/10.1186/s41935-019-0128-4

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• Trace evidence
• Glass fracture
• Fractography
• Multiple fractures
• Fracture pattern

Evidence in cold-case murder trial: Shattered glass and a clock that's gone 2 minutes in 29 years

• Published: Oct. 29, 2014, 9:43 p.m.

Ronald Meadow

• Douglass Dowty | [email protected]

Syracuse, NY -- Prosecutors spent much of today showing the jury evidence from 29 years ago in the murder trial of Ronald Meadow, accused of strangling his estranged wife, Colleen.

Among the mounds of evidence, there was a glass wine decanter, a clock radio and hair samples, all kept in evidence bags after being collected from the March 5, 1985 murder scene. During testimony, Assistant District Attorney Robert Moran walked the officer who supervised the evidence collection through the process.

Now retired, Drew Buske relied heavily on his notes from the time to recall what the items were. Some of the dozens of items directly related to the prosecution's case against Meadow, others were simply items collected that helped paint a picture of the victim's life and her Syracuse apartment.

Murder suspect Ronald Meadow, guarded by jail deputies, appears before County Court Judge Anthony Aloi (not pictured).

Onondaga County District Attorney William Fitzpatrick is the lead prosecutor in the trial against Meadow, who was arrested late last year for the cold-case murder of his wife, Colleen, in her Syracuse apartment in 1985. She was found hog-tied and face-down on the floor.

Defense lawyer Ed Menkin was quick today to point out problems with the three-decade-old evidence.

The glass wine decanter had shattered. No one knows how. Buske speculated someone had dropped it. Menkin guessed that was probably true.

The clock radio, clearly seen in crime scene photos at 10:01 p.m., had mysteriously advanced to 10:03 since then. Menkin guessed it had been plugged in at some point; Buske said he didn't know.

And "tape lifts" were used to collect hair from the scene for identification. Now, much better DNA technology is available.

All in all, Menkin's cross-examination highlighted the problems with relying on evidence so old and, in some cases, mishandled.

Fitzpatrick acknowledged in his opening statements that police standards in the 1980s were not as exacting as they are today. Technology wasn't as advanced. Some evidence wasn't collected properly. But he told the jury that didn't keep him from proving the case.

Menkin spent a long time trying to poke holes in the veracity of the evidence introduced today, trying to cast doubt on the prosecution's case. The evidence had been moved several times since 1985. Sealed bags had been opened and re-opened. Officers had little to no recollection of the scene outside of their notes.

At the end of the day, Fitzpatrick called Colleen Meadow's sister, Nancy Moran, to the witness stand.

She said that Colleen described on several occasions Ron beating her. One time he beat her sister, Moran said, was when they were on a trip. Ron didn't like the way she was driving, so he got in the driver's seat and punched her, Moran testified her sister had told her.

Moran never saw Ron Meadow abuse Colleen, she said. But Moran is the third person to say that Colleen Meadow had confided in them about her husband's abusive behavior.

Colleen Meadow

Colleen's aunt said Tuesday that Colleen told her about Ron Meadow handcuffing her to a chair early in their marriage. And her boss at Niagara Mohawk said Colleen told her that Ron had threatened to kill her.

Each of those times, County Court Judge Anthony Aloi told the jury they could consider that testimony as evidence of the couple's relationship and Ron Meadow's potential motive. But they could not consider that to mean the defendant had a propensity or predisposition to commit murder.

Testimony resumes at 9:30 a.m. Thursday.

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Forensic analysis of glass evidence: Past, Present and Future

Introduction

Glass is the most frequently occurring transfer evidence found at crime scenes. Generally, it is encountered in scenes involving burglary, hit and run, shooting scenes and housebreaking and theft etc. Broken glass is easily transferred to objects and individuals present at the scene of crime, thus it is possible to link a suspect to the event and the items as well as to the individuals present at crime scene [1]. Glass analysis mainly involves classification, discrimination and individualization of the glass evidence [2]. Classification can also be called identification. As different kinds of evidence samples may be encountered at the same crime scene it is important to identify the exact source of that evidence sample. Discrimination comes after classification, as it excludes the evidence if it does not match the source. Individualization is a determination process that every sample is unique even among other samples of same class. According to the law of individuality, every evidence sample have unique characteristics that differentiate it from the class of evidence. Physical matching is the only method of glass individualization. Physical matching of the known broken sample fragments with the questioned sample enables the individualization of glass evidence and eliminates all other sources [2]. In case if physical matching is not possible then other methods of analysis are performed. Other forensic analysis of glass consists of comparison of various physical properties, optical properties and elemental analysis techniques [3].

History of forensic analysis of glass evidence

Various kinds of glass evidences can be encountered at the crime scene such as small glass fragments, broken glass pieces, window planes, headlights, etc. Every glass has different composition and that plays important role in the forensic analysis of glass. Glass has different kinds of fractures and forensic analysis can determine the direction of glass fracture. Early forensic analysis includes physical techniques, microscopic techniques and elemental analysis.

Physical examination: Every glass analysis starts with identification of the glass by physical and optical properties such as amorphous structure and isotropism, hardness. Initial stages of analysis involve physical observation such as colour, surface feature, fluorescence and thickness of the glass sample. Physical examination plays important role in the comparison process of glass evidence recovered from the scene of crime and the questioned sample [4].

Basic schem atic diagram for glass analysis

Density and refractive index measurements are conventional methods that were first used in glass analysis. Most forensic science laboratories use Sink-float method to determine density.  In this method, a questioned glass fragment is dropped into a liquid mixture of bromobenzene and bromoform. If the glass density matches the liquid mixture density, the glass fragment neutrally floats in the liquid mixture. This method had some demerits, thus an alternative method for the determination of density is derived which is based on Stoke’s law [5].

Refractive index (RI) has been determined by immersion method in the initial years of forensic glass analysis. This method of determining RI was based on the technique described by Winchell and Emmons in 1926. This method involves, a glass fragment being immersed in a suitable oil and then heating until the glass observed under a microscope disappeared. At this temperature refractive index of both oil and glass fragments is the same [3].

Microscopic techniques re used for the preliminary examination of surface features of glass fragments. Light microscopes and stereo-microscopes are used widely for this purpose. Light microscopes are used to identify the layers of glass fracture.  Glass fragments are first visually examined under fibre optical light and then observed under stereo-microscope [6].

Elemental analysis is the confirmatory examination for determining the chemical texture of glass evidence. Most of the elemental analysis are destructive in nature. Elemental analysis includes neutron activation analysis, atomic absorption spectroscopy, scanning electron microscopy (SEM), inductive coupled plasma spectroscopy (ICP), etc.

which were primarily used in the initial days of glass analysis.    

Present day forensic analysis of glass

Basic steps of forensic glass analysis are same in the present day as well. Parameters of glass examination are same only the methods of examination are evolved. Physical matching or jig-saw method is still in use. Measurement of density is reduced in present days due to its limitations but still, it performed in some of the laboratories [2]. Density gradient tube filled with liquid has been discovered for the determination of density. Method of determination of refractive index has been evolved in the new era. Glass refractive index measurement (GRIM) is an automated system for determining refractive index using the immersion method. Emmons procedure has been developed for the determination of refractive index. This method uses a hot-stage microscope in association with different source lamps [1][7]. Microscopic advancement has been increased in recent years which helps in the preliminary examination of glass evidence. Elemental analysis has highly emerged in the present day. Nowadays scanning electron microscopy in conjugation with energy dispersive x-ray spectroscopy (SEM-EDX) is primarily used for elemental analysis of glass [8].  Conventional method inductive coupled plasma (ICP) spectroscopy has some disadvantages and to overcome these disadvantages ICP is coupled with optical emission spectroscopy (OES) [ICP-OES], and ICP-OS is also coupled with mass spectroscopy [ICP-OES-MS] for better result [2][9]. Laser ablation inductive coupled plasma mass spectroscopy [LA-ICP-MS] has been discovered for the detailed elemental analysis of glass [10].

Future forensic analysis of glass

According to the present scenario, the glass industry is manufacturing advanced glasses which will need more advanced techniques for glass analysis. Glass manufacture process has been evolved over the last few years. Conventional glass composition varies with modern-day glass composition. As with changing and emerging composition of glass, method of its elemental analysis is also emerging. In the future glass analysis will not be limited to present-day technologies. Researches in the field of elemental analysis are witnessing new advancement like total reflection x-ray fluorescence (TRXRF), scanning electron microscopy x-ray (SEM-XRDF) diffraction techniques, etc. [2]. 

Case study: Susan Nutt (1987)

In February 1987, Craig Elliott Kalani 19-year old killed in a hit-and-run case.  He went for a night walk along with his dog in his neighbourhood in northwest Oregon at 9:30 PM, but never returned to his home. As the investigation proceed police found some glass pieces near his body and some of the glass fragments in Craig’s pocket.

After the collection of glass fragments, police started to search a vehicle that had damages consistent with a hit and run case. They found a car with a similar kind of damages to a woman named Susan Nutt. In order to connect Susan to the crime, it was needed to match those damages of the car to the glass fragments found at crime scene. During forensic analysis, analyst found that the glass fragments from the crime scene matched the Susan’s car damages in a preliminary examination i.e., Physical match. During elemental analysis, analyst found that windshield glass from the crime scene and Susan’s car contained the same 22 chemical elements. And that concluded that the glass fragments found at the crime scene belonged to Susan’s car.

The glass evidence helped to convict Susan Nutt. She was sentenced to up to five years in prison and five years in probation.

Glass has been used as evidence in criminalistics for over a century and its evidentiary value is still increasing day by day. The face of forensic analysis of glass has been changing over recent years due to advances in the identification of physical matching, physical and optical properties and elemental analysis. Advancement in forensic glass analysis has been reached the next level in the modern era and it will keep enhancing in upcoming years.

• Forensic glass analysis James E. Girard – Criminalistics: forensic science, crime and terrorism 2 nd 97-112
• The forensic analysis of glass evidence: past, present, and future BW Kammrath , AC Koutrako – Forensic Science: A multidisciplinary approach, 2016 – books.google.com 299-329.
• Elemental analysis of glass and paint materials by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for forensic application Tatiana Trejos, Waleska Castro, Jose r. Almirall October 2010 – 17-19.
• Forensic Glass Comparison: Background Information Used in Data Interpretation Maureen C. Bottrell April 2009 vol 2 no. 2. ( https://archives.fbi.gov/archives/about-us/lab/forensic-science-communications/fsc/april2009/review ).
• Density determination of irregular shaped and small glass fragments by Stoke’s law: An alternative technique for the forensic analysis of glass Panadda, C. Ratchapak and P. Nathinee 2018.
• Microscopic analysis methods https://www.contractpharma.com/issues/2014-10-01/view_features/using-microscopical-analysis-methods/ (Last accessed on 24-10-2021)
• Forensic classification of glass employing refractive index measurement Umi K. Ahmad, Nur F. Asmuje, Roliana Ibrahim, Nor U. Kamaruzaman. Malaysian journal of Forensic Science 2012, 3(1).
• A Review of Scanning Electron Microscopy Investigations in Tellurite Glass Systems Ali E. Ersundu, Miray Celikbilek, Suheyla Aydm 2012.
• Analysis of major elements in glass sample with a sequential ICP- OES Jobin Yvon S.A.S Horiba Group, Longjumeau, France.
• 10 modern technologies in forensic science https://www.forensicscolleges.com/blog/resources/10-modern-forensic-science-technologies (Last accessed on 24-10-2021)
• Forensic glass analysis by LA-ICP-MS: assessing the feasibility of correlating windshield composition and suppler Abbegayle J. Dodds, Edward M. “Chip” Pollock, Donald P. Land October 2010.
• https://prezi.com/fj2a8w7v6fbu/craig-elliott-kalani-hit-run/ (Last accessed on 29 th October 2021).

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Green River Murders

In 2003, Gary L. Ridgway pleaded guilty to 48 counts of homicide, prompted by his desire to avoid the death penalty. Ridgway’s attorney was quoted in the press as stating that the paint evidence was crucial in his client’s decision to change his plea (Ryland and Suzuki, 2012).

It was the identification and analysis of microscopic particles of paint, orders of magnitude smaller than those that searched for by most crime laboratories that lead to this break in the case.  Ultimately, Microtrace was able to link these particles to a specific, and unusual, type of paint that was used in the location where Gary Ridgway worked.   The involvement of Microtrace in this investigation has been documented in a variety of print articles and television programs.  One summary can be read here , while a selection of others are listed below:

• Green River, Running Red: The Real Story of the Green River Killer
• From the Green River: Forensic Evidence and the Prosecution of Gary Ridgway
• Biography – The Green River Killer
• Three more charges against Ridgway in Green River case
• Prosecutor’s Summary of Evidence in the Green River Murders
• Cold Case Files – The Green River Killer
• Radio Interview: Crime Solving with Microscopic Evidence
• Murders under a Microscope
• Behind the scenes of Ridgway’s stunning confession
• Ridgway faces 3 new charges
• Introduction to the Analysis of Paint Evidence

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Interpol review of paint, tape, and glass evidence 2019–2022

Jose almirall.

a Florida International University, Department of Chemistry and Biochemistry, Center for Advanced Research in Forensic Science, Miami, FL, USA

Tatiana Trejos

b West Virginia University, Department of Forensic and Investigative Science, USA

1. Introduction

This review paper covers advances in scientific methods applied to Glass, Paint, and Tape reported since the 19th Interpol Forensic Science Symposium in October 2019. The review includes peer-reviewed literature, published reports, books, and book chapters on the subjects. Forensic examiners should also be aware of the publication of standard practices, guides, and test methods (e.g., ASTM) and the developments within the manufacturing industries, including production volumes, production locations, and the current trends in the manufacture of these widely used materials.

The leading peer reviewed journals reviewed for this paper were Forensic Chemistry, the Journal of Forensic Sciences, and Forensic Science International. In addition, more than seventy (70) different analytical chemistry or other science journals have published peer-reviewed communications on the advances in forensic paint, tape, and glass examinations in the past 3 years.

Jeff Teitelbaum, Forensic Research Librarian at the Florida International University's Global Forensic and Justice Center, contributed with the primary literature search using keywords for forensic paint, glass, and tape. In addition, literature from several search engines such as Science Direct, SciFinder, Web of Science, and Google Scholar from the last three years were also considered. The literature review is primarily focused on peer-reviewed articles and books published since the summer of 2019 to the summer of 2022.

This review made evident a growing interest in the trace evidence community on chemometrics and statistical interpretation of the data. The increasing literature in this field can facilitate wider adoption in operational forensic laboratories. Several studies have emphasized using statistics for pattern recognition, data reduction and classifications, optimization and validation of analytical procedures, and assessing the evidential value of the traces. An array of methods was reported, including principal component analysis, discriminant analysis, neural networks, and likelihood ratios, to mention some.

1.1. Books, review articles, and consensus-based standards

Various books and book chapters on glass, paint, tape, and, more generally, trace materials evidence were published in the review period:

• 1) The first edition of the Handbook of Trace Evidence Analysis contains seven chapters on trace evidence, including five pertinent to this review [ 1 ]. Chapter 1 focuses on trace evidence recognition, collection, and preservation. Chapter 2 thoroughly reviews polarized light microscopy for trace evidence, while Chapters 3 and 6 focus on paint, polymers, and glass evidence, respectively. Finally, Chapter 7 provides a discussion of interpretation approaches in trace evidence.
• 2) The second edition of Paint Analysis: The Handbook for Study and Practice discusses in detail the analysis of paints and coatings, emphasizing analytical testing, sampling, failure analysis, and quality control in the coating industry [ 2 ]. Part IV of the book covers 12 chapters dedicated to analytical methods, including optical light microscopy, fluorescence microscopy, infrared spectroscopy, surface infrared spectroscopy, infrared microscopy, Raman spectroscopy, time-of-flight secondary ion mass spectrometry, scanning electron microscopy, electron microanalysis, X-ray photoelectron spectroscopy, thin-layer chromatography, and gas chromatography-mass spectrometry.
• 3) The book Resins for Waterborne Coatings presents a comprehensive discussion of market trends, regulations, industry terminology, and the chemistry and formulations of various resins such as alkyds, epoxy, silicone, alkali silicates, amino resins, and hardeners [ 3 ].
• 4) Chapter 21 of the third edition of Forensic Science: The Basics by Siegel and Mirakovits discusses paint and fiber analysis fundamentals, including some case studies [ 4 ].
• 5) Chapter 14 of the 13th edition of Criminalistics: An Introduction to Forensic Science, by Saferstein and Roy, provides an overview of paint, soil, and metal evidence [ 5 ].
• 6) In Chapter 8 of the Analytical Pyrolysis Handbook: Examination of Forensic Evidence, the authors describe the application and interpretation of Py-GC-MS data for the forensic examination of various trace materials, including architectural, industrial, and automotive paint, adhesives, rubbers, and plastics, to mention some [ 6 ].
• 7) Chapter 14 of the Practical Forensic Microscopy: A Laboratory Manual outlines a laboratory exercise to develop basic skills on general characteristics of paint by cross-sectioning, solubility testing, and microscopic examination [ 7 ].
• 8) Chapter 5 of Crime Scene Management within Forensic Science discusses the fundamentals of composition and analysis of paint, soil, and glass evidence [ 8 ].
• 9) In Chapter 6 of the Forensic Science Handbook, Suzuki provides a comprehensive discussion of the theory, instrumentation, and interpretation of FTIR spectra, with two sections dedicated to paints, polymers, and adhesives [ 9 ].
• 10) In Chapter 2 of the Forensic Science Handbook, Wright and Thornton describes the forensic examination of pains [ 10 ].

Also, various review articles were published in the past three years. In 2020, Trejos et al. wrote an overarching review on five trace subdisciplines: hair, fiber, tape, paint, and glass [ 11 ]. This review discusses the current state of trace evidence, along with a historical evaluation of the scientific foundations and advancements in the field.

In the same year, Lavine et al. presented a literature review of the criteria used in forensic science to compare IR spectra [ 12 ]. The article focuses on library searching (spectra comparison to a database), pattern recognition for discrimination purposes, and the use of likelihood ratio to assess the strength of spectral similarity in comparative analysis. Various similarity metrics algorithms for library searching were discussed, including the correlation coefficients calculated from the mean-centered absorbances or the absorbance values, the Manhattan city block distance (absolute absorbance differences), and the Euclidean distance. The authors recommended caution when using the hit quality index to account for sample contamination, signal to noise, instrument type, and measurement geometry. Linear and quadratic discriminant analysis, support vector machines, SIMCA, and principal component analysis are among the pattern recognition algorithms discussed in this manuscript.

Duarte et al. published a comprehensive review of the automotive paint analysis literature from 2010 to 2019, focusing on analytical instrumentation and chemical analysis [ 13 ]. Among the methodologies listed were the most utilized PLM, FTIR, Py-GC/MS, SEM-EDS, UV–Vis, and XRF, and other newer methodologies in paint examinations using Near Infrared, ICP-OES, ICP-MS, DART-MS, Fluorescence Microscopy, Optical Coherence Tomography, SERS, and LIBS. Kaur et al. also published a literature review on automotive paint focusing on instrumental analysis from 2015 to 2020 [ 14 ]. Like the Duarte et al. review, the authors agree on the techniques and relative frequency of publications per methodology, with a large overlap in the citations provided.

Finally, in 2022, Bailey et al. published a review of analytical chemistry focused on surface analysis in forensic science, including a section on paint and glass literature in the past decade [ 15 ].

Among some of the consensus-based standard guidelines and practices that were recently published or updated during this review period are:

• 1) ASTM E2808-21a Standard Guide for Microspectrophotometry in Forensic Paint Analysis [ 16 ].
• 2) ASTM E1610-18 Standard Guide for Forensic Paint Analysis and Comparison [ 17 ].
• 3) ASTM E2937-18 Standard Guide for Using Infrared Spectroscopy in Forensic Paint Examinations [ 18 ].
• 4) ASTM E3234-20 Standard Practice for Forensic Paint Analysis Training Program [ 19 ].
• 5) ASTM E3296-22 Standard Guide for Using Pyrolysis Gas Chromatography and Pyrolysis Gas Chromatography-Mass Spectrometry in Forensic Polymer Examinations [ 20 ].
• 6) ASTM D5380-21 Standard test method for identification of crystalline pigments and extenders in paint by X-ray diffraction analysis [ 21 ].
• 7) ASTM E3296-22 Standard Guide for Using Pyrolysis Gas Chromatography and Pyrolysis Gas Chromatography-Mass Spectrometry in Forensic Polymer Examinations [ 22 ].
• 8) ASTM E3260-21 Standard Guide for Forensic Examination and Comparison of Pressure Sensitive Tapes [ 23 ].
• 9) ASTM E3233-20 Standard Practice for Forensic Tape Analysis Training Program [ 24 ].
• 10) ASTM E1967-19 Standard Test Method for the Automated Determination of Refractive Index of Glass Samples Using the Oil Immersion Method and a Phase Contrast Microscope [ 25 ].
• 11) ASTM E2330-19 Standard Test Method for Determination of Concentrations of Elements in Glass Samples Using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for Forensic Comparisons [ 26 ].

A list of the ASTM standards that have been approved by the Organization of Scientific Area Committees (OSAC) can be found on the Trace Materials Subcommittee of the OSAC: https://www.nist.gov/organization-scientific-area-committees-forensic-science/trace-materials-subcommittee .

Moreover, the ENSI Paint and Glass Working Group has developed the following best practice manual and guidelines:

• 12) EPG-BPM-001 Best Practice Manual for the forensic examination of paint [ 27 ].
• 13) EPG Guideline-001 Guideline for the initial inspection search and recovery of forensic paint samples [ 28 ].
• 14) EPG Guideline-002 Guideline for the forensic examination of paint by FT infrared spectroscopy [ 29 ].
• 15) EPG Guideline-003 Guideline for the forensic examination of paint by Raman spectroscopy [ 30 ].
• 16) EPG Guideline-004 Guideline for the forensic examination of paint by SEM-EDS [ 31 ].
• 17) EPG Guideline-005 Guideline for the forensic examination of paint by Pyrolysis GC-MS [ 32 ].

1.2. Paint evidence

1.2.1. analytical methods and spectral comparisons.

FTIR spectroscopy is an informative and rapid technique that enables the non-destructive analysis of small samples. Thus, it is not surprising that IR spectroscopy continues as one of the techniques more widely reported in the literature for paint examinations, although many other techniques are being investigated. Raman spectroscopy is the second most used technique described in the paint literature.

In 2021, Kwofie et al. demonstrated the advantages of a sample preparation approach that avoids embedding media and therefore prevents spectral interference from the epoxy casting material [ 33 ]. In this study, they perform the paint cross-section by securing the paint chip between two rigid pieces of polyethylene before cutting in the microtome. Then, the authors performed IR image maps of the automotive paint fragments. The spectra of the individual paint layers were reconstructed by alternating least squares (ALS) and compared against a library. Superior library matches were feasible with the proposed method compared to embedding the paint in epoxy for microtome cross-sectioning. In a follow-up study, the same group evaluated the FTIR and machine learning algorithms for the make/model classification of OEM automotive paints [ 34 ]. The validation dataset consisted of 4-layer paints from the RCMP collection from 26 OEM vehicles (2000–2006). The results showed that the machine learning approach using search prefilters to focus on binder signatures could be applied to the ALS reconstructed IR spectra of each layer to identify OEM paint manufacture information.

In 2021, Suzuki published a series of papers describing his work on the characterization of perylenes organic pigments by FTIR [ [35] , [36] , [37] , [38] ]. The survey included OEM automotive paints from 1974 to 2019, obtained from the CTS Reference Collection of Automotive Paints, paint manufacturers, the FBI Laboratory National Automotive Paint File, local automobile rebuild shops, and vehicles in salvage yards. In the first manuscript, the author thoroughly discusses the FTIR spectra and main functional groups of five perylenes, information about finishes containing these pigments, and strategies to identify the perylenes when present with other components, including red mica. This study was focused on Perylene Red Y (C.I. Pigment Red 224), which was found mainly in vehicles manufactured before 1989 [ 35 ].

A second manuscript describes the characterization of Perylene Maroon (C.I. Pigment Red 179), a pigment commonly found in red o maroon metallic basecoats [ 36 ]. The study also describes the IR identification of red alumina pearlescent pigment found in finishes manufactured after 2000. The IR spectra of 143 red or maroon metallic OEM basecoats were examined, and the Perylene Maroon was found to be prevalent in these finishes. A third publication reports the identification of Perylene Bordeaux (C.I. Pigment Violet 29) [ 37 ]. This pigment was found in both metallic and nonmetallic automotive finishes with maroon hues. Unlike the other two perylenes previously discussed, perylene Burdeaux is used in light to medium amounts and combined with other pigments, making the IR identification less straightforward. A variety of examples of IR spectra with pigment combinations are illustrated and discussed. The fourth perylene, Perylene Red (C.I. Pigment Red 178), was found exclusively in nonmetallic finishes with red hues [ 38 ]. This pigment was used in OEM finishes from 1984 until the early 1990s. Overall, this series of publications provide information about the use of IR spectra to identify these commonly used pigments, along with strategies that can be used in casework for comparative analyses and vehicle identifications.

Various paint discrimination studies have been reported in this period. In 2020, the Forensic Chemistry and Physics Laboratory in Singapore conducted an in-house population study of 256 vehicle paints of six major colors [ 39 ]. After examination and comparison by microscopy, SEM-EDS, and FTIR, 99.98% of the paints were differentiated. In addition, a population study of the automotive distribution in Singapore was obtained from data from the local transport authority, surveys conducted on random vehicles, and the laboratory collection of automotive paint samples. Distributions of makes were compared to those reported in the literature for Australia, US, and Canada, with notable differences observed. Higher relative percent of Toyota and Honda makes were documented in the Singapore survey, while no General Motors were observed, which was predominant in the other compared regions. Substantial differences were also observed in the frequency of decorative flakes on black topcoats compared to other published studies, indicating the importance of using region-relevant information to interpret paint evidence.

Sharma et al. [ 40 ] studied the discrimination of sixty red spray paints from 20 manufacturers by ATR-FTIR and principal component analysis (PCA). Data pre-processing for PCA included baseline offset and linear baseline correction, smoothing with Savitzky–Golay and second polynomial order, normalization by range, and multiplicative scatter correction (MSC). PCA achieved correct discrimination of all samples and correct classification of eight blind paints. Among the substrates examined, cement, fabrics, and paper produced poor spectra due to the matrix interferences, while gloves, metal, plastic, leather shoes, tile, wood, and hair substrates all rendered good results and similar spectra to the neat spray paints. Good reproducibility was reported for the intra-brands samples (three per brand).

He et al. reported using Microscopic Laser Raman spectroscopy (MLRM) to compare white architectural paints [ 41 ]. Analyzing 252 architectural paints from seven manufacturers using Bayes discriminant analysis yielded 100% discrimination of the samples. The data preprocessing that provided the best results included Newton interpolation polynomial correction combined with Savitzky-Golay and polynomial smoothing. Principal component analysis was used to reduce data dimensionality, and Bayes discriminant analysis (BDA) and multilayer perceptron neural network (MLP) were evaluated for sample classification. The study demonstrated the utility of MLRM for rapid screening of architectural paints and the use of statistical models for classification purposes.

Duarte et al. analyzed a collection of 143 white automotive paints by ATR-FTIR and coupled microscopy [ 42 ]. Spectra were collected from vehicles involved in car accidents and analyzed in five locations. Principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) was used for sample classification. The model classification accuracy range from 79% to 100%, depending on the set used for calibration, testing, or validation.

In a study by Verma et al., a set of 40 automotive samples from two Indian manufacturers were evaluated by solubility tests. A subset of 10 samples was further examined by ATR-FTIR [ 43 ]. Samples from the two manufacturers were differentiated based on solubility and chemical composition. In another study by the same group, a small set of nine paint samples collected from Indian vehicles manufactured by Maruti Suzuki was analyzed by SEM-EDS. All paints were cross-sectioned, embedded in epoxy, and reported to contain two layers. The authors say the presence of thulium, which they hypothesized may be linked to this manufacturer [ 44 ].

In a research study by Sabaradin et al., 80 simulated automotive paint samples were analyzed using Py-GC-MS, with an emphasis on evaluating the discrimination of paint transfers of two layers: base coat and primer [ 45 ]. The simulated samples were prepared on aluminum sheets, where ten different primers were applied, followed by eight types of red base coats. The chemical composition of the various constituents is reported and yielded grouping of the samples into three distinctive classes via PCA and cluster analysis.

Zieba-Palus investigated the variability of the chemical composition of automotive paints across various metallic and plastic body parts [ 46 ]. A set originating from 30 different vehicles was analyzed by FTIR and Raman Spectroscopy. Most bumpers consisted of 2-layer systems, while the metallic parts of the vehicles were predominantly 4-layers. Although some metal and plastic parts of the same vehicle contained similar compositions, most samples contained different chemistry identified by FTIR (e.g., binders) and Raman (e.g., organic pigments). Although some differences are expected due to the various functionalities of the body parts, the authors noted that the samples were not guaranteed to be OEM. Therefore, some of the differences may also be due to aftermarket repairs. The study complements existing literature on this topic and reinforces the importance of collecting representative samples near the damaged area to avoid false exclusions during a known-questioned comparison.

Coelho et al. proposed a simplified screening approach to analyze automotive paints by ATR-FTIR as a whole specimen instead of conducting individual layer analyses [ 47 ]. The study consisted of fragments collected from six vehicles' hood and side panels. The preliminary findings report notable differences in the various paint sources.

In an article by He et al., ship deck paint was the subject of discrimination studies by ATR-FTIR [ 48 ]. The experimental design included 150 ship deck green paints from five brands. From these samples, 125 were randomly used as the training set for the machine learning algorithms, and the remaining 25 samples (5 of each of the five brands) were used for the validation set. The spectra were preprocessed using automatic baseline correction, peak area normalization, multiple scattering correction, and smoothing. Then, three algorithms were evaluated for classification, principal component analysis (PCA), Fisher discriminant analysis (FDA), and K-nearest neighbor analysis (KNN). The classifier algorithms were evaluated in terms of accuracy, precision rate, recall rate, and F-measure. The FDA using the first or second derivative spectra, performed best in correctly classifying the samples by brand and manufacturer (100% correct classification of the validation samples).

In 2020, Wei et al. used chemometric analysis to classify car bumper splinters from hit and runs using ATR-FTIR [ 49 ]. Fisher discriminant analysis (FDA) and support vector machine (SVM) were used to classify 156 car bumper samples from 10 different brands. The classification models were evaluated using the full IR spectra or selected fingerprint regions, with accuracy ranging from 88.5% to 100%, depending on the algorithm employed.

Qiu e al. used data fusion of Micro-laser Raman spectroscopy (MLRM) and ATR-FTIR for characterizing and differentiating 160 bumpers from 8 different manufacturers [ 50 ]. Classification algorithms included PCA, Multi-layer perceptron neural network (MLPNN), and FDA. The authors concluded that the fusion spectra models perform better than single spectra datasets, with the FDA with feature-level fusion providing the best discrimination.

Besides IR and Raman Spectroscopy, few studies utilized mass spectrometry methods for paint examinations. In a DART-MS review paper, Sisco and Forbes discuss the existing literature for the application of DART-MS for identifying organic pigments and other organic compounds from paint evidence [ 51 ]. In 2022, Gupta and Samal published a review of current DART-MS applications in forensic materials, including paint and tape from improvised explosive devices [ 52 ].

In 2021, Alderman et al. investigated Dynamic Headspace GC-MS using activated carbon to analyze volatile organic components in paints and architectural coatings. It was determined that quantitative analysis was feasible for seven distinctive VOCs [ 53 ]. In another study, volatile organic components from spray paints were studied by SPE-GC-MS [ 54 ]. The headspace above paper sprayed with paint was sampled every 30 min for 4 h using a 65 μm thick polydimethylsiloxane/divinylbenzene SPME fiber and a single quadrupole GC-MS. The study found that VOCs are lost rapidly, but low concentrations remain in the headspace beyond 4 h. Thus, VOCs could be used to establish the time since deposition on a sprayed surface.

1.2.2. Studies related to databases and interpretation of trace evidence

The PQD database continues to be a valuable investigative tool in cases such as hit-and-runs. The database, maintained by the Royal Canadian Mounted Police (RCMP), released an update in 2018 and included a paint search exercise, referred to as a search quiz. The exercise can be used to assess a paint examiner's competence to conduct a PDQ search for training and as a peer-review process to compare consistency in search findings and reporting between analysts and technical reviews. In 2021, Wright published an approach the FBI followed for conducting searches using this quiz tool. She proposed its use for PDQ searches' competency assessment and to establish the required documentation of a technical review [ 55 ]. Two analysts conducted the make-model-year searches by the component's text-based system (Layer System Query, LSQ) and spectral library searches using the Bio-Rad® Know-It-All™ software. The article discusses each examiner's independent findings, examples of reporting language utilized at the FBI, and some recommendations.

Klaasse et al. at the Netherlands Forensic Institute presented a prototype multifunctional database, named TraceBase, for forensic trace analysis suitable for investigative and comparative case evaluations [ 56 ]. The novelty of the database architecture relies on its capability for data input and output from various trace materials and analytical methods. It is designed as an open-source server-based back-end where a modular approach can retrieve the data. The authors illustrate the utility of the databases with examples for textiles and glass. The database has the flexibility to introduce the data in various formats. For instance, in a textile case, infrared data can be included as full IR spectra, text-based interpretation of IR components, or not be included in the search. The proposed database can potentially facilitate the assessment of the evidential value in comparative studies characterization or identification of unknown samples to provide investigative leads.

The problem of subjectivity in spectral comparisons that heavily relies on human intervention is reported in the comparison of paint and tape for various spectrochemical methods (e.g., comparison of IR, XRF, EDS, Raman). In a recent review paper, Lavine and co-authors discuss the need for statistical support in comparing IR spectra to assess the quality of a spectral comparison and minimize the appearance of personal bias [ 12 ]. Quantitative metrics for the comparison of spectra have also been proposed. For instance, Lavine and co-workers and other research groups have broadly reported the statistical comparison of IR spectra of automotive paints for library searching and pattern recognition purposes.

A study by Falardeau et al. addresses the interpretation of paint at the activity level. This study conducted experimental work on the aerosol distributions of fluorescent paint in open and closed environments to evaluate spray paint evidence at the activity level [ 57 ]. The surrounding areas were covered with paper sheets, and after spraying, the paper and clothes were observed under regular light and photographed. Luminescence images were also recorded at 450 nm and 570 m and then transformed into black and white images. A computer-based program extracted the droplet features from photographic images to study their size and distribution. For open environments, the authors found some correlations between the droplets’ density and the proximity areas that can lead to differentiation of a bystander versus a person performing the graffiti.

Another study addressing the interpretation of paint evidence and other trace materials at the activity level is presented by Letendre et al. [ 58 ]. Factors related to background, persistence, transfer, contamination assessment, and activities simulating casework activities and common substrates found at crime scenes were discussed in the manuscript. For paint, the authors gathered information from 134 studies, including population and discrimination studies, background occurrence, and transfer of these traces.

Menżyk et al. discussed the evidential value of polymeric materials using chemometrics and a likelihood ratio approach [ 59 ]. The study collected polymeric objects from automotive and household polymer and paint items and analyzed them by NIR and FTIR. Data preprocessing for NIR included standard normal variate transformation (SNV), first derivatives (FD) with a window width of 13 points followed by SNV, and localized standard normal variate transform (LSNV). For FTIR data, the authors utilized the first derivative of the spectra followed by SNV. Likelihood ratios were estimated after dimensionality reduction and feature selection for the probabilistic interpretation of spectral data.

As evidenced from the examples highlighted above, databases, chemometric methods, and more comprehensive interpretation approaches show great potential for better use of the data collected from forensic analyses.

1.2.3. Weathering/stability

The pigment stability during artificial weathering on wood coatings was studied with hyperspectral imaging, reflectance spectrometry, and FTIR [ 60 ]. Exposure included cycles of water condensation and UV under controlled weathering conditions for 40 days. The findings indicate that natural indigo was more resistant to degradation than commercial pigments and that the stability is affected by interactions of raw materials. For instance, it was experimentally observed that titanium dioxide accelerated the aging of the paint coatings.

Sanmartín and Pozo-Antonio studied the weathering of graffiti spray paints exposed to UV radiation [ 61 ]. The experiment included various surfaces (granite, rhyolite, biocalcarenite, and glass) exposed to long, medium and short UV wavelengths. Thirteen samples were analyzed for each of the four substrates in triplicate, for a total of 208 samples. One sample per group was treated as a control and therefore was not exposed to UV radiation. Analysis by gravimetric measurements, digital imaging, color spectrophotometry, and infrared spectroscopy was conducted at the beginning of the study and four and eight weeks after irradiation. Among the four spray paint colors evaluated (red, black, silver, and blue), only the black paint did not show physical or chemical changes. Short and medium UV radiation caused the most significant physical and chemical changes.

1.2.4. Paint case reports

Kaur et al. present a brief overview of the forensic examination of paints, including a discussion of some cases from the literature [ 14 ].

Lee et al. presented a case report investigating three maritime collisions. After a forensic examination of paint residues recovered from the ship of interest, the other vessel(s) involved in two of the three collisions were identified [ 62 ]. The samples were taken from various collision sites (e.g., the deck, guardrails, port-side, starboard) and from 2 to 3 suspected ships per case. The analytical scheme included scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), attenuated total reflection–Fourier transform infrared spectroscopy (ATR–FTIR), thermogravimetry (TG), derivative thermogravimetry (DTG), and pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). The authors presented a detailed description and interpretation of the data and their rationale for their conclusions. This study sets an essential precedent for marine paint examinations and comparisons, which is relatively limited in the literature.

1.2.5. Artwork and cultural heritage

A vast number of paint publications were focused on the authentication of artwork and cultural heritage paint. In a point of view article, Oliviera Andrade discusses the current state and challenges of the forensic analysis of art and cultural heritage items [ 63 ]. The authors highlight the need to conduct multidisciplinary and inter-agency collaborations to identify fine counterfeits and alterations, including a vast battery of orthogonal methods.

Sirro et al. utilized Raman spectroscopy on a counterfeit case of fake paintings of the 20th-century Russian avant-garde [ 64 ]. The study showed that Raman was sensitive to time markers of the zinc oxide. The results of Raman spectroscopy were complemented with pyrolysis gas chromatography, mass spectrometry, and X-ray analysis. The authors analyzed a collection of over 400 paintings to cover a broad period of dye technology, including the beginning and end of the 20th century (1900–1997). The manuscript describes the basis for differentiating chemical profiles from genuine and fake specimens.

Various analytical tools were presented in the literature for the analysis of artwork and cultural heritage paints, including FTIR [ [65] , [66] , [67] , [68] , [69] , [70] ], portable microscopy, and XRF [ [70] , [79] ], SEM-EDS [ 67 , 68 , [70] , [71] ], Raman spectroscopy [ 67 , 72 , 73 ], far-infrared spectroscopy in ATR mode [ 66 ], hyperspectral reflectance imaging in the visible [ 74 , 75 ] and near-infrared, XRF [ 71 , 76 ], portable macro X-ray fluorescence [ 75 , [77] , [78] , [79] ], pyrolysis GC-MS [ 68 , 80 , 81 ], micro reflectance imaging spectroscopy (micro –RIS) [ 82 ] and time-of-flight secondary ion mass spectrometry imaging (TOF-SIMS) [ 83 ]. Botteon et al. evaluated the use of portable micro-spatially offset Raman spectroscopy (micro- SORS) to analyze pieces of artwork from the 16th century [ 84 ]. Rooney et al. [ 85 ] and Taugeron et al. [ 86 ] assessed the utility of NMR in studying cultural heritage objects in paint layers.

In 2022, La Nasa et al. used microscopy, SEM-EDS, and pyrolysis GC-MS to characterize the paint components from two Messerschmitt Bf 109 historic airplanes, one from the Deutsches Museum collection and one privately owned [ 81 ]. Depending on the area of the plane, 2 to 13 layers were fully characterized and attributed to the first factory paint, the Legion Condor, Spanish Airforce, or museum preservation paints. This is the first study of this kind reporting the characterization of original aircraft paint materials fabricated in the 1920s and repainted during various subsequent periods.

In the same year, Tutt and Hoffmann presented an interesting approach to the forensic examination of historic vehicles, an area of forgery that is recently growing due to the increased market value of these collection automobiles [ 87 ]. A multi-technique and interdisciplinary approach was utilized to illustrate various case examples.

1.2.6. New trends in paint formulations

Nanomaterials are becoming more prominent in paint formulations in automotive, marine, and other industrial applications because their mechanical, thermal, electrical, chemical, and physical properties favor the robotic nanospray painting process and overall end-product quality [ [88] , [89] , [90] , [91] , [92] , [93] ]. The automated spray painting is most common on the anti-chip coat, primer surface, base coat, and clear coats, although they are applicable to other paint layers. Several studies discuss the use of paint infused with carbon nanotubes (multiwall carbon nanotubes, WWCNTs) by ultrasonication and magnetic stirring processes to enhance the quality of spray coating [ 94 , 95 ]. Modifications of carbon nanotubes (CNTs) and epoxy-CNT composites have also been investigated [ 93 , 96 ]. The MWCNTs are rolls of graphene produced by chemical vapor deposition, often in polyurethane base coats. Typical sizes range from 30 to 80 nm in diameter and are 5 μm long [ 95 ]. Advantages of the nano paints are the improved quality, including anti-corrosion, self-cleaning, coolant/isolating properties, smoother surfaces, and increased life span. Also, they provide a more environmentally friendly and cost-effective approach. As a result, they are anticipated to become more commonly observed in casework items. These nanomaterials have been analyzed by Raman spectroscopy, XRD, SEM-EDS, Transmission Electron Microscopy TEM, and Differential Thermogravimetry DTG, to mention some [ 95 , 97 ].

Advances in anti-corrosion coatings have gained attention recently, as many corrosion protection systems are based on chromates and volatile organics that are no longer environmentally sustainable. Graphene has been used for the cathodic protection of zinc-rich epoxy coatings [ 98 , 99 ].

Sukanya et al. reviewed the state on the development of anticorrosive films other than graphene, including layer double hydroxides (LDHs), transition metal dichalcogenides (TMDs), MXenes, layered hexagonal boron nitride (h-BN), and graphitic carbon nitride (g-C3N4) [ 100 ]. The LDHs are species that contain divalent or trivalent cations such as Mg 2+ , Zn 2+ , Fe 2+ or Ni 2+ , Al 3+ or Fe 3+ and cations such as carbonates, chlorine, sulfates, and RCO 2 - . The TMDs, are often represented as MX 2 , where M is a transition metal such as Mo or W, and X represents atoms such as S, Se, or Te. The MXenes are another emerging family of coating materials comprised of metal carbides or nitrides (e.g., Ti 3 C 2 T x ). The MXenes nanosheets have a large surface area with a surface chemistry that favors mechanical properties with good thermal and electrical conductivity. The MXenes can be functionalized with binders such as epoxy and silane. Finally, graphitic carbon nitride (g-C3N4) and h-BN are analogous to graphene in terms of mechanical, thermal, hydrophobic, and permeability properties and prevent the diffusion of corrosive analytes. These can also be functionalized with epoxy and polyurethanes and combined with iron oxide nanoparticles (Fe 3 O 4 ) to enhance the corrosion-protective coatings.

Research continues on the development of eco-friendly solvent-free powder coatings. Li et al. reported using waterborne epoxy coatings with functionalized nanomaterials [ 101 ]. For this purpose, MXene-packed structure nanosheets were modified with L-Cysteine to favor anticorrosion and weather protection. The coatings were analyzed by FTIR, XRD, EDS, and XPS, and the anticorrosion properties were measured via electrochemical methods. Wang et al. evaluated silanized Mxene/carbon nanotube composites to aid anticorrosion of polyurethane coatings [ 102 ]. The carbon nanotubes and the functionalized surfaces of Ti 3 C 2 T x and S-CNTs form a shield layer that prevents penetration of corrosive analytes.

New technological advances have also made possible the use of powder coating in non-metallic surfaces, such as medium density fiberboard (MDF) [ 103 ], using low-temperature curing processes. The coatings are created with polyester-hybrid technology with a complete binder package that adheres to MDF and metal with film thicknesses of up to 125 μm per application.

Significant advances for automatic repairing of defects have been made in the last decade within novel self-healing polymeric materials. Hong et al. developed a disulfide polyurethane diol adduct (DSPUDA) with reversibility of bond cleavages that promotes the auto repair of scratches [ 104 ]. Nano-scratch tester and an atomic force microscope are among the instrumentation utilized in this study. Cui et al. also investigated novel scratch resistance and self-healing automotive clear coatings using polymer/graphene composites [ 105 ]. Zotiadis et al. reported successful repairing of microcracks on alkyd-based coatings using epoxy microcapsules loaded with poly urea-formaldehyde [ 106 ].

Another area of interest in coatings technology is the development of UV curable powder coatings. Czachor-Jadaka et al. reviewed the advances in UV curable powder coatings and their advantages compared to thermally cured coatings [ 107 ]. A detailed discussion of the literature on the chemical structure of resins is presented in this review, including unsaturated photoinitiators and additives used in the formulation of UV-curable polyester, urethane acrylates, acrylate and methacrylate resins, epoxy, and polyamides.

The global market of metallic pigments is growing and driving innovative formulations. It is expected to reach $1.20 billion by 2022 [ 108 ]. Among the leading pigments, aluminum and copper pigments account for one-third of the total market, including newer technologies compatible with lower solvent carriers and waterborne coatings. Current trends for more sustainable paint industries are driving away from lead chromate and other leaded pigments.

The automotive refinish coatings market is also following innovations pushed by greener options, with polyurethanes and acrylic latex waterborne basecoat technology being the most popular in collision repair centers [ 109 ]. Primer aerosol coating technology that cures by UV lamp in a few minutes has also been introduced in the auto refinish market.

Thejo Kalyani et al. reviewed luminescent paints' scientific foundations and applications [ 110 ]. Although this type of paint is not commonly encountered in casework, forensic examiners may find helpful information in this article as the number of applications is numerous, and the likelihood of occurrence in particular cases may increase in the future. Among the many uses of luminous paints and pigments, the authors described persistent paints used in roads, building signs, construction materials to visualize materials failures, clocks, lamps, biosensors, forgery identification, and anti-counterfeiting. In the March 2022 volume of the Coatings World Magazine, PPG announced the launch of the first-of-a-kind retroreflective coating (Envirocron LUM powder coating) for a variety of applications, including bicycles, scooters, safety equipment, tools, fences, shopping carts, and road guardrails and signs.

Recycling in the paint industry is also part of global sustainability projects. To minimize the environmental footprint of automotive paint waste, a group in Malaysia evaluated the feasibility of recycling paint sludge disposal to create concrete products [ 111 ]. From a forensic point of view, these recycled materials must be on the lookout for forensic practitioners.

The architectural coatings industry has also changed its formulations over the last decade. One of the most notable is the shift from oil/solvent-based to waterborne paints [ 3 , 112 ]. Environmental regulations have also influenced VOC reduction in latex-based waterborne coatings. As a result, zero-VOC coalescents have been developed, and resins have been adapted to require little to no solvent to form the film. Various paint manufacturers incorporated different low and zero-VOC products into the market. Surfactants like alkylphenol ethoxylates have been banned in Europe and are encouraged to be replaced in some areas in North America due to concerns of bioaccumulation in the environment. Therefore, alternative APE-free resins and surfactants are being used in architectural paints. Alternative surfactants of particle size smaller than 200 nm are required to maintain the stability of these APE-free formulations. Pyrithione compounds and various zinc, silver, or copper preservatives have replaced organic solvents and formaldehyde-containing preservatives. Another critical transition in architectural coatings is the paint-and-primer-in-one products which provide faster applications for the end-users. This has required using pigments with higher hiding power like pigment red 254 or pigment yellow, or dispersing agents that distribute the titanium dioxide in the dried film better.

1.3. Tape evidence

Among the many types of tapes recovered at crime scenes and examined at forensic laboratories, the most popular are electrical, duct, and packaging tape. These materials are often used to bind or gag victims in kidnappings, sexual assaults, and homicides. Also, they are employed in the fabrication of improvised explosives devices, the packaging of drugs, and the sealing of objects involved in criminal activities. Therefore, the physical and chemical examination of pressure-sensitive tapes can provide valuable investigative and intelligence leads and assist the trier of fact with their decisions. Moreover, tape evidence is prone to trap other traces that can become relevant during an investigation, such as DNA, hairs, fibers, fingerprints, and drugs or explosives residues, to mention some. Thus, some studies have focused on how to preserve and recover those traces.

From 2020 to 2022, the forensic examinations of tape have gained attention in various areas, such as the assessment of emerging methods, evaluation of error rates of conventional analytical protocols, statistical interpretation approaches, and evidence preservation. The following sections discuss the most recent literature on these subjects. However, since this is the first time that tape has been incorporated in the INTERPOL reviews, we have included some older pertinent literature.

1.3.1. Emerging analytical methods

Most of the research on emerging analytical methods was focused on electrical and packaging tapes, rather than duct tapes, possibly due to the more significant amount of distinctive physical features observed in the latter. The analytical methods for improved characterization and interpretation of tape evidence reported in this review have been centered on spectroscopic and mass spectrometry techniques.

Characterization and analysis of organic compounds have been reported by infrared spectroscopy (FTIR, ambient ionization methods, and mass spectrometry). Infrared spectroscopy has been long used as a primary tool for the classification and comparison of tapes, and newer studies increase this body of knowledge with a particular emphasis on data analysis and chemometrics. Zięba-Palus utilized infrared spectroscopy for the classification and comparative analysis of packaging, insulation, and duct tapes of various colors [ 113 ]. In this study, the backing and adhesive of 50 samples were investigated by FTIR operated in transmitted mode. Although previous studies have demonstrated the utility of FTIR in the laboratory analytical workflow [ 23 , [114] , [115] , [116] ], this manuscript provided helpful insights into the identification of chemical components. Backings were separated by polymer type into four major groups, with some samples further discriminated based on the spectral comparison in two of those groups. Adhesives were clustered in five groups based on the presence of isoprene rubber, styrene-butadiene rubber, esters, or acrylates. Some discrimination was observed for tapes belonging to the same subgroup by their relative amounts of styrene, carbonates, titanium dioxide, kaolin, and compounds containing a carbonyl group.

In 2022, Nimi et al. examined 75 tape samples originating from 25 brands of electrical tapes by ATR-FTIR [ 117 ]. Data reduction and classification methods (principal component analysis, PCA and linear discriminant analysis, PCA-LDA) were used to group samples based on their spectra. Correct classification rates ranged from 80 to 89% for the adhesive and 84–100% for the backing side. Also, the potential interference from substrates like cardboard, glass, plastic, and steel was investigated and showed that background contaminations influenced the accuracy of adhesive while the backing spectra remained unaltered.

In a study by Oliva et al., laser-assisted sampling (LAS) and direct desorption flowing atmospheric-pressure afterglow mass spectrometry (LAS-FAPA-MS and FAPA-MS) were evaluated for tape analyses [ 118 ]. The authors indicated that the advantages of LAS-FAPA-MS compared to regular Pyrolysis GC-MS are the minimal sample preparation, real-time analysis, and higher degree of spatial information. The ambient ionization apparatus was coupled to a high-resolution mass spectrometer (LTQ- Orbitrap XL) and operated in positive ion mode. In this study, the backing and adhesive layers of 15 masking, electrical, and duct tapes were analyzed, and their mass spectra were compared for classification and comparison purposes. Direct desorption-FAPA-MS provided information on the surface's polymer additives, while LAS-FAPA-MS complemented the spectral data with higher m / z peaks and more fragmentation. The identification of the m / z fragments for 21 compounds is thoroughly discussed in the manuscript, providing a valuable body of foundation for the compound characterizations. The principal component analysis of the combined spectral data showed discrimination capabilities ranging from 80 to 100%, depending on the tape type.

Zhang et al. investigated the use of a diode laser-assisted micro-pyrolysis program (LAMP) coupled with flowing atmospheric-pressure afterglow ambient mass spectrometry (FAPA) for the direct characterization of polymers and polymer additives [ 119 ]. The method was effective in desorbing, pyrolyzing, and ionizing polymer species and their additives. Pyrolysis products were observed in positive and negative modes and confirmed with high-resolution mass spectrometry. Advantages of LAMP compared to regular pyrolysis are superior throughput, faster temperature control, analysis in real-time, and MS imaging with high spatial resolution.

The application of high-resolution magic angle spinning (HR-MAS) 1H NMR spectroscopy was evaluated for tapes [ 120 ]. The results for 90 black electrical tapes were compared to those obtained by the FBI after following their typical analytical scheme (microscopy, FTIR, SEM-EDS, and Py-GCMS. One advantage of HR-MAS is that it is suitable for insoluble materials if they are swellable. The sample preparation consisted of placing approximately 10 mg of the tape in a 4-mm ZrO2 rotor and filling it up with about 70 μL CDCl3. The data was analyzed using hierarchical clustering (HCA). The performance of HR-MAS was superior to conventional methods (98.4% for HR-MAS vs. 96% for all other techniques combined).

Besides the characterization of organic constituents, several studies focused on improved methods for inorganic analysis. The value of elemental analysis of tapes by SEM-EDS has been previously reported in the literature and widely adopted in the field [ 23 , 116 ]. Additional research explored alternative elemental techniques that can achieve higher discrimination and complement conventional analytical schemes, namely X-ray Fluorescence Spectroscopy (XRF) [ 121 , 122 ] and Laser-Based methods such as Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS and Laser-Induced Breakdown Spectroscopy (LIBS) [ [123] , [124] , [125] , [126] ].

The elemental analysis of electrical tape by X-ray Fluorescence (XRF) has demonstrated its forensic utility for classifying unknowns and comparing known and questioned tapes. In a study by Prusinowski et al., three XRF systems were used to evaluate a range of configurations typically used at forensic laboratories [ 121 ]. The discrimination for a set of 40 electrical tapes manufactured from various sources and grades increased from 78.8% by SEM-EDS to 84.6% by LA-ICP-MS and 81.5–91.0% by XRF for 150um spot size iXRF with SiLi, 25 μm spot size benchtop XRF with SD detector, and 1 cm spot size XRF with SiLi detector, respectively. No false exclusions were observed in an additional dataset of 20 fragments of tape originating from the same roll and measured in various replicates on the same and different days.

In a follow-up study by Brooks et al., 114 electrical tape backings originating from 94 different samples and 20 items from the same roll were used as part of the method validation [ 122 ]. Improved classification capabilities were observed for XRF over SEM-EDS, as shown by its ability to detect more elements (14 vs. 6) and distinguish the samples into a larger number of groups (61 distinctive groups vs. 15). The superior performance for characterizing elemental profiles also improved discrimination power (96.7% u-XRF vs. 87.3% SEM-EDS). Also, the performance, advantages, and limitations of different spectral comparison methods are reported in this study, such as spectral overlay, spectral contrast angle ratios, and Quadratic Discriminant Analysis (QDA).

Martinez et al. reported a series of four studies [ [124] , [125] , [126] ] that demonstrated the powerful capabilities of LA-ICP-MS for identifying major and minor elements on tapes and using their elemental profiles for investigative leads and source comparisons. In the first study, the backings of 90 black electrical tapes were analyzed by LA-ICP-MS [ 123 ]. The results were compared to a typical analytical scheme for tape examinations (microscopical examination, Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Pyrolysis–Gas Chromatography-Mass Spectrometry (Py–GC–MS)). The study demonstrated that LA-ICP-MS alone captured a vast amount of chemical information of the formulation components and resulted in improved discrimination and superior characterization compared to the combined analytical protocols. The results suggested that LA-ICP-MS has the potential to be adopted at the front of the analytical workflow for fast decision-making and informative leads. The LA-ICP-MS method alone provided 94% correct discrimination of the tapes known to originate from different rolls and 100% correct association of the tapes known to originate from the same roll.

The second study expanded the application of LA-ICP-MS and LIBS to packaging tape. The methods allowed the characterization of ten relevant elements by LA-ICP-MS and seven by LIBS, including lithium, potassium, and sodium, which LA-ICP-MS did not easily detect [ 124 ]. The selected elements demonstrated good reproducibility within a single source and good discrimination between different sources. The LIBS and LA-ICP-MS data provided some orthogonality, with added discrimination when combined.

In the third study, the LIBS and LA-ICP-MS protocols were subject to two interlaboratory exercises in an effort toward method standardization [ 125 ]. Seven laboratories participated in the exercises, which evaluated the capabilities of SEM-EDS, LIBS, and LA-ICP-MS for the forensic comparison of electrical tapes in mock cases that compared known versus questioned items. The results show good agreement among the participants when using the same analytical instrumentation. All the laboratories conducting SEM-EDS, LIBS, and LA-ICP-MS correctly associate the K-Q pairs of tapes originating from the same rolls (no false negatives reported). False positive rates ranged from 13% to 17% by SEM-EDS, depending on the exercise, while no false positives were reported by any of the LIBS and LA-ICP-MS participating laboratories. The increased discrimination observed by the laser-based methods results from the superior sensitivity and selectivity of the methods. For instance, in this dataset, SEM-EDS detected up to seven elements in the samples of interest, while LIBS and LA-ICP-MS characterized up to 17 and 32 elements, respectively.

Finally, the fourth study presented by this research group uses likelihood ratios to assess the probative value of tape evidence [ 126 ]. The authors conducted data reduction of the LA-ICP-MS spectra by PCA, then used scores of the first five principal components as input to estimate the LR. A posthoc calibration assisted in yielding log LR in more realistic ranges per the dataset size. Low false inclusion and false exclusion error rates of 3.7% and 2.2% were reported, demonstrating the utility of the proposed approach to assess the weight of high-dimensional data.

Kuczelinis et al. developed and characterized a polymer-based in-house calibration standard for examining electrical tapes by LA-ICP-MS [ 127 ]. A collection of 87 PVC black electrical tapes from different rolls were analyzed using a semiquantitative method to characterize up to 30 elements in the backings. In addition, a same-source set of 24 samples (3 fragments from 8 sections) from each of three different rolls was evaluated for false exclusions. The selected match criteria yielded less than 0.7% false exclusions and less than 2.5% false inclusions. In another study by the same team, a novel method was developed using picoliter-volume standard addition to quantify elements in polymeric materials by LA-ICP-MS [ 128 ].

These studies suggest that forensic laboratories can benefit from shifting from SEM-EDS to more sensitive methods (u-XRF, LIBS, or LA-ICP-MS) for the forensic characterization and comparison of electrical tapes.

1.3.2. Physical fits of tape materials

Regardless of the general probative value assigned to physical fits (a.k.a. fracture match), standard criteria and protocols to determine what constitutes a match are still in the early development stages. As a result, we observed a trend in the physical fit literature toward demonstrating the scientific foundations of fracture fit examinations. Various approaches were developed to mitigate bias and assess the relevance of the evidence.

Brooks et al. review the current state of physical fit research in trace evidence materials [ 129 ]. The manuscript discusses the foundations and limitations of the discipline for casework protocols and case reports, fractography and qualitative-based studies, and quantitative assessments of physical fits.

Recent studies formally evaluate the incidence of error rates and accuracy in determining matches and non-matches of fractured trace materials. The reported datasets range between 30 and 2200 samples, evaluated by single or multiple analysts.

A group at the University of California at Davis conducted pioneer research in the quantitative assessment of duct tape physical fits [ 130 ]. The authors studied four different types of separation methods (hand-torn, Elmendorf torn, scissor cut, and cutter cut) on over 2000 duct tape pairs of varying grades and colors. A first effort to provide a quantitative measure of the quality of the match was calculated using a match score as the relative “matching length” divided by the overall width of the tape. Statistical analysis was focused on reporting performance rates. The dataset produced mean accuracy ranging from 98 to 100% for torn tape and 98–99.8% for cut tape. False-positive rates lower than 0.7% and 3.3%, and false-negative rates lower than 2.7% and 0.3% were reported for hand-torn and cut-tapes, respectively.

In 2020, Prusinowski et al. developed a systematic method to quantify the quality of a duct tape physical fit using a similarity score named Edge Similarity Score (ESS) [ 131 ]. The ESS uses the duct tape scrim bins as the smallest comparison unit in a reproducible manner. Then, the ESS is estimated as the relative percent of scrim areas across a questioned fractured edge that “fit” the known comparison tape. This simple approach provides a quantitative estimation of a fit's quality while assuring that the peer-review process can be conducted in a systematic, transparent, and reproducible fashion. Performance rates were estimated for a blind dataset of over 2200 tapes, including sensitivity, selectivity, and accuracy. The effects of the separation method (cut vs. hand-torn), stretching, and tape grade (low, medium, and high grade) on ESS distribution are reported. The study demonstrated that accuracy from 84.9% (higher quality hand-torn set) to over 99% (low and mid-quality sets) is feasible, with no false positives reported. Moreover, the ESS score can be used as a metric to inform and support the examiner's decision. Score likelihood ratios were used as a proxy for the weight of the evidence. Relative scores served as a good predictor for fracture match determinations and provided a means for statistical support for the examination of fracture fits. ESS higher than 80% provided a score likelihood ratio (SLR) that supported the conclusion of a match, and ESS lower than 25% provided an SLR supporting the decision of a non-match.

Another interesting approach to assessing the evidential value of duct tape physical fits was presented by van Dijk et al. [ 132 ]. Here, the authors developed a novel method using likelihood ratios and Bayesian networks to evaluate the weight of the evidence. The study focused on duct tapes with weft-insert patterns, consisting of weft fibers inserted into the gaps between the chain-like warp loops. Briefly, 136 pieces of various duct tape grades were separated by three people, using either a tearing from top-down or bottom-up, creating 272 edges, with a total of 127 true fit pairs. Various BN models were used to estimate the probabilities under the competing hypothesis, Hp and Hd. One of the assumptions made for the models is that the loop breaking patterns comply with the Markov property, where the state of a loop is assumed to only depends on the state of the loop above it. The probability estimates were based on data from the features imparted on the broken loops on torn ends, theoretical input, and expert knowledge. The performance of likelihood ratios was assessed using ECE plots and demonstrated that the system was well-calibrated. Also, this study presented a proof of principle to address the question of how the value of the evidence is influenced by the presence of a partially fractured edge.

1.3.3. Interpretation of tape evidence and tape as substrate for other traces

Weiten et al. presented an interesting discussion of the application of Bayesian Networks to interpret traces left behind on tape surfaces at the source and activity level [ 133 ]. Fingermarks and biological fluids were evaluated at the source level. Then, activity factors were considered in their interpretation models, such as recovery, transfer, persistence, and estimation of the position of the sampling location on the original roll based on fracture fits.

A case study involving the use of duct tape, fingermarks, and DNA was used by de Koeijer et al. to illustrate a practical interpretation approach using Bayesian Networks to combine different evidence types of evidence and explain their combined strength to the courtroom [ 134 ].

Tape lifting has also been proposed as a forensic approach to recovering particulates and traces that offers the benefits of being cost-effective and creating a secure environment for the residues of interest [ [135] , [136] , [137] , [138] , [139] ]. Gwinnett et al. reported using tape (Easylift) for lifting and monitoring micro polymers [ 140 ]. The study reported that in situ analyses of microplastics and fibers were possible through the adhesive since it was compatible with polarized light microscopy (PLM), confocal Raman spectroscopy, fluorescence microscopy, microspectrophotometry (MSP), and hyperspectral microscopy. Kanokwongnuwut et al. evaluated 14 tapes to recover and visualize cellular material using DD staining and fluorescence microscopy [ 141 ]. Three types of tapes showed utility in the collection and screening of touch-DNA, as they did not inhibit direct STR amplification or masked the fluorescence examinations.

In 2020, Chadwick and all performed an extensive study to evaluate if chemicals used to enhance fingermarks negatively impact the physical, optical, and chemical comparative analysis of duct tape evidence [ 136 ]. Untreated tapes were compared with those treated with fingermark enhancers, namely cyanoacrylate, cyanoacrylate stained with rhodamine 6G, and Wet Powder™. Physical and optical examinations were conducted on a stereomicroscope and Video Spectral Comparator (VSC), while chemical analyses were done via ATR-FTIR. Compared to cyanoacrylate-based chemicals, wet powder suspensions were more suitable as they did not affect the physical and chemical characterization of the tape adhesive or backings. The results demonstrate that the applied chemicals had minimal impact on most of the physical or optical characteristics of the tapes selected. Still, differences in the infrared spectra can lead to misidentifications. However, the authors propose an operational workflow that can help forensic examiners to determine the type of developing treatment performed on a tape and use this information during the interpretation of the data rather than evaluate the results in isolation.

1.4. Glass evidence

1.4.1. industry data, forensic laboratory management, book chapters and review papers.

The annual sourcebook for the glass industry, published in July 2022 by the National Glass Association (NGA), contains a list with contact information to over 1200 companies in more than 700 product categories including glass manufacturers, distributors, and suppliers; https://www.glassmagazine.com/issue/july-2022 . The NGA forecasted continued growth in glass production for developed economies, in particular for the U.S. float glass industry with expected growth between 20 and 45% from 2018 to 2022 despite the challenges posed by the pandemic. A downloadable global float plant database that includes dozens of different float glass manufacturers operating more than 230 plant locations with more than 450 float glass lines is available from NGA. The World of Glass 2021 Report [ 142 ] lists notable expansion in glass manufacturing in the U.S. including four new float lines in North America.

The Project FORESIGHT Annual Report for 2020–2021 (published in May 2022) [ 143 ] is a business-guided self-evaluation of forensic science laboratories around the globe, with more than 190 laboratories providing data. The report includes metrics to assist forensic science managers evaluate work processes and laboratory productivity. The aggregate productivity for trace evidence sections, including all materials analyses (e.g. paint, tape, glass and fiber evidence but not fire debris analysis) is reported for casework submissions from July 1, 2020 to June 30, 2021. Some important takeaways from the report include that trace evidence cases include more “items”, “samples” and “tests” than almost every other evidence type submitted to the laboratory. The median number of trace evidence “tests” performed per 100,000 population served in a locality is reported as ∼27 “tests”, in comparison to ∼828 “tests” for DNA casework/100,000 population and ∼1500 Drugs-Controlled Substances/100,000 population. The median “cost” for the average trace evidence case is reported as ∼ $ 5000. in comparison to ∼ $ 3700/case for DNA casework and ∼ $ 400./case for drugs-controlled substances casework. The complexity of a trace evidence “case” results in a more expensive examination that is less requested in a locality.

A very basic overview of paint, soil and glass evidence was published as a chapter in a recent book [ 144 ]. While this chapter does not report anything new to add to the literature, it does explain the use of these types of evidence in a forensic laboratory. A different, similarly introductory chapter on glass analysis was published in another book [ 145 ] and a third chapter, also introductory in nature was published in a different book [ 146 ]. A more comprehensive and advanced chapter in another book [ 147 ] includes more up-to-date information on glass analysis and interpretation with an emphasis on the interpretation of glass evidence using recently reported methods.

Two different laboratory experiments were published in a teaching laboratory manual. One set of experiments featured glass breakage determinations [ 148 ] and one set of experiments featured glass examinations [ 149 ].

An excellent recent review of the literature [ 11 ] focuses more generally on the scientific foundations of trace evidence analysis. The review provides information on manufacturing, physical and chemical analysis, transfer and persistence and recent trends in the interpretation of trace materials evidence. In addition to paint, tape, and glass evidence, this very thorough review, consisting of more than 450 references, also includes sections on hair and fiber evidence examinations.

1.4.2. Physical and optical measurements and examinations

A study that examined the glass fracture patterns/characteristics after a shotgun blast to different types of glass (e.g. soda-lime and tempered glass) of different thicknesses and at different distances from the shotgun to determine hole diameters and “dicing” resulting from tempered glass, a not surprising observation [ 150 ].

Another study [ 151 ] evaluating the bullet hole morphologies in glass to associate distance from firing, projectile type, speed and angle of entry was also published. The authors aim to improve crime scene reconstruction events based on a better understanding of the morphological features of the bullet holes.

Haag and Haag [ 152 ] also describe bullet interactions with glass in order to better understand the nature of glass particles imbedded in recovered bullets to differentiate glass-imbedding from other silica-containing minerals such as sand and quartz primarily by polarized light microscopy.

A thesis by Beach [ 153 ] describes how a projectile impact on glass can result in different types of fracture patterns. The study includes how variations of glass type, glass thickness, curvature, distance from a firearm muzzle, contact angle, and the type of projectile can correlate to fracture patterns on the glass. The study also aims to correlate impact energy to the degree of glass fracture including the impact of muzzle-to-target distance for different firearm and ammunition combinations and including the use of a Doppler radar system to measure the projectile velocity.

Dondeti and Tippur [ 154 ] describe failure behavior and fracture mechanics of hairline cracks in glass due to stress sites. The authors evaluate crack initiation and quasi-static crack growth from a self-healed crack in soda-lime glass plate by calculating the stress intensity factor (SIF) history for the stress event.

Jiang et al. [ 155 ] aimed to better understand the dynamic tensile characteristics of glass by investigating the effects of loading rate, glass size and stress waves on the dynamic tensile behavior of brittle glass. Dynamic flexural stresses were induced with stress pulses and the tensile characteristics were viewed by high-speed photography. The authors report that the dynamic tensile stresses within a glass specimen reached a maximum at the same time as a crack starts.

Grant et al. [ 156 ] used attenuated total reflectance Fourier transform infrared (ATR-FTIR) to examine automotive window tints and then applied chemometric tools to differentiate the polymer types of different tint products. The authors report being able to associate unknown tint samples to a known brand.

Viviani et al. [ 157 ] evaluated the effects of explosions on laminated glass plates constructed from glass panes bonded by thin polymeric interlayers, in the pre-glass-breakage phase. The authors describe the viscoelastic properties of the interlayer and thereby producing a model of the coupling between the glass and the polymer laminate that can assist with future design of laminated glass.

Voros et al. [ 158 ] further examined the effect of annealing on the refractive index of glass that was subjected to heat such as in a house fire. The authors collected toughened non-toughened glass microfragments and simulated exposure to a fire by heating the glass in a furnace for various times at 450 and 650° C and cooling down quickly to model different heat expositions. The authors report a significant change in the RIs in all cases. However, after annealing the samples from the same source were associated and samples from different sources were discriminated by RI as previously reported by others.

Podor et al. [ 159 ] directly observed the formation of crystals in glasses, phase separation, chemical reactivity or glass foaming directly by use of an Environmental Scanning Electron Microscope (ESEM) equipped with a high temperature furnace. These observations can lead to a better understanding of how heat can impact the physical and optical properties of glass.

A study by Nentwig et al. [ 160 ] assessed the medical and biomedical impact from blows to a human head using different glass bottles to evaluate the injuries to facial and cranial bones resulting from the impacts. The authors concluded that blows with a 0.5-l beer bottle or with a 0.33-l Coke bottle to the head can transfer up to 1.255 N of force and thus are able to cause severe blunt as well as sharp trauma injuries.

Brooks et al. [ 129 ] conducted a thorough review of the literature on physical fit determinations, including work on tape, textiles, and polymers but also glass. The authors focused on case reports, fractography studies, and quantitative assessment of a fracture fit. The authors note a recent shift in research that focuses on quantitative, performance-based assessment of error rates associated with physical fit examinations and with the application of likelihood ratios to determine evidential significance. The authors also include reports of probabilistic interpretations of large sample sets and the implementation of automatic edge-detection algorithms to support expert opinions.

Thompson et al. [ 161 ] report on the development of a computational framework that uses fracture mechanics and statistical analysis to provide a quantitative match analysis for match probability and including error rates for the match analysis. The framework employs the statistics of fracture surfaces at microscopic scale, defined as greater than two grain-size or micro-feature-size, and assumes that a fragment “have the premise of uniqueness which quantitatively describes the microscopic features on the fracture surface for forensic comparisons”. The methods used include 3D spectral analysis of overlapping topological images of the fracture surface to classify specimens using statistical learning and matrix-variate models. A set of thirty-eight different fracture surfaces of steel articles were correctly classified. The authors claim that this framework lays the foundations for forensic applications with quantitative statistical comparison across a broad range of fractured materials with diverse textures and mechanical properties.

Voros et al. [ 162 ] simulated a case involving refractive index measurements of very small (250 μm) fragments recovered from garments after breaking a pane of float glass. Refractive index was measured for fragments that were crushed and not crushed. The authors note some non-matches for surface fragments of non-crushed samples. The authors conclude that crushing improves the probability of “matching” and improves the edge count values during the RI measurement.

Voros et al. [ 163 ] also report on the value of annealing glass that has been subjected to thermal stress before measurement of the refractive index, as previously reported by others. The experiments were carried out on fragments in the range of ∼100 μm.

1.4.3. Chemical measurements

Ernst et al. proposed an approach to compare ceramic frit in vehicle windows by XRF [ 164 ]. The authors investigated the discrimination of ceramic frits from twenty-five vehicle windows by physical observations, microscopy, and micro-X-ray fluorescence (XRF). The set included twelve windshield panes, eight rear windows, four side windows, and one sunroof obtained from 22 different vehicles manufactured from 2002 to 2016. Three XRF instruments with different configurations were utilized to evaluate the elemental profiles, including mono and polycapillary systems equipped with SiLi or SD detectors. The XRF yielded discrimination ranging from 97 to 100%, depending on the instrument and match criterion utilized.

Cagon et al. [ 165 ] compares the performance of four non-destructive techniques for in situ characterization of leaded glass windows. Macroscopic X-ray fluorescence imaging (MA-XRF), UV–Vis–NIR, Raman spectroscopy, and infrared thermography (IRT) are used to assess the ability of these techniques to group colorless glass panes based on differences in composition. IRT, MA-XRF and UV–Vis–NIR spectroscopy was able to distinguish at least two glass groups with MA-XRF providing the most detailed chemical information, not surprisingly. Potasium (K) and calcium (Ca) were used along with decolorizers (Fe, Mn, As) to further group glass samples. In addition, UV–Vis–NIR detected cobalt and iron where Raman spectroscopy was reported as hampered by fluorescence caused by the metal ions of the decolorizer in most of the panes but nevertheless was able to identify one group.

Wimpenny et al. [ 166 ] report on a study focused on the chemical and isotopic compositions of major and trace elements and their relationship within a population of fallout samples resulting from a nuclear test event. The authors aim to better understand how fallout melt glass formation in near surface environments is influenced by that environment and demonstrate how major and trace element abundances can provide useful insights into chemical processes within the fireball post event. Isotopically enriched uranium (presumably from the weapon) and natural composition uranium (from a combination of anthropogenic and environmental materials from within the blast zone) were detected. The association of the composition of local soils from the event site suggest that local soils are the most probable source of entrained material into the fireball and the source of material into which the bomb vapor was incorporated. Processes such as condensation from the fireball were modeled to better understand the composition of macroscale fallout melt glass.

Jacquemin et al. [ 167 ] report on the use of Raman imaging to investigate the homogeneity of the Raman response at the surface of casted aluminosilicate glass pieces. Samples were probed at constant focus depth across 7 × 7 cm 2 surfaces using 500 μm spatial steps, resulting in large and detailed Raman images. The authors show that the modification of the Raman response is small across the scanned area and that the information is carried by the spatial representation of the selected Raman parameters, in particular the Si–O stretching mode involving Q2 tetrahedral units, and the Si–O–Si bending vibrations envelope in the low-wavenumber range were of interest. The evolution of Raman parameters across the surface of the sample permitted the identification of areas consisting of material from different stages of casting. The authors report the ability to observe structural and chemical changes originating from the manufacturing process of the glass pieces as revealed by Raman imaging.

Merk et al. [ 168 ] used laser induced breakdown spectroscopy (LIBS) and Raman, in combination, to discriminate sixteen different glass samples by use of principal component analysis (PCA). The LIBS/Raman results were compared to μ-XRF and SEM-EDS in this work. The authors report very good discrimination (up to 99%) when LIBS and Raman are used jointly. The authors only report the source of the Raman signals as “fluorescence” but do report the use of trace elements (e.g., Fe, Ti, Ba, Sr) for discrimination purposes.

Corzo and Steel [ 169 ] reported on the use of micro X-ray fluorescence spectrometry (microXRF) for the analysis of small (<1 mm) glass fragments that are partially transparent to the X-ray beam with an aim to better understand the signal-to-noise ratio (SNR) of the determination. The authors note that, in addition to fluorescence, the primary beam X-rays may scatter within the chamber and provide noise in the measurements. The fragments were mounted on a 3D-printed plastic mount to allow fragments to be raised as high as possible from the sample stage, thereby minimizing stage scatter and improving the SNR for most elements with the greatest improvement (>30%) observed for the lower atomic number elements (Na and Mg) and higher atomic number elements (Sr and Zr). An additional simple method to improve SNR was the use of primary beam filters with elements containing characteristic lines in the high-energy range (Rb, Sr, and Zr) showing the greatest improvement (>70%) in SNR.

Samanta et al. [ 170 ] utilized Instrumental Neutron Activation Analysis (INAA) for multi-elemental analysis of soda-lime glasses. Five automobile glass fragments were analyzed by INAA targeting seven trace elements of interest. Concentrations of Sc and elemental concentration ratios such as La/Sc indicated that five glasses fall into two major groups, which was confirmed by statistical cluster analysis.

Kaspi et al. [ 171 ] report on the use of particle-induced X-ray emission (PIXE) for elemental analysis of glass fragments. The authors claim that the use of a PIXE workflow, in combination with machine learning (ML) can “produce models with better than 80% accuracy in identifying glass” sources but the study fails to describe the underlying analytical figures of merit for the PIXE method such as bias, precision and limits of detection for the list of elements reported. The authors also fail to compare the PIXE performance to existing, more routinely used techniques for which analytical standard methods of analysis already exist (e.g. micro-XRF, solution ICP-MS and LA-ICP-MS).

A follow-on paper by the same group [ 172 ] reports on the “classification” of glass from car windows from different cars using PIXE as well as “possible glass corrosion”. The authors claim to have developed a database for use in a classification model. The authors do not address the practical aspects of maintaining proton-ion beam PIXE instrument in a typical forensic laboratory and do not report the analytical figures of merit for PIXE on standard reference materials, for example, in order to evaluate the precision, bias and limits of detection of PIXE as a semi-quantitative method for this application.

Costa et al. [ 173 ] report the use of total reflection X-ray fluorescence (TXRF) for the analysis of Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Ni, Cu, Zn, Rb, Sr, Ba and Pb concentrations in glass samples. The authors report that the limit of detection (LOD) and limit of quantification (LOQ) were “adequate” for determination of trace elements in glass. The authors also evaluated the accuracy and precision of the determination by analysis of a standard reference materials of glass (NIST 612). The authors report that, for the majority of the elements, good agreement was achieved between the certified value and the value obtained in the NIST 612. The relative standard deviation (RSD%) was achieved between 3.6 and 10.3%. The authors also report no significant differences observed between the proposed method compared to ICP-MS analyses. The TXRF method was applied to the analysis of 31 glass samples and with aid of an exploratory principal component analysis (PCA) resulting in “a perfect discrimination” of the glass from smartphones was obtained. In addition, the authors report that soda-lime glass “could be reasonably distinguished” from smartphone screens.

Sharma et al. [ 174 ] report on the use Particle Induced Gamma-ray Emission (PIGE) and Instrumental Neutron Activation Analysis (INAA) for the characterization of windshield glass samples derived from six different vehicle manufactures. PIGE was used to measure the concentrations of the four major elements (Si, Na, Mg and Al) and a total of nineteen (19) elements including sixteen (16) trace elements were analyzed using INAA in a research reactor. The authors used statistical tools such as K-mean, Cluster Analysis and Principal Component Analysis (PCA) for grouping studies. The authors report that the PCA results confirmed that windshield glasses from six manufactures clearly associated to the six different groups.

Almirall et al. [ 175 ] reported on the elemental concentration values for seventeen (17) major and trace elements typically present in soda-lime glass manufactured using the “float " process and used in the quantitative analysis and forensic comparison of glass samples using laser ablation (LA) micro sampling coupled to inductively coupled plasma mass spectrometry (ICP-MS). This is the first reporting of the chemical characterization of a new set of float glass standards intended for use as matrix-matched calibration standards in the forensic analysis and comparison of glass by LA-ICP-MS using a standard test method (ASTM E2927-16e1 ). Three different compositions were manufactured at low, medium, and high concentrations of 32 elements by Corning. This work describes an international collaboration among seven (7) laboratories to evaluate the homogeneity of the three new glass materials and reports the consensus concentrations values of 17 elements at three concentration levels. LA-ICP-MS analysis was reported by eight (8) laboratories and one laboratory reported micro-X-ray Fluorescence Spectrometry data for the same glass standards (CFGS1, CFGS2 and CFGS3). The analytical results reflected <3% relative standard deviation (RSD) within each lab and <5% RSDs among all labs participating in the study for the concentration ranges using sampling spots between 50 μm and 100 μm in diameter. These results suggest that the new calibration standards are homogeneous for most elements at the small sampling volumes (∼90 μm deep by ∼80 μm in diameter) reported and show excellent agreement among the different participating labs. Consensus concentration values were determined using a previously reported calibration standard (FGS 2) and checked with a NIST 1831 standard reference material.

Becker et al. [ 176 ] reported on the use of a single-pulse laser ablation sampling coupled to an inductively coupled plasma time of flight mass spectrometry (Single-Pulse LA-ICP-TOFMS) method in an effort to be able to analyze very small glass samples. The authors compared the results of the single-pulse LA-ICP-TOFMS and report good performance in associating glass fragments from the same source. The authors were able to reduce the necessary sample volume from the typical 400 μm × 200 μm x 100 μm normally used for LA-ICP-MS to 100 μm × 100 μm x 33 μm which corresponds to a reduction in sample mass from ∼20 μg to ∼0.8 μg. This development allows for the measurement of smaller fragments than previously possible.

Von Wuthenau et al. [ 177 ] reported on the use of LA-ICP-MS to analyze glass from perfume bottles to detect counterfeit perfume by comparing authentic perfume glass bottles to the counterfeit glass bottles, without having to open the bottles of perfume. Perfume glass bottles manufactured in different production facilities in Germany, India, Peru and Poland were used for the study. A total of 63 elements could be measured but only 15 elements (Li, Na, Al, Ti, V, Co, Rb, Sr, Mo, Ba, La, Ce, Pr, Er and Pb) were used to associate glass to sources using statistical evaluation of the data ( t -test, ANOVA, principal component analysis (PCA)). The use of LDA permitted the differentiation of six different production sites from four different countries with a prediction accuracy of 100%.

Martinez-Lopez et al. [ 178 ] reported on the use of μ-XRF and LIBS analyses to characterize the homogeneity (elemental variation) of glass compared to the more established method for quantitative analysis using LA-ICP-MS. The aim of the study was to support that the μ-XRF and LIBS methods would produce sufficiently sampling precision (in comparison to LA-ICP-MS) to enable microsampling using μ-XRF and LIBS. The authors report that the variability of the elemental composition within 100 fragments from two different panes of the same windshield was found to be less than 10% RSD for both μ-XRF and LIBS and less than 5% RSD for LA-ICP-MS. Comparison methods simulating casework situations in which one questioned fragment is compared to more than one known fragment resulted in better performance as the number of fragments of the known sample increased (to up to 4 fragments, 12–20 measurements). Error rates below 3% were obtained for μ-XRF and LIBS when selecting the appropriate number of fragments, measurements, and comparison criterion.

1.4.4. Data analysis and interpretation

de Zwart and van Der Weerd [ 179 ] reported on the use of filtered contents of a database such that only items that are considered relevant to the population are selected for the background database in the analysis. Six different scenarios related to fibers, textiles, and glass evidence are described along with the hypotheses and relevant populations that may be evaluated by an expert. The focus of the study is to filter items to develop a more relevant population and provide an overview of the selected items and feedback to the examiner.

Rodrigues and Bruni [ 180 ] used previously published Energy Dispersive X-ray Fluorescence (EDXRF) data from 28 windshield glass samples from 26 different vehicle models to differentiate between the internal and external panes of the windshield. The oxides of Na, Ca, Mg, Mn, Al, Si, K, Ti and Fe were previously measured by other researchers for both the inner and outer panes of the windshield (for a total of 56 analyses). The authors used unsupervised Principal Component Analysis (PCA) and supervised Soft Interclass Modeling Classification Analogy (SIMCA) methods in the analysis together with Receiver Operating Characteristics (ROC) curves to evaluate the results. The authors report that PCA indicated the presence of two groups of glasses in three main components with the distances and interclass residues in SIMCA showing no outliers. The ROC analysis indicated a sensitivity of 0.793, a specificity of 0.815, and an efficiency of 0.804 for predictions. The authors concluded that this approach was successful in discriminating between the inner and outer panes of the windshields.

Fortunato and Montanari [ 181 ] used previously published refractive index and eight metal oxide composition data generated from SEM-EDS analysis of glass fragments to propose the use of a transvariation-based one-class classifier as a measure of typicality in a “one-class” classifier system. The aim of the transvariation-based one-class classifier was to identify the best boundary around the target class, i.e. to recognize as many target objects as possible while rejecting all those deviating from this class.

A presentation by Corzo [ 182 ] reported on the use of micro-X-ray Fluorescence Spectrometry (μXRF) analysis of glass samples to develop a database of μXRF data for glass fragments to improve evidence interpretation. The author argues that the development of such a background database can then be used to calculate coincidental match probabilities and likelihood ratios to then assign a significance to the glass evidence.

Malmborg and Nordgaard [ 183 ] used the open-source software (SaiLR) to calculate likelihood ratios (LR) from probability distributions of reference data based on SEM-EDS measurements of metal oxides in glass fragments. The authors report that the best performance of the model was achieved by focusing on the oxides of calcium, magnesium, and silicon. The authors also report that although performance improved with normalization of data, the difference was small. Limits of LR output were set to 1/512 ≤ LR ≤ 158 using the empirical lower and upper boundaries (ELUB) LR method. The authors report that the limited range was primarily a consequence of notable within-source variation, but it could also be due to the low discrimination power of the SEM-EDS method to discriminate between glass sources. The authors acknowledge that previous reports using LA-ICP-MS data outperform the reported results. The output from the LR calculation within SaiLR is based on a two-level multivariate kernel density (MVK) model and provides good discrimination between the same and different source comparisons but it does not include a calibration step. The MVK model without calibration has been previously shown to be very sensitive to the dimensionality of the data. The MVK model within SaiLR may be suitable for very low dimension problems (∼2–3 variables) but reducing the available number of variables also reduces the discrimination power.

Lucy, Martyna and Curran [ 184 ] published an R package, also based on SEM-EDS data, to facilitate the calculation of a multivariate Likelihood Ratio for glass data. This package also uses the MVK model followed by ECE calibration as reported by others.

Park et al. [ 185 ] reported a “data in brief” note on the development of a glass “database” constructed of 48 panes of glass produced on consecutive days within a manufacturer and representative glass samples from a second manufacturer. The authors used LA-ICP-MS data from 18 elements using the ASTM E2927 method of analysis. The authors make the raw data available to researchers, but the “database” is not representative of the population of glass as may be needed to calculate significance from coincidental match probabilities and likelihood ratio calculations.

Wen ate al [ 186 ] report an alternative continuous Bayesian approach, the Dirichlet Process Mixture Model (DPMM), to model the relevant rarity of glass based on the refractive index (RI) measurements. A DPMM was developed based on a finite mixture model with additional prior specification on the mixture proportions. Glass is a common type of physical evidence in forensic science. The authors report that the key advantage of the method is that it allows for a more flexible model of the probability density distribution of refractive index measurements.

Akmeemana, Corzo and Almirall [ 187 ] report the use of a new R-based Shiny graphical user interface (GUI) to calculate calibrated likelihood ratios (LRs) using three (3) different background databases of glass composition measured by LA-ICP-MS using the ASTM E2927 method. The authors also report the development of a new vehicle survey glass database generated at Florida International University (FIU) generated from LA-ICP-MS analysis, a database comprised of a combination of casework and survey samples collected from solution-digestion ICP-MS analysis from the Federal Bureau of Investigation (FBI) Laboratory, and a previously reported casework sample database collected from LA-ICP-MS analysis at the Bundeskriminalamt (BKA) Laboratory. The LRs are calculated using a previously reported two-level multivariate kernel (MVK) model and calibrated using a previously described Pool Adjacent Violators (PAV) algorithm. The log LR (LLR) were calculated and compared to the match criterion recommended in the ASTM E2927-16e1 method, using these three background databases using a typical glass evidence case scenario. This paper also reports how the LLR values increase as the size of the background database increases, as expected.

Almirall and Akmeemana [ 188 ] have made available the Shiny Glass application freely through the FIU library for researchers and practitioners to use. The FIU vehicle glass database comprised of more than 330 different vehicle glass samples of known origin and generated using the ASTM E2927 LA-ICP-MS standard method has also been made freely available for download in an effort to encourage future researchers to use LA-ICP-MS data instead of the less discriminating SEM-EDS data commonly used. This R shiny application can be used to calculate the likelihood ratios using a multivariate kernel density (MVK) model. This app was developed to easily access the R code written previously by members of Prof. Almirall's research group at Florida International University. In this procedure, MVK-calculated source likelihood ratios are calibrated using a pool adjacent violators (PAV) algorithm.

Gupta et al. [ 189 ] reported on the use of a novel dimensionality reduction method to calculate likelihood ratios (LRs) from multivariate elemental concentrations of glass using LA-ICP-MS data. The LRs were calculated using principal component analysis (PCA) and post hoc calibration steps resulting in very low (<1%) false inclusions when comparing glass samples known to originate from different sources and very low (<1%) false exclusions when comparing glass samples known to originate from the same source. The LRs calculated using the novel PCA approach are compared with previously reported LRs calculated using a more computationally intensive Multivariate Kernel (MVK) model followed by a calibration step using a Pool Adjacent Violators (PAV) algorithm. In both cases, the calibrated LRs limited the magnitude of the misleading evidence, providing only weak to moderate support for the incorrect hypotheses. Most of the different pairs that were found to be falsely included were explained by chemical relatedness (same manufacturer of the glass sources in very close time interval between manufacture). The authors state that the computation of LRs using dimensionality reduction of elemental concentrations using PCA may transfer to other multivariate data-generating evidence types.

When considering only subsets of glass fragments recovered from garments and considering the presence of background glass on garments Vergeer et al. [ 190 ] propose to incorporate modelling the probability of an allocation of background fragments into groups given a total number of background fragments by a two-parameter Chinese restaurant process (CRP) for the glass evidence. The proposed solution consists of relaxing the assumption of conditional independence of group sizes of background fragments. Under the assumption of random sampling of fragments to be measured from recovered fragments in the laboratory, parameter values for the Chinese restaurant process may be estimated from a relatively small dataset of glass in other relevant cases. The authors use a dataset of glass fragments collected from upper garments in casework and show model fit to provide a calculation of an LR at activity level accompanied with a parameter sensitivity analysis for reasonable ranges of the CRP parameter values.

Ramos et al. [ 191 ] propose models that compare favorably to previously proposed feature-based LR models, by improving the calibration of the computed LRs based on quantitative elemental analysis using LA-ICP-MS and the ASTM E2927 method. The authors assume that the within-source variability is heavy-tailed to incorporate uncertainty when the available data is scarce as it typically happens in forensic glass comparisons. The authors also address the complexity of the between-source variability by the use of probabilistic machine learning algorithms such as a variational autoencoder and a warped Gaussian mixture. The authors report improved overall performance of the likelihood ratios generated by the new model over previously reported approaches and credit the improvement to “a dramatic improvement in the calibration despite some loss in discriminating power”.

Corzo et al. [ 192 ] report on the results from an interlaboratory study involving seventeen (17) laboratories that participated in three interlaboratory exercises to assess the performance of refractive index, micro X-ray Fluorescence Spectroscopy (micro-XRF), and Laser Induced Breakdown Spectroscopy (LIBS) data for the forensic comparison of glass samples. Glass fragments from automotive windshields were distributed to the participating labs as blind samples and participants were asked to compare the glass samples and report their findings as they would in casework. For samples that originated from the same source, the overall correct association rate was greater than 92% for each of the three techniques (refractive index, micro-XRF, and LIBS). For samples that originated from different vehicles, an overall correct exclusion rate of 82%, 96%, and 87% was observed for refractive index, micro-XRF, and LIBS, respectively. Wide variations in the reported conclusions existed between different laboratories, demonstrating a need for the standardization of the reporting language used by practitioners. Moreover, few labs used a verbal scale and/or a database to provide a weight to the evidence.

Lambert et al. [ 193 ] reported the results of a different interlaboratory study involving ten (10) different laboratories using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and the standard test method (ASTM E2927-16e1 ) for the analysis and comparison of glass evidence. The primary aims of the interlaboratory study were to evaluate the performance of the new CFGS2 calibration standard for the quantitative analysis of simulated casework samples, evaluate the comparison criterion as recommended by the ASTM E2927 method and the incorporation of a likelihood ratio (LR) calculation as a more quantitative determination of the strength of evidence. Each laboratory calculated a LR to report the significance of glass source comparisons for a set of glass samples of known origin. Two different types of background databases were used for the calculation of the LR to evaluate the effect of the size and composition of the databases on the calculation of the LR. As expected, glass that originated from the same windowpane was found to be indistinguishable using the ASTM E2927 match criteria and resulted in a high LR value (strong support for an association) and glass that originated from different vehicles are distinguished (strong support for an exclusion). Glass samples that originated from different vehicles but that were the same make, model and year (or comparisons between the inner and outer pane of the same windshield) were chemically similar and reflected a low LR. Good agreement among the laboratories was reported with <5% relative standard deviations (RSDs) among participants.

Akmeemana et al. [ 194 ] report on the utility of likelihood ratio (LR) calculations using novel datasets of glass samples of known manufacturing history. The LRs calculated from comparing elemental analysis of glass using ASTM E2927 LA-ICP-MS data for glass manufactured at three different plants over relatively short periods (over 2–6 weeks) range from very low values (LR 10 −3 ) when the glass are manufactured at different plants or manufactured weeks-months apart in the same plant to very high values (LR 10 3 ) when the glass samples are manufactured on the same day. Although the glass samples being compared may not originate from the same broken window source, they do exhibit chemical similarity within these lower and upper bounds and the LRs presented closely correlate chemical relatedness to manufacturing history, specifically the time interval between production.

Acknowledgements

The contribution of Jeff Teitelbaum, Global Forensic and Justice Center at Florida International University is gratefully acknowledged for conducting the initial literature search for articles related to paint, tape, and glass evidence examinations.

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• Published: 20 May 2023

A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models

• Roberta Zanini   ORCID: orcid.org/0000-0002-3190-9191 1 , 2   na1 ,
• Giulia Franceschin   ORCID: orcid.org/0000-0003-1817-2962 1   na1 ,
• Elti Cattaruzza   ORCID: orcid.org/0000-0003-0643-0266 2 &
• Arianna Traviglia   ORCID: orcid.org/0000-0002-4508-1540 1  

npj Materials Degradation volume  7 , Article number:  38 ( 2023 ) Cite this article

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Glass has been used in widespread applications within several sectors since ancient times and it has been systematically studied under different perspectives. However, its thermodynamic properties and the variety of its compositions, several aspects related to its durability and its alteration mechanisms remain still open to debate. This literature review presents an overview of the most relevant studies on glass corrosion and the interaction between glass and the environment. The review aims to achieve two objectives. On one hand, it aims to highlight how far research on glass corrosion has come by studying model systems created in the laboratory to simulate different alteration conditions and glass compositions. On the other, it seeks to point out what are the critical aspects that still need to be investigated and how the study of ancient, altered glass can add to the results obtained in laboratory models. The review intends also to demonstrate how advanced analytical techniques commonly used to study modern and technical glass can be applied to investigate corrosion marks on ancient samples.

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Introduction

Throughout history, glass has played a central role in the production of goods for widespread use, making it a material that has been extensively studied from various perspectives. However, due to its thermodynamic properties and the vast range of existing compositions, there are still several aspects of its nature that are not fully understood. For instance, evaluating glass durability and alteration mechanisms remains a challenge, as it requires taking into account numerous factors, some of which are difficult to replicate in laboratory experiments. One of the most prominent challenges is the time required for the transformation of the glass structure, which is directly responsible for the kinetics and dynamics of the processes involved.

Technological innovation has recently introduced new tools for studying the durability of glass and shedding light on the deep connections in extremely complex materials between compositions, structure, and the surrounding environment. As an example, a recent review 1 reported about the possibility of employing mechanistic models to study aqueous glass alteration. Thanks to the availability of such novel techniques, glass alteration mechanisms and kinetics can be hypothesised using non-conventional experimental methods (simulations) and without direct experimental validations. In parallel, the long-term perspective provided by studying ancient vitreous objects can inform and underpin the predictions obtained through simulation and laboratory methods 2 , 3 , 4 , 5 , 6 .

Most of the studies performed to understand glass corrosion are based on artificial ageing experiments that are designed to monitor, step by step, the process of glass alteration and the physicochemical evolution of the glass structure 7 , 8 , 9 , 10 . Because of the great variability of the experimental conditions involved in these studies, however, results are not yet conclusive, and the glass transformation process is still only partially described in literature.

Standard approaches to the study of glass corrosion are limited in that they are valid in some specific experimental conditions, but not in others. Indeed, most of the available research focuses on the corrosion mechanisms of glass with specific compositions (lead-rich or borosilicate families), or under conditions of saturated humidity (relative humidity, RH ≥ 100%) 11 , 12 , 13 . In both cases, the atomic diffusion occurring in the material alteration processes is assisted by two key parameters, which are the presence of ionic species with high mobility and the presence of liquid water on the surface, respectively. Only an handful of published works 2 , 7 , 8 –focused on ageing under conditions of RH < 100% and on a wider variety of glass compositions, like those of the silica-soda-lime (SSL) glass family– are available nowadays in literature: they represent valuable experimental approaches to determine and separate the relative influence of different parameters on the chemical and environmental durability of glass.

In view of the above, this review focuses on the literature available on glass alteration and related structural investigations that have contributed to the comprehension of the modifications occurring in the glass network when glass degrades. In addition, an examination of the existing –but limited– literature on the analysis of ancient glass will bring to the attention of the chemistry community a vast amount of unexplored information that is relevant also for the study of contemporary glass types, to validate the prediction of glass behaviour obtained through laboratory experiments. The most advanced techniques to study alteration on heritage glass will be described with the aim of highlighting how deepening the understanding of altered ancient glass properties is key for a full understanding of the phenomenon of natural ageing of both modern and ancient glass objects.

For better clarity, the term a rchaeological glass will be used to refer to glass specimens that have aged in soil or marine environments for centuries, whereas the term historical glass will be used when speaking about glass aged under the effect of environmental moisture fluctuations (in museums or private collections). The term ancient glass is more generally used to refer to both the above mentioned cathegories.

By using a combination of comprehensive characterization of ancient samples and laboratory-based ageing experiments, researchers can obtain valuable information about glass alterations that have occurred over hundreds of years while also monitoring the alteration process step-by-step. This integrated approach offers a unique opportunity to consolidate and supplement current theories of glass corrosion and validate predictive models using real-world examples of long-term alteration. Overall, this approach provides a more complete understanding of the complex process of glass corrosion and can help guide the development of more durable glass materials.

Glass structure and its dissolution prediction: key highlights

Glass is an amorphous material that shares structural features at the atomic level with a supercooled liquid, while exhibiting mechanical properties typical of solids.

Zachariasen 14 was the first to describe the amorphous structure of glass in 1932. His studies led him to conclude that there is no long-range atomic periodicity in glass structure. He also defined the requirements for a particular oxide to exist in the vitreous state. Silicon dioxide, also known as silica, is the primary network former in both ancient as well as in modern glass.

In silica-based glass, the bonding forces involved in the formation of the glass network are those of crystalline SiO 2 , which has a tetrahedral structure with 4 oxygen atoms located at the corners of a tetrahedron and a Si atom placed in the middle of the structure 15 (Fig. 1 ). The glass network can be figured as a built-up of such tetrahedral elements, which share each corner with neighbouring tetrahedra, one per corner. The remaining corners are available to form other chemical bonds. SiO 2 is considered a primary network former, but other oxide types exist with these same properties, such as B 2 O 3 or P 2 O 5 . The addition of alkali oxides, such as Na 2 O, K 2 O, Li 2 O, to the glass network former is responsible for breaking some of the Si-O-Si bonds bridging silicon atoms and for including the alkaline cations into the glass structure. These network modifiers generate the formation of non-bridging oxygens (NBOs) in addition to the bridging oxygens (BOs) of the silica network. NBOs hold a negative charge that is locally compensated by one highly mobile alkali cation with a positive charge (i.e., Na + ). The main effect of such network modifiers is to decrease the viscosity of the glass melt, thus facilitating its workability at lower temperatures. Alkaline earth oxides like CaO are added to the glass batch as network stabilisers 16 . They connect to two NBOs and are commonly considered as lower mobility ions (i.e., Ca 2+ ) that are possibly effective in inhibiting the diffusion of other cations across the silica network, hence improving the chemical resistance of the glass (Fig. 2 ). It is now clear why 90% of commercial glass is made of a mixture of silica (as network former), sodium and calcium oxide (as network modifiers). Nevertheless, the positive influence of alkaline earths in increasing glass stability against alteration has not been fully demonstrated yet, and several works indicated that Ca 2+ and Mg 2+ ions are at least as mobile as alkalis within the hydrated layer in atmospheric conditions 17 , 18 .

Schematic representation of a tetrahedral silica unit (not to scale).

For clarity’s sake, only the BOs and NBOs helping to identify the Q n configuration in the silica network are coloured in red for BO and in yellow for NBO.

The structure of glass is not in thermodynamic equilibrium. The durability of glass depends on both the kinetic and thermodynamic stability of its oxide components. In a state of thermodynamic equilibrium, the chemical potential of the species on the glass surface and those in solution are equal, and as a result, no net mass transfer occurs. However, glass does not exist in a state of thermodynamic equilibrium and is therefore prone to undergo chemical reactions that can result in degradation over time. The understanding of the kinetic and thermodynamic stability of glass components is crucial for developing more durable and long-lasting glass materials.

An essential factor to be taken into consideration to predict glass dissolution is the knowledge of the relative concentration of bridging and non-bridging oxygen atoms 19 . The latter are bonded to only one silicon atom and their quantity within the glass network is proportional to the concentration of modifier ions.

Considering the two possible configurations for an oxygen atom (BO and NBO), the silicon atom may be found in five different tetrahedral arrangements: Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 , where the subscript indicates the number of bridging oxygens (Fig. 2 ). The structure of the glass network is the result of the distribution of the rings and voids regulated by the interconnection between these different silicate tetrahedra. The size of the voids in the network controls the rate of water diffusion, which is kinetically favoured when the dimension of the voids is comparable to the diameter of the water molecule (0.28 nm). In complex glass (mixed alkali glass), modifier cations can totally or partially fill the voids, but when the material is exposed to high relative humidity conditions, these alkali ions are leached from the glass surface and replaced by hydrogen ions as part of molecular water. The ion-exchange reaction drives the hydrolysis of the glass network with kinetics depending on both the distribution of local structural units (Q n ) and the modifier content. In addition, the exchange of high-radius cations as K + from the bulk is considered to leave a bigger void in the glass network compared to the exchange of smaller cation as Na + , thus facilitating the entrance of water molecules into the deeper areas. In general, it can be said that the higher the concentration of NBO, the higher the number of ion exchange sites available and the rate of ion-exchange and network hydrolysis, following the reaction trend: Q 1  > Q 2  > Q 3  > Q 4 19 , 20 . For this reason, in the discussion about the kinetics of the processes of ionic exchange and hydrolysis reactions it is essential to take into consideration the chemical composition of glass.

Taking into account what has been said above, knowing the chemical composition of complex glass and its Q n concentration and distribution is fundamental for the appreciation of its chemical stability and leaching resistance, so as to adopt an adequate preventive conservation strategy. Nevertheless, the glass reactivity does not depend on Q n species only. Understanding the correlation between local structural features of the glass and the activation energies of individual bonds is also crucial to predict the dissolution mechanism of the glass network. Potential Mean Force (PMF) calculations estimated the activation barrier for Si dissolution in presence of aluminium (Al). They revealed that Al is easily dissociated from glass network, but Si dissociation is hindered when Al is present as a second neighbour 21 . As a result, Al causes opposing effects on glass durability if added at low and high concentrations: the addition of Al in small concentration increases the durability by reinforcing the strength of Si and increasing the polymerisation of the glass network, while at high Al concentration, the preferential release of Al results in the weakening of the silicate network. This predicting method can be extended to understand the role of Na, Mg, or B in more complex glass compositions.

Since the prediction of dissolution of glass network is a complicated topic of research, many recent works 22 , 23 , 24 reported the use of machine learning based approaches to account for the percentage of bridging oxygen species, network connectivity, average ring size, as well as the composition modification due to the preferential release of modifier cations during the incongruent dissolution.

Several glass studies demonstrated the usefulness of Raman spectroscopy as an analytical technique to discriminate the characteristic vibrational modes of each Q n configuration 25 , 26 , 27 , 28 . Through the deconvolution of the Raman bands typically associated to the glass network it is possible to determine the single Q n distribution and associate the variation of the area of the related Q n band with the chemical composition of the sample analysed 29 , 30 . When using this analytical approach, Raman spectroscopy can be adopted as a technique to distinguish a stable glass from an unstable one by means of the rigorous deconvolution of the vibrational bands of the glass network 29 , 31 , 32 .

Nowadays, an analytical protocol that combines the potentialities of Raman spectroscopy described above with the advantages of using portable instrumentation to evaluate the chemical stability of glass, to predict the glass network dissolution for preventive conservation purposes, or even to establish the suitability of glass as storage material for nuclear waste is not available. Moreover, the in-depth spatial resolution of Raman spectroscopy is inadequate if one wants to determine the layered structures on an altered surface, whose features, in fact, sometimes vary on the nanometre scale. The association of other complementary analytical techniques would be ideal to optimise the reliability of Raman spectroscopy results.

X-ray absorption near edge structure (XANES) spectroscopy at the Si K-edge is another interesting technique that has been used to study the polymerisation degree of SiO 4 tetrahedra silica glass. In XANES spectra, position and structure of the absorption edge are largely determined by the charge of the absorber atom and by geometry of the first coordination shell, which depends on the coordination of the nearest neighbour atoms, the degree of polymerisation, and the presence of network modifiers and network substitutes 33 . A study conducted on silicate glass reported that, with the increase in the polymerisation degree of the silica tetrahedra, the Si K-edge shifts towards higher energy, while it shifts towards lower energy when Si is substituted for another network former (Al) 34 .

The chemical changes occurring at the surface of a corroding glass often cause an alteration of the local environment of metal atoms, especially of the metal-oxygen pair distribution. These characteristics have been measured using conventional and glancing angle extended X-ray absorption fine structure (EXAFS) techniques, able to give information about number and distance of the atoms surrounding the absorber one. Examining the modifiers distribution in the vicinity of the surface gives important hints on how the surface is modified as corrosion advances 35 .

As transition metal cations are generally of interest for the application of X-ray absorption techniques, their study can be easily exploited to monitor the decay of ancient glass. The modifications of the chemical environment of chromophore species (i.e., transition metal cations) can be recorded using both XANES and EXAFS techniques by monitoring the spectra of a selected metal species in an altered glass sample. By means of this method, it was possible to establish a relationship between the oxidation state of Fe and Cu cations during glass decay. Whereas, the Mn oxidation state was not directly correlated with the glass decay of the samples studied 36 .

XANES and EXAFS analysis are not common and straightforward techniques, since they require the preparation of tailored samples from analysed objects (which is not always possible when dealing with cultural items) and the access to a synchrotron facility. However, this type of analysis provides important information to help the identification of structural and chemical changes in altered glass samples when investigating the molecular changes around cations with high field strength and well-defined short-range order.

In the following section, the main models of glass corrosion will be explored: they are based on the observation of the interactions between glass structure and external environment and on the different mechanisms in place during the process of glass alteration.

Exploring the mechanisms of glass alteration through the different existing models

The terminology used in the published works is imprecise. The terms corrosion , alteration , degradation , and deterioration are often found to be frequently used interchangeably as synonyms, despite having different shades of meaning. For the sake of clarity, all these terms will be used here as synonyms when talking about the phenomena that induce a change in glass physicochemical properties, regardless of the intrinsic or environmental factors that have determined such change. The term dissolution will be, instead, used to refer only to the rupture of the Si-O covalent bonds and the breakup of the structural silica network, and the term leaching will be used to identify the step of degradation that consists in the loss of alkali and alkaline earth metals ionically bonded to the silica network that precedes the network dissolution 37 . Even if leaching is often associated with the initial steps of glass degradation, the loss of alkalis can be a secondary phenomenon in specific conditions of high temperature (RH < 100%) and unstable glass compositions 16 . In other particular conditions, the degradation process occurs without any loss of alkalis, because the water penetrated into the glass network is unable to solvate them 8 , 9 , 18 .

The interaction between water and glass can activate two different degradation phenomena, i.e., leaching and network brakeage, mainly depending on the pH of the solution in contact with the surface. During leaching, the aqueous solution in contact with the surface typically has pH<9 38 , 39 . In this condition (pH<9), ion exchange that involves alkalis (Na, K) and alkaline earth metals (Ca, Mg) occurs, forming ionic bonds with the oxygen of the glass network and the H + ion from the aqueous solution 16 . This is a diffusive phenomenon and the thickness of the glass region involved in the reaction (indicatively a few microns) depends on the glass composition and on the time and temperature of exposition. The altered layers that form on the surface can act as a diffusion barrier to further extraction, even if hazardous cracks that allow the penetration of water molecules into the pristine glass may form. The alteration due to leaching does not affect the Si atoms: the network distribution does not change, only the Si-O-M bonds do.

On the other hand, the ions interdiffusion during leaching leads to a pH increase (eventually above 9) due to the formation of Na + OH - species in solution and Si-OH acid from the reaction between Si-O-Na and H 2 O. An alkaline environment results in more aggressive attacks to the glass network, since it promotes the dissolution of the Si–O bonds 40 . The reaction with the hydroxyl ions (OH – ) breaks the Si-O–Si bonds and silanol groups Si–OH are formed.

The sites left free by the leaching of cations from the glass surface can be easily filled by hydrogen ions, which have small ionic radius. The hydrolysis process induces the introduction of H 2 O molecules and OH - ions into the opened silica structure, thus increasing the rate of the hydration process and the ion exchange.

The interactions and reactions that occur between aqueous solution and glass have been over the years the subject matter of extensive research focusing on the mechanism involved in this process at different scales of observation, research that has been key to support the formulation of the classic theory of alteration and Interfacial Dissolution Precipitation model for dissolution of vitreous materials.

The Classic Inter-Diffusion (CID) model of glass corrosion is based on diffusion-controlled hydrolysis and ion exchange reactions 41 , which lead to the formation of structurally and chemically distinct zones (Fig. 3 ).

The latter consists of a hydrated, cation-depleted layer resulting from a selective cation release. Courtesy of Gin et al. 48 .

The classic theory of glass corrosion is underpinned by different formulated models of ion-exchange that take into consideration the effects of the preferential dissolution of more soluble cations during the initial part of the leaching process 42 , 43 .

Many experimental and theoretical results obtained from observation in liquid conditions (and not in unsaturated humidity ones) have reported that the leaching mechanism involves the preferential release of alkali and alkali-earth ions rather than that of network formers, such as Si or Al ions, with a consequent formation of alumina/silicate-rich layer on the glass surface 38 , 43 .

In general, the concept of preferential leaching is based on the thermodynamic and kinetic stability of the different glass modifiers. At lower temperatures and for ions with the same charge, the diffusion of larger ions (for example Ba 2+ or Ca 2+ ) becomes energetically unfavoured, while the smaller ions (for example Mg 2+ ) can move more easily through the glass network 44 . In any case, double-charged ions usually show less diffusivity in SSL glass than single-charged ones, mainly because of the marked effect of the very intense local electric fields acting on them. If the leaching mechanism proceeds, the solution becomes richer and richer in OH – and its pH increases favouring the dissolution of silica through the break of O-Si-O network. Preferential leaching supports the selective removal of specific cations (non-stoichiometric release) and designs a theory about incongruent dissolution of glass that explains the formation of altered surface layer.

The technological developments of the last decades have made it possible to reveal increasingly more detailed evidence on the process that controls glass corrosion 11 . The chemical reactions proposed in the general mechanism of glass corrosion (hydration, hydrolysis, and interdiffusion) are still considered valid in the most recent studies. However, over the last years the attention of the scientific community has been focusing more on understanding how these reactions evolve kinetically and thermodynamically during the alteration process, and how they influence the structural and microstructural properties of the alteration layer at the atomic scale 1 , 45 .

In 2015, a nanometre-scale study 46 of glass corrosion was performed using a combination of high mass and spatial resolution techniques, proposing a revised theory of glass corrosion called the Interfacial Dissolution-Reprecipitation (IDP) model. The IDP model is based on the congruent dissolution of silicate glass coupled in space and time with the reprecipitation of amorphous spherical silica aggregates of variable size. In opposition to the traditional glass alteration model, this recent theory supports the stoichiometric dissolution of glass without interdiffusion-controlled ion-exchange mechanisms at the glass reaction front 46 .

Hellmann et al. validated this model through the study of artificially aged borosilicate glass altered at 50 °C in deionised water, using a unique combination of techniques with high spatial and mass resolution 46 . By following the mobility of the major constituent elements of complex borosilicate glass, an identical release behaviour was noticed for modifier and former ions, regardless of their charge. These results validate two processes at the basis of this novel corrosion mechanism, i.e., the stoichiometric release of all the glass elements and the precipitation of amorphous silica with the formation of an altered surface layer. Furthermore, the interface between pristine glass and altered zone was demonstrated to be chemically and structurally well defined, with elemental gradients in the nano- to sub-nano-metric range.

A schematic representation of the IDP model is presented in Fig. 4 according to the results of oxygen and silicon isotope tracer experiments in ternary borosilicate glass 47 .

a Initial congruent dissolution of glass is the first step occurring at the glass-water interface. This stage continues until the amorphous silica solution is supersaturated and etching pits are formed on the surface; ( b ) Si-rich interfacial solution layer is formed depending on the ratio of silica in solution to silica released during glass dissolution. Under this condition, the localised saturation of silica in solution promotes condensation and nucleation reactions that lead to the polymerisation of monomeric silica to form dimers and oligomers; ( c ) Precipitation of silica in the form of spheres on the dissolving glass surface occurs after silica supersaturation and nucleation in the solution; ( d ) Formation of SAL (Surface Alteration Layer) composed of altered amorphous silica proceeds along with congruent glass dissolution and further diffusion of dissolved species through the developing SAL; ( e ) Diffusive transport of water and dissolved species through the corrosion rim continues depending on the porosity of the SAL, i.e., higher porosity values correspond to higher diffusion rates; ( f ) Precipitation of secondary minerals like zeolites and clays sometimes occurs within and at the silica surface.

As described above, the IDP model entails the formation of alteration layer through the precipitation of hydrated species from the thin film of water to the hydrolysis front, with high degree of liberty to reorganise (Fig. 5 ). This leads to a sharp concentration profile of highly soluble cations and an interstitial water layer that would allow an easy separation of the altered layer from the pristine glass 48 .

Courtesy of Gin et al. 48 .

The rate of the reactions involved in the glass corrosion (ion-exchange, hydration, and dissolution) depends on factors such as glass composition, temperature, and pH. All of them may occur simultaneously during process of alteration in contact with liquid water and can be rate-limiting in function of the experimental conditions. In conditions of unsaturated humidity, studies demonstrating the existence of a rate drop followed by a residual regime are lacking. For these reasons, both the two models presented in literature (CID and IDP) are not able to describe universally the distribution of mobile ions and hydrous species inside the alteration layer.

A work published in 2017 49 reported the in-depth characterisation using atom probe tomography (ATP), transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) of the alteration layer formed under close-to-saturation conditions. The results revealed the presence of an alteration layer with a more complex structure, made of three different sub-layers (Fig. 6 ): (i) close to the pristine glass, a thin hydrated layer containing all the glass components, (ii) moving towards the surface, a passivating layer with constant concentration of glass formers (Si, Al) and decreasing concentration of modifiers (Na, Ca) which is delimitated by a rough interface where alkaline and alkaline earths are preferentially leached out, and (iii) an external nanometric layer where Si undergoes hydrolysis and condensation reactions 49 . These results contradict the IDP model of glass corrosion recently designed, which highlights that many gaps are still present in the explanation of the mechanism of glass alteration.

Courtesy of Gin et al. 49 .

The authors of this review would like to emphasize that the universal application of the IDP model and other intermediate models to all silicate glass types is still an open question. This is because most recent research has focused solely on the artificial alteration of borosilicate glass, and the applicability of these models to other types of silicate glass remains uncertain. The characterisation of synthetic and geological glass altered over long time periods can be key to understand both the complex mechanisms responsible of the long-term transformation of glass network and the physicochemical features of the material transformed by the alteration. Multi analytical and high spatial and mass resolution investigation performed on surface of ancient glass provide clues about the characteristics of the altered layers and about how they reorganised their internal structure during long time periods. Instead, looking at the interface between the alteration layer and the pristine glass provides information about the mechanisms of glass alteration 2 . In this way, this approach promises to be efficient in validating the above mentioned debated kinetic models. The knowledge of the long-term behaviour of glass structure is also pivotal to predict the network dissolution in burial conditions in nuclear waste management studies 4 , 50 .

Evidence of degradation on ancient glass

Archaeological glass shows multiple and clearly visible symptoms of deterioration that can help identify well-distinguished classes of glass alteration 51 depending on the conditions it aged in (i.e., in soil, underwater, under extreme environmental conditions) for several centuries. Dulling, iridescence, weeping, pitting, discolouration and cracking of the surface affecting the specimens are common phenomena that can be observed on glass that has been recovered from archaeological excavation 52 . The formation of one or the other of them depends both on the physicochemical properties of the glass and on the environmental factors it has been exposed to. More than one of the manifestations of alteration can be found in a single glass object, thus making it sometimes difficult to identify the most appropriate strategy for conservation and/or stabilisation.

The atmospheric deterioration of glass appears markedly different from the degradation that occurs when glass objects have been buried for centuries, for example under soil, and it is rarely observed on this type of glass that has remained interred up to discovery and recovery. Crizzling , also known as glass sickness or glass disease, has been identified as the major alteration symptom for glass objects stored in museums and in private collections.

Alteration of ancient glass in archaeological stratigraphic contexts

The term dulling is used to refer to the loss of clarity and transparency typically observed in ancient glass and caused by the formation of layers of alteration products on the glass surface 52 .

As discussed above in the section dedicated to the general mechanism of glass corrosion (Section “Exploring the mechanisms of glass alteration through the different existing models”), in presence of neutral or acidic conditions, elements like alkalis are typically leached out from the first glass layers onto the surface. These leached species, reacting with humidity and moisture of the environment, tend to form corrosion products (like salts) that build up on the object’s surface and determine at first the loss of the original clarity 53 . In addition, at advanced stage of alteration, also hydrated silica (silica-gel) particles can also reprecipitate on the glass surface, leading to the formation of thicker alteration layers and causing an additional loss of glass transparency and the appearance of translucency 54 . This phenomenon is due to a combination of effects occurring between the local presence of water and the composition of glass, which determines the diffusion of ionic species from the first atomic layers under the surface to the environment and the consequent reprecipitation of hydrated silica and other alkali-derived compounds. The extent of the visual effect is much more considerable as the ion exchange proceeds and the thickness of the deposited layers on surface increases.

In more advanced stages, dulling can lead to the formation of iridescence patinas (Fig. 7 ) that may eventually detach in the form of crusts from the original glass substrate. In 1863 Brewster 55 demonstrated that this iridescent effect is due to the diffraction of incident light from layers of weathering products containing metal oxides formed after ion leaching. The rays of light are reflected from thin alternating layers of air and weathered glass crusts. These densely overlapping layers gradually penetrate deeper into the glass and they eventually change in colour towards darker hues 56 . The cationic species leached from the glass are often prone to reacting with the anionic species derived from reaction between OH - (especially in basic conditions) and atmospheric acid gases, thus forming salts with hygroscopic properties on the surface. This generates a phenomenon called weeping , which was first described by Organ in 1956. Weeping can lead to the formation of crystals or solutions of salts, depending on their deliquescence relative humidity 57 .

Images collected with Olympus BX43F optical microscope, x10 magnification. A detail of the indented rim where the overlapping of thin layers of altered patina is clearly visible (right panel).

The alteration phenomenon called pitting , is described as micro, small or large based on pit size, and it can occur simultaneously at different individual sites that later merge into interconnected complex structures producing an altered top layer, which causes the loss of glass transparency 52 .

Contrary to dulling, pitting is a visible mark of the weathering process, which occurs in alkaline solution 58 and commonly found on excavated glass. As described in Section “Exploring the mechanisms of glass alteration through the different existing models”, during the alteration process in alkaline solution, the prevailing deterioration mechanism is the dissolution of the silica network through the breaking of Si-O-Si bonds. Subsequently, the prolonged presence of a layer of moisture on the surface of glass causes an increase in the pH of the attacking solution, and ultimately pits are formed as a result of local dissolution of the silica network 52 .

A model that explains the formation of altered pits was recently developed by observing the decay process of different silicate glasses in river and marine aquatic environments 59 . The experimental results showed that the alteration of SSL glass is characterised by a two-step mechanism. The first step, called “hydration period” , is short and causes the formation of isolate fissures, while the second step, called “pit development period” , involves the creation of basic species (OH - ) during the dealkalinisation process that progressively break the silica network, thus widening the fissures to form pits. According to the results of this work 59 , the formation of pits is correlated to a dynamic loss of mass, i.e., the slow rate of the first step of hydration allows the diffusion of solution and the consequent basic attack inside the fissures, causing local network dissolution.

Figure 8 shows the surface of a Roman archaeological sample affected by pitting.

Image collected with Olympus BX43F optical microscope, x10 magnification.

Discolouration can be often seen on archaeological glass surface in combination with other types of weathering phenomena described above. It is closely related to darkening, which occurs when the oxidation of specific leached ions, such as iron, manganese, and copper, changes the colour of the weathering crusts, or to the production of hydrogen sulphide by sulphur-reducing bacteria in anaerobic environment and the formation of lead sulphide 53 . The latter case occurs only when the glass contains a high concentration of Pb oxide, and it is buried under anaerobic condition. In other cases, the presence of manganese and iron causes the darkening of glass with the formation of brownish pits 60 , as those visible in Fig. 9 . Ancient glass contains these elements in the form of impurities present in raw materials (sand and wood ash) or as a result of their deliberate addition as chromophores or/and decolourant agents in the form of minerals (i.e., pyrolusite) 61 .

In the case of archaeological glass that has been interred for centuries, cracks may be present on the surface of the glass fragments: these are caused by the shrinkage of the alkali-deficient layer due to temperature and humidity changes. Typically, these cracks are filled with mineralised material, likely originating from the soil in which it was buried. The mechanism of their formation is currently being studied since it has not yet been fully understood.

Alteration of glass in atmospheric conditions

Crizzling has been identified as the appearance of minute cracks on the surface of glass (Fig. 10 ) developing over time. These cracks penetrate deeper in the body of the object, ultimately resulting in its physical collapse. This phenomenon is due to two main factors: the unstable composition and the storage in fluctuating humidity environments 62 . Better storage conditions can slow down, but not stop the deterioration, because the role of glass composition remains a key factor in the evolution of the crizzling alteration 63 .

Object exposed at the Museo Civico di Modena (Italy). Picture by Renaud Bernadet.

Early publications reported the chemical effects causing crizzling through experiments that reproduced the condition of glass alteration in the laboratory. The results showed that crizzling is mostly associated with glass compositions characterised by high alkali and low CaO contents, and/or a high K/Ca ratio 64 .

In 1975 Brill 63 first used the term crizzled to describe glass with a decrease in its transparency due to the formation of fine cracks on the surface. He noted that certain glass objects that were in contact with water for centuries do not exhibit a high degree of degradation; however, once they are exposed to museum storage conditions (light, low RH and temperature), they display the formation of crizzling. This alteration mechanism is due to the dehydration of the glass surface, i.e., the low RH in museum display cases (15-20 %) causes a loss (up to twenty percent in weight) of the water that penetrated the gel layer of altered glass, bringing on a significant loss of volume in the gel layer itself which ultimately results in the cracking of the glass surface 62 , 63 . In general, the cracking of the hydrated gel layer formed on the surface of unstable glass can be attributed to several factors, including the dehydration of the gel layer itself - as mentioned above - the leaching process, which can lead to network contraction after the replacement of larger alkali ions (Na + ) by smaller hydrogen ions (H + ), and the different coefficient of expansion of the bulk glass and the gel layer 16 .

The guidelines of the Corning Museum of Glass 52 describes the process of crizzling indicating five stages. In the first stage (Initial stage) the glass has a blurred appearance due to the presence of leached alkali on the surface. During this phase, it is still possible to wash the surface and the glass can return to its original appearance. Conversely, in the second stage (Incipient crizzling) the haziness remains also after washing and the glass surface exhibits fine cracks like tiny silvery lines. Cracking progresses in stages three (Full-blow crizzling) and four (Advanced crizzling) until it gets to the deepest regions, leading to the loss of small fragments. Eventually, crizzling is so deep that the glass loses its structural integrity, even without any external contribution (Fragmentation stage).

Often crizzled glass has a pinkish hue. When alkaline leaching occurs and the glass structure is open, the manganese ions present in the surface cracks oxidise, yielding a pink colour 62 . This phenomenon is more evident in ancient glass which contains manganese as a decolourant.

To limit the evolution of the crizzling process, preventive conservation is an essential strategy for the safety of museum glass objects. The Corning guidelines set the optimal RH range for glass conservation between 35 to 65%, however crizzled glass or glass with a particularly fragile composition require specific individually controlled cases with a stable RH condition in the range of 40–50% inside 65 .

In addition to preventive conservation, the development of an innovative and compatible consolidation treatment for the protection or repair of cracked glass is an open challenge for the scientific community of cultural heritage conservation.

Intrinsic and extrinsic parameters influencing glass deterioration

As already mentioned, atmospheric conditions, such as temperature, the pH of the environment, salts and ions concentration, relative pressure under burial or marine conditions, and the presence of water in liquid form (RH ≥ 100%) or vapour (RH < 100%), strongly influence the kinetic of the glass surface alteration and its chemical transformation.

Many published papers of archaeological interest use the term weathering to refer to the typical degradation process that affects archaeological glass that has been exposed to particularly unfavourable environmental conditions (especially in burial and underwater contexts) 52 . Weathering is a degradation process occurring through contact with water in the environment, both in the vapour and in the liquid state, whereas the term atmospheric deterioration is used to describe glass that aged under the effect of water in the form of moisture (especially in protected environments like museum display cases), which means in the form of vapour state interaction.

The study of extrinsic factors, that are closely linked to the environmental conditions acting during the alteration process over centuries requires the development of appropriate artificial ageing protocols, which allow the modelling of the phenomenon as a function of these parameters. Real cases of glass degradation are the result of the combined actions of the intrinsic and extrinsic factors mentioned above, which generate entangled mechanisms of ionic interdiffusion from the glass network to the environment and vice versa. From an experimental point of view, studying this interconnected process of ionic interdiffusion and the formation and growth of novel phases implies the need of relying on simplified model systems to investigate the effect of specific variables to the detriment of others, which are kept constant.

The study of the effect of the various factors on glass degradation has attracted the curiosity of researchers since the beginning of the XX century. Already in 1925, G. W. Morey 66 stated that the subject was still in an empirical state, despite the considerable number of works carried out. At that time, to understand the effect of water on the alteration process many experiments were conducted by varying temperature, the pH of the environment and glass composition.

The following paragraphs describe the effect of various parameters that have a primary influence on the process of glass alteration. The studies there cited use ancient glass samples as evidence of the long-term effect of these parameters.

Effect of glass composition

Many ancient finds of SSL glass are macroscopically preserved intact in their shape, despite the physicochemical alterations caused by the burial environment over the centuries. Currently, 90% of globally manufactured glass is still based on the SSL composition, which has been kept largely consistent over the centuries except for a few modifications introduced at the beginning of the 20 th century to enhance chemical durability and resistance to devitrification 67 . Besides SSL glass, many other types of ancient glass exist, such as potash lime glass (K 2 O – CaO – SiO 2 ), lead silicate glass (PbO – SiO 2 ), or potash lead silica glass (K 2 O – PbO – SiO 2 ) 68 . The optimal preservation of certain ancient samples and the complete collapse of others is the result of a complex interplay between their intrinsic material properties and the extrinsic factors acting on them.

Intrinsically, glass physicochemical properties play a significant role in determining its degradation behaviour. Such properties typically correspond to the chemical composition of glass, the nature of its surface, the presence of impurities, inclusions, inhomogeneity, and phase separations. In particular, the concentration of silica, alkali (soda, potash), stabiliser (lime, lead), as well as the inclusions of trace elements and additives like metal oxides, added into glass as chromophores, opacifiers and decolourants all strongly affect material durability 53 . Small variations in the concentration of these components determine strong variations in glass durability. Diffusion through the leached layer is more likely to occur for smaller ions, such as Na, Mg, Li, rather than for larger ones, i.e., Ca or Ba. Although all types of alkaline silicate glass are susceptible to weathering degradation, from a thermodynamic point of view stability increases as in the following: K 2 SiO 3  < Na 2 SiO 3  < Li 2 SiO 3 38 .

Silica rich glass, such as Roman SSL glass, is more durable than poor silica glass, like medieval glass 69 . In addition, K + , which is contained in medieval glass as a monovalent cation modifier, is more susceptible to leaching out from the deeper region of the glass network during weathering alteration than bivalent cations such as Ca 2+ , which are present in Roman glass 70 , 71 . This behaviour is due to the bivalent cations forming stronger bonds with non-bridging oxygen, as previously mentioned. The preferential leaching of K + over Ca 2+ during the weathering process was also confirmed by an experimental study that investigated the weathering phenomena on naturally weathered potash-lime-silica-glass 72 . This behaviour of K + cations was also observed during leaching experiments in aqueous acidic solution.

The study of the varieties of composition of ancient glass and their resistance in RH > 100% have made it possible to identify the presence of compositions that are more chemically stable than others and to define general conditions to discriminate between stable and unstable glass 73 .

In 1978 74 , Hench systematically studied the surface of glass exposed to pure water and distinguished between 6 main types of surfaces with increasing inclination to deterioration in relation to their composition. The surface layers of the different glass types may have protective or non-protective properties for the glass substrate, depending on the capacity of reducing ion leaching and glass dissolution. In this work Hench distinguished stable and unstable glass based on the different compositions of the alteration products that form the first surface layers when glass is altered in liquid conditions. However, the protective character of hydrated layer has not been demonstrated in unsaturated humidity conditions yet; and, moreover, a work of Sessegolo et al . 75 studied medieval stained-glass windows in unsaturated conditions with isotopic water experiment and demonstrated that the alteration layer is not protective against vapour transport and interdiffusion.

A further way to discriminate a stable glass from an unstable one could be to use the ternary diagram of Fig. 11 , which was formulated in 1975 by Newton et al . 76 with a view to help predict the weathering behaviour of different types of glass. Plotting the concentration (mol. %) of network stabilisers (RO), network modifiers (R 2 O), and silica (SiO 2 ) determines the chemical stability of a given glass composition, i.e., highly durable glasses are placed near the centre. This diagnostic model may work well when binary or ternary glass is considered, however complications may occur when classifying ancient glass, which has a more complex composition, because it may be necessary to consider the combination of multiple formers (SiO 2 ad Al 2 O 3 ), monovalent oxides (R 2 O) and divalent oxides (RO).

R 2 O represent the content of network stabilisers (monovalent oxides), RO the content of network modifiers (divalent oxides), and SiO 2 the content of silica.

By using electron microprobe analysis (EMPA) and hydration-dehydration experiments on ancient glass Brill observed that a deficit of stabiliser (CaO content less than 4 wt%) combined with an excess of alkali (over 20 wt%) in SSL glass easily leads to an extensive surface deterioration known as crizzling 63 . He defined as unstable glass the one with total alkali oxide concentration over 20%; this composition range determines silica network configurations that are open enough to facilitate the migration of monovalent cations. In line with these findings, other studies reported that an increase in the CaO to SiO 2 ratio increases glass stability, while a concentration of calcium oxide over 15 wt% entails rapid glass instability 38 . Recently, a comparison in terms of glass chemical stability was made using artificial mock-ups with different glass compositions which were exposed to high humidity environment and different levels of formic acid 77 . The results showed that glass with a higher content of stabilisers exhibits greater stability, especially glass with calcium.

The presence of potash as a modifier and the low silica content in the composition make glass particularly fragile and susceptible to chemical alteration. Numerous works reported the considerable damage that affects medieval glass windows with Si-K-Ca-based composition, which is considered one of the most unstable 78 , 79 , 80 . Typical alteration marks are mainly pitting 81 or the formation of a corrosion crust 71 on the surface due to the combined attack of water and increased pollution in the air. The weathering crust is often very heterogeneous and fractured because of the wetting-drying cycle conditions and generally consists of calcite, gypsum and/or syngenite formed by the reaction between the alkaline and alkaline-earth elements released during ion-exchange and sulphur dioxide in the atmosphere 6 .

Effect of minor components in glass composition

In addition to the major elements discussed above, other minor elements may be present in the composition of ancient glass as decolorising, colorising or impurities of the raw materials used for their production. These minor elements can in turn contribute to the alteration process of the glass itself by giving rise to distinctive corrosion marks.

Several authors have remarked that manganese that accumulated between the altered layers may have originated from the burial environment 82 , 83 . Dark deposits have been detected inside the dealkalinisation layer on the surface of Roman glass samples 84 . Secondary electron images have uncovered that these deposits are formed from the interconnection of spherical particles of about 2 µm in size, with iron and manganese oxides as their major components. These studies clarify that during the leaching process Fe (II) and Mn (II) ions are hydrated and oxidised, giving rise to the formation of dark amorphous products that precipitate into the pores of the leached silica film.

It is well-known that glass technologists used to add Mn, as well as Sb, to the glass melt as a decolorising agent, but it is also known that, in this type of glass, the appearance of brownish areas on the altered surface is due to an oxidation process of Mn(II) to higher oxidation states. It is generally observed that in the dark areas Mn is present in +IV oxidation state 85 , 86 . Nevertheless, a recent work performed using synchrotron radiation X-ray absorption spectroscopy (XAS) analysis on historical stained-glass windows demonstrated that the most extended brown altered areas mainly contain Mn mainly under a +III oxidation state 87 .

Through the analysis of 14th–17th century window glass, Schalm et al. 82 concluded that the formation of Mn-rich inclusions takes place simultaneously with the growing of leached layers settling along their interfaces and their concentration is mainly caused by the environment (soil) in which the glass was buried for several centuries.

Although brown/black staining has mostly been attributed to manganese compounds, iron (and titanium) compounds were often reported in association with the manganese 53 . Other elements which are often present in historical glass network as impurities can improve the chemical stability of glass. For example, low amounts of ZrO 2 (2 %wt) increase the acid and alkaline durability of glass 88 , and a hydrated ZrO 2 surface can act as a barrier to further dissolution of other ionic species. More recently, it was demonstrated that the substituting an insoluble oxides like zirconia to a fraction of silica slows the glass dissolution kinetics, but prevents the alteration gel reorganisation inhibiting the pore closure mechanisms and leading to greater degree of corrosion 89 .

These results show that it is essential to know the exact composition of an ancient object to determine a range of the aging behaviour that it is prone to. To do that, it is important to also consider the presence of elements in lower or trace concentrations to fully describe the corrosion phenomenon and formulate models that are closer to real cases of ancient glass alteration.

Effect of liquid vs atmospheric water

Water was observed to be the primary environmental ageing factor that causes glass deterioration by Lavoisier already in the early 1770s. Lavoisier also indicated two different mechanisms of water penetration into the glass network, i.e., through network voids between oxygen atoms in molecular form, and by hydrolysis and condensation reactions with the metal-oxygen bonds 19 .

The alteration of silicate glass differs when it occurs in liquid-phase or in vapour-phase regime. In contact with water the glass surface undergoes chemical attack through ion-interdiffusion and hydrolysis of metal-oxygen bonds 6 . In vapour conditions (RH < 100%) the hydration process does not release elements into the fluid, but it involves a redistribution of elements in the alteration layer (expect for volatised elements) 90 . The formed hydrated layer has chemical composition and porosity that are different from those obtained in liquid conditions, thus resulting in the glass durability properties specific for liquid or vapour-phase regimes. Even if the molecular process is the same, the interplay between the intrinsic and extrinsic variable changes affecting the macroscopic transformation of the material 45 .

During the phenomenon of water diffusion, no matter in which RH condition, the glass surface is covered by different monolayers of water, depending on the water partial pressure in the atmosphere. As soon as the silica network of a freshly made glass encounters moisture from the environment the chemisorption of water molecules starts from the first layers of the surface, which is consequentially more subjected to physical and chemical transformation. Water is chemisorbed on the glass surface and a thin film of water of few molecular layers builds up onto the chemisorbed layer, depending on RH condition.

One of the first characterisations of altered glass surface was conducted by Hench in 1975 56 , using a combination of analytical techniques that enables to obtain physicochemical information from different depths of the altered glass surface. He systematically studied the glass surface altered in liquid water, using techniques like Auger electron spectroscopy (AES), infrared reflection spectroscopy (IRRS), and electron microprobe (EMP), and distinguished 6 main types of surfaces with increasing inclination to deterioration (Table 1 ). Of these, only Type I is stable and shows no main differences with bulk glass. The other types are considered unstable and prone to deterioration over time. Type II and type IV are of interest for historical deterioration. Type II is formed when the glass contains a high level of network formers, and it involves the formation of a silica-rich layer on the surface that acts as a protective film, preventing the leaching of alkali ions and the rupture of the glass network (Si-O-Si). On the other hand, no protective film is formed for Type IV and so the leaching of alkali can proceed. This latter situation is typical of unstable glass.

According to Hench’s classification, ancient glass has a surface that corresponds to Type II and Type IV surfaces, which are characterised respectively by the presence of silica-rich surface protective layers (when the concentration of network formers is high enough) and the presence of non-protective surface layers that allow the alkali leaching to proceed (when the soda to silica ratio is high enough) 56 .

In the case of discontinuous contact with aqueous media, which is typical of the atmospheric alteration, the formation of laminated structure on the altered glass surface has been explained by the intermittent water supply 6 . Through the observation of ancient glass surface by TEM, a sequence of bands of different thickness has been recently distinguished 91 , i.e., thinner bands called lamellae (20-50 nm) and thicker ones called laminations (0.1-4 µm), which consist in groups of laminae with the same orientation. The amorphous laminae, which are depleted of alkaline ions, are formed from the local rearrangement of glass elements resulting from the repetition of several cycles of interdiffusion and glass dissolution processes. Moreover, cracks are developed perpendicularly and parallelly to the laminae and laminations. Based on this study, two main processes take place within the cracks, i.e., the migration of atmospheric solutions deeper into the bulk glass, thus moving the alteration front further, and the precipitation of secondary mineral phases which favours the mechanic separation and the loss of glass fragments.

In 2016, a study of the altered layers observed in ancient glass proposed a model to explain the formation process of this laminated degradation 92 . Thanks to optical microscope, field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDX) analyses, it was possible to describe surface lamellae as an alternation of random amorphous silica nanoparticles with different packing densities and with thickness between 0.1 and 10 µm. Moreover, the growth of nanosized silica particles on the surface of the altered glass was observed to be one of the by-products of the leaching process of glass components that occurs under alkaline conditions.

While glass alteration in contact with water in a liquid state was widely studied during the 1900s, testing approaches in unsaturated atmosphere have been seldom explored until more recently 93 . In 2020, Majerus et al. 7 published an overview of the experimental protocols for glass alteration tests in unsaturated atmosphere. All the tests reported were conducted under RH < 100% condition and, when a saline solution was present, their configuration ensured that the samples were never in contact with it. These results obtained under RH < 100% suggested that the glass alteration mechanism was different from the one obtained in contact with liquid water (RH > 100%). In contrast to saturated humidity conditions, in unsaturated environments, the hierarchical order of glass alteration processes involves network hydrolysis first and interdiffusion second, particularly for glass with compositions classified as unstable.

Alloteau et al. recently demonstrated that in water vapour conditions, the process of glass hydration is independent from dealkalinisation: at higher temperature (80 °C) hydrolysis predominates over diffusion processes and solvation, whilst at lower temperature the two processes proceed together in parallel. To state that, they performed ageing test in static conditions (no T and RH cycling) at controlled temperature and humidity (RH 85%) on three different types of unstable historical glass (soda-lime silicate from antiquity, mixed alkali silicate from middle-age/renaissance and potassium silicate from XVI-XVIII century). The conditions were set to avoid any liquid water flow on the samples during tests.

Sessegolo et al. 75 evaluated the respective contribution of rain, wet periods and unsaturated humidity on the kinetics of formation of the alteration layer of potash-lime-silica glass. The results demonstrated that the fluctuation of dry and wet periods (non-static condition) leads to the formation of an altered layer rich of cracks, pits, and scales, which constitute a network of pores and fractures behaving as a major vector for interdiffusion of liquid and vapour water. The characterisation of this fracture network is particularly important in case of prolonged alteration in atmospheric conditions, where liquid and vapour water phases are systematically combined in the associated diffusion processes. In another subsequent work 94 the authors proposed a model to estimate the thickness of alteration of stained glass windows subjected to liquid water (such as rain), comparing the expected value of alteration thickness extrapolated linearly over 650 year with those observed on ancient stained glass. The values predicted by the developed model match well with the alteration layer thickness observed on real ancient medieval samples 6 , 95 , 96 .

Effect of saline and burial environments

In the last two decades, several works have been done to explore the effect of ageing in burial or marine environments on the alteration mechanisms of glass and on the formation of new alteration phenomena. A considerable number of works focused on the prediction of the durability of high-level nuclear waste glass and the migration of radioactive and non-radioactive elements into the burial soil 9 , 97 , 98 .

Verney-Carron et al. 5 studied ancient Roman glass blocks naturally aged in seawater obtaining the kinetic parameters to develop a geochemical model to simulate the alteration of archaeological glass. Comparing the results obtained from the characterisation of ancient glass with those from the computer simulation, this work demonstrated the importance of archaeological glass for validating the predictive capacity of geochemical long-term model, thus bridging the gap between the results obtained from short-term experiments and long-term alteration of complex systems.

Palomar devoted many studies to weathering and to the comprehension of the environmental effect on the stages of glass alteration 59 , 99 , 100 . In particular, she reported the effect of coastal atmosphere on glass degradation 101 , which is a scarcely investigated subject. The alteration of glass surface exposed to coastal environment is mainly caused by the high presence of liquid water that covers the glass surface, thus inducing a hydrolytic attack and the dealkalisation process, and by the high wind speed, which favours the transportation and deposition of sodium and chlorine ions on the glass surface. Marine aerosol in elevated concentrations represents a hazardous agent for the chemical stability of glass, since its action could increase the hygroscopicity of the glass surface and open the glass structure, allowing the alteration to proceed deeper into the glass.

Another visible mark of degradation observed on the surface of glass altered in saline environment is the formation of peculiar chemical ring-like patterns that have been explained according to the Liesegang theory (Fig. 12 ). Diffusion, reaction, nucleation and crystal growth are all phenomena that have been used to formulate models that explain the Liesegang rings formation 102 , 103 . Dal Bianco et al . observed these weathering rings present on glass fragments from the Roman ship Iulia Felix found on the Grado lagoon, in North-East Italy, and dated back to the 2 nd century AD 104 , 105 . Their characterisation study showed a maximum diameter of the rings of about 1 mm and the interconnection of interface lines during the simultaneous growth of adjacent rings. This study did not report an exhaustive theory about the formation process of rings, but the authors observed that the structure was similar to the descriptions of Liesegang kinetic evolution of precipitates in gel found in other research works 106 , 107 . This assumption is acceptable since hardly corroded glass structure can be assumed as a gel where weakly soluble salts periodically precipitate due to the reaction between two soluble substances, one of which is dissolved in the gel medium. The final appearance of the precipitates depends on their solubility and on the initial concentration ratio of the reagents, but generally their aspect is concentric around the centre in which precipitation starts 108 .

Image collected with an Olympus BX43F optical microscope, x10 magnification. Interconnection between adjacent rings can be observed.

Nowadays, the formation of this type of rings on archaeological glass has been observed on the surface of samples recovered in submarine environment 104 , 109 . The authors of this review suggest that a saline-rich environment, whether seawater or wet soil, play a role in the formation of the rings. The soluble substances present in these environments may react with soluble substances from the aged glass forming salt precipitates on the surface of glass that act as centre of nucleation for the growth of the concentric structure.

The mechanisms that generate the formation of rings on altered glass surface have not been analytically confirmed, and the study of the evolution of the kinetics that controls ring growth on archaeological glass remains incomplete, despite the availability of modern techniques. Although the layers observed and experimentally described by Schalm 110 can be compared to the bands structure obtained from Liesegang kinetics, there is a lack of systematic experimental work in the literature to confirm whether this theory can be applied to the dissolution-precipitation mechanisms of glass.

Chemical studies on the composition of the soil where archaeological glass aged for centuries might validate the hypothesis of ionic exchange between the elements of the soil and the elements of the glass network, providing the opportunity to understand how the interaction with the external environment can drive the process of alteration in different types of glass.

Many published research studies mention that also for glass samples that altered buried in soil the most common pathology observed is the formation of dealkalinisation layers 111 , 112 , 113 . One of these performed a stratigraphical analysis using a non-invasive technique (laser induced breakdown spectroscopy, LIBS) to observe the progressive dealkalinisation of glass bulk composition, reporting an increase in the calcium and sodium intensity signals on the glass surface 84 .

In the field of ancient glass, experimental studies simulating ageing in soil are less common 114 , 115 , 116 . In a recent work, Palomar et al . tried to replicate ageing in soil in a natural burial environment to understand the corrosion mechanisms acting on different ancient glass types (Roman, medieval, lead crystal glass, common window glass) 117 . The burial tests were set up to last 300 days and were carried out at 60 °C, to accelerate the alteration processes. The pH of the burial soil and the glass composition showed to have a key role in controlling the reactions between the constituent glass elements and those from the environment.

When considering burial artificial alteration of mixed alkali glass compositions with low silica concentration, acidic or neutral conditions (pH 6.5 – 7.5) lead to the formation of micro pits and cracks; an iridescent and translucent layer is formed, instead, on the surface of glass under alkaline soil condition (8.0 – 9.0) 118 . Silica-soda-lime glass shows a different behaviour. In fact, in acidic soil the formation of both isolated and interconnected fissures may be observed. However, under neutral and alkaline soil conditions an increase in the number and depth of pits, whose rate of accretion depends on the content of alkaline oxides in the glass, can be detected, with a considerable increase in the diffusion of surface degradation under the alkaline condition 117 . This experimental evidence is a clear example of the necessity to consider glass composition and environmental factors simultaneously when approaching the understanding of the glass corrosion mechanism.

Effect of the environmental pH

A pivotal factor in determining the rate of glass corrosion is the pH of the attacking solution both in case of vapour and liquid conditions 10 . Under conditions of low pH (acidic solution), the deterioration mechanism predominantly involves the ion-exchange process due to the abundance of hydronium ions in solution and the formation of silanol groups [Si-OH], generating a hydrated gel on the surface which slows down degradation 117 . Differently, at high pH, the interaction between the glass surface and the alkaline solution leads to the dissolution of the silica network through the rupture of the Si-O-Si bonds, which implicates a more aggressive condition.

In works performed in 2016 92 and 2021 110 , Schalm et al. observed that glass surface is transformed in both acid and alkaline condition, but different morphologies can be developed depending on the pH of the solution. At pH<7 the transformed glass has a homogeneous morphology determined by the dissolution of leached silicate network and precipitation of dissolved compounds as amorphous silica forming linear and randomly branched chains of alteration product that can be described as a silica gel. At pH between 7 and 10 of the local solution the transformation process leads to the formation of silica nanoparticles packed into alternating density lamellae. The authors successfully reproduced the lamination in laboratory experiment performed around pH 10, where silica solubility is highly variable and induces the cyclical dissolution-precipitation of silica. The mechanism of formation of consecutive lamellae with alternating packing density is proposed to be dependent on the pH oscillation in the local solution during the alteration process. In extreme alkaline condition (pH>10) the fast dissolution of silica network occurs without any material precipitation.

A recent work 94 highlighted how the pH-dependency is linked to the specific composition of glass and consequently to the solubility of different glass constituents. The results show that the pH-dependency of Ca-Mg-silicates highly differs from the behaviour of aluminosilicates, and that Si-K-Ca medieval glasses have a very low pH dependency at alkaline conditions.

Effect of pollutants

Air pollution was identified as a particularly dangerous agent that enables to speed up and enhance alteration processes. In the museum context, the presence of carboxylate acids pollutants is generally the main cause of glass corrosion and of the formation on the glass surface of efflorescence salts as deterioration products 119 . With regards to historical stained glass, an experimental work carried out over a six-year period to quantify the influence of various air pollutants from different local environments (Europe and North America) on the degradation of potash-lime-silica glass, which has a similar composition to that of medieval stained glass, showed the formation, after exposure to rain and solar radiation, of crystalline carbon-rich products unlike those of samples aged under sheltered conditions 120 .

Melcher and Steiner 120 performed a 6-year experiment to evaluate the effect of acid gas and pollutant on replicas of stained glass, comparing the leaching depth formed on potash-lime-silica glass replicas to former leaching studies performed on medieval stained glass. This work demonstrated that the hypothetic leaching depth cannot be directly related to pollution data because, while the leaching depth increases with time, the leaching rate decreases. Even a paper of Robinet et al . 30 highlighted the role of the organic pollutants formic acid, acetic acid and formaldehyde in the alteration of unstable soda silicate glass. He confirmed that museum wooden cabinets emitting organic pollutants must be avoided since they foster the progressive alteration with an estimated rate of ~2 nm/day.

Advancements in monitoring ancient glass conservation state

The kinetics of glass corrosion, as well as the sequence of events involved and the prevalence of one interconnected mechanism over another, are dependent on both the chemical composition and structure of the glassy material and the environmental conditions to which it is exposed. These conditions include the amount of water that reacts, the chemistry and pH of the solution, and the duration of exposure. Given the complexity of these factors, the alteration of glass is a complex phenomenon that requires careful study. Therefore, particular attention should be paid to the analysis of ancient glass samples that have undergone modifications over time scales that cannot be replicated in the laboratory. These samples provide valuable insights into the long-term alteration mechanisms of glass and can aid in the development of more accurate predictive models for understanding the complex behaviour of glass materials in various environmental conditions.

The characterisation of altered ancient glass provides concrete evidence of the transformation of the vitreous structure, the nature of the dissolution products, and the way in which a specific glass composition reacts to a particular environment 3 , 4 , 7 , 70 , 121 , 122 . Over the course of history, the evolution of glass manufacturing has led to the production of various types of glass, such as Roman SSL glass, medieval stained glass, or Venetian crystal glass ( Cristallo ). Artefacts (and their fragments) made from all of these types of glass inherited from the past represent an exceptional opportunity to better understand the processes involved in glass corrosion on a long-term scale. This is due to their specific chemical stability, resulting from their unique chemical composition, and their aging in soil, underwater, or confined spaces with specific microclimates for centuries.

The analysis of glass surfaces is currently considered the most effective scientific approach for studying chemical and physical variations that occur at the interface between the atmosphere and the glass surface. Advanced surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS) and ToF and/or dynamic SIMS, provide high-resolution data that enable the investigation of the chemical composition of the glass surface’s first nanometres and facilitate monitoring of its modification during the alteration process 67 , 123 . SIMS is widely considered one of the most suitable techniques for studying corroded glass 124 , due to its ability to detect hydrogen and investigate the glass surface up to a depth of a few microns. Another technique commonly reported in the literature for studying the weathered surface of ancient glass is laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which allows for the acquisition of 2D and 3D elemental distribution with spatial resolution as high as the spot size used for the analysis. Only one work, published in 2013 125 , presents the procedure to obtain elemental maps to investigate surface layer phenomena on pitted ancient glass. The results of this application showed that the dealkalinisation of glass sample occurred on surface, as the direct or indirect consequence of the hydration process, resulting in the final formation of pits and of the so-called Liesegang rings on the surface of the sample. Despite the novelty of this research, the spot size of the laser beam used in this work (diameter of 80 µm) is higher than the average size of the corrosion marks present on ancient samples. The spatial resolution on the reconstructed maps can be improved by using smaller spot size of the laser beam (down to 10 µm) making it possible to obtain a resolution suitable for appreciating the chemical variability of extremely heterogeneous samples. This can potentially increase the understanding of the mechanisms of formation of these altered phases.

Critically speaking, the extensive study of glass surface composition using these advanced analytical techniques is still seldom considered when investigating the corrosion mechanism of glass both on ancient samples and through an artificial ageing approach in the laboratory 30 , 126 , 127 . Each surface technique has its own peculiar applications and limitations 128 , but a combination of different techniques allows a complementary and more comprehensive characterisation of corroded glass.

Collecting morphological, structural, and chemical information through a multi-scale analytical approach is necessary to gain a comprehensive understanding of the evolution and kinetics of glass alteration, including the modification of glass composition. Well-designed experiments are crucial due to the co-operation and mutual influence of different mechanisms involved in glass corrosion. This approach allows for investigation from a macroscopic to a nanometre scale observation. The information gathered can help determine how and when to intervene for conservation and preservation of glass objects, whether they are of industrial or historical significance.

The evaluation of storage parameters such as relative humidity, temperature, and light, as well as the condition of the glass, are crucial for preventing degradation of glass objects in museum collections or other sites of cultural interest. This approach is particularly important for understanding the influence of intrinsic and extrinsic factors in the processes of alteration and finding the best conditions for conservation. Surveys 129 , 130 , 131 conducted in various museums in Europe have shown that a significant number of glass objects are in critical condition due to extensive degradation, highlighting the need for preventive actions. For example, at the Victoria and Albert Museum, out of 6,500 glass vessels optically investigated, more than 400 showed clear signs of glass deterioration 54 . Similarly, the glass storage conditions of the Royal Palace of Madrid and the Technological Museum of Glass (Segovia) were evaluated during the period from September 2019 to November 2020, detecting a high concentration of formic acid in the display cabinets and wardrobes, mainly due to the presence of wood, which results in a higher glass surface hygroscopicity (pH=8) 132 .

Several methods have been used to categorise unstable glass from museum collections based on its appearance 65 or by analytical techniques –like X-ray fluorescence (XRF) 133 , ion beam techniques (particle -induced X-ray emission, PIXE, and particle-induced gamma ray emission, PIGE) 134 or spectroscopic techniques (Raman and Fourier-transform infrared, FTIR) 135 , 136 – in order to determine glass composition. Using these techniques to analyse cultural heritage objects can be complex due to many operative limitations, as for instance the need of carrying out micro-sampling (which is often not possible) or moving the objects to the specialised laboratories where the instruments are hosted. With the aim to distinguish stable from unstable glass in museum collection, the ideal analytical technique should provide the chemical composition of the sample, be highly sensitive, and have a very fast time of analysis to characterise as many samples as possible in a short period of time, while operating in a non-invasive way. Consequently, it can be safely stated that no scientific and straightforward approach for understanding the chemical nature and composition of unstable glass in a non-invasive way and for large museum collections of glass objects has been developed yet.

In the light of the above, it is evident that the most effective approach to study glass degradation involves the integration of two main distinct but interconnected methods:

the evaluation of the surface of ancient glass artefacts;

the evaluation of intrinsic and extrinsic causes for glass alteration.

Advanced analytical techniques can be used to characterize ancient glass surfaces and observe the products of the long-term transformation of the glass structure resulting from the alteration process. In parallel, laboratory-based aging experiments can be used to evaluate intrinsic and extrinsic causes of glass alteration, distinguishing the effects of different parameters and identifying the most relevant factors that influence glass alteration kinetics. Parameters such as the content of alkalis and stabilizers in the glass composition, as well as temperature and humidity, strongly impact the chemical durability of glass by affecting the concentration of alkalis and hydroxyl or non-bridging oxygens in the hydrated layer. It is difficult to determine which factor has the most significant influence on the process of glass alteration because the final degradation symptoms are the result of a mutual effect between all the aforementioned.

As evidenced, over the years many experiments aimed at understanding the phenomenon have been reported in literature and almost as many methods have been proposed. This abundance of results represents an outstanding opportunity for future research studies in this field, which will be able to rely on advanced analytical approaches and more accessible high-resolution techniques even for the domain of cultural heritage science. The results obtained from the study of ancient materials are pivotal to validate the long-term capacity of kinetic models -which are based on experimental data only - and to strengthen current theories of glass corrosion.

Moreover, comparing the characteristics of artificial replicas with those of ancient glass enables a better understanding of the glass corrosion phenomena, that in turn can underpin both the formulation of new protective solutions to preserve and protect glass artefacts in the long-term and the design of technologies exploiting glass properties in a variety of applications.

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Zanini, R., Franceschin, G., Cattaruzza, E. et al. A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models. npj Mater Degrad 7 , 38 (2023). https://doi.org/10.1038/s41529-023-00355-4

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"Earth's changing climate is a critical issue and poses the risk of significant environmental, social and economic disruptions around the globe. While natural sources of climate variability are significant, multiple lines of evidence indicate that human influences have had an increasingly dominant effect on global climate warming observed since the mid-twentieth century." (2015) 8

The Geological Society of America

"The Geological Society of America (GSA) concurs with assessments by the National Academies of Science (2005), the National Research Council (2011), the Intergovernmental Panel on Climate Change (IPCC, 2013) and the U.S. Global Change Research Program (Melillo et al., 2014) that global climate has warmed in response to increasing concentrations of carbon dioxide (CO2) and other greenhouse gases ... Human activities (mainly greenhouse-gas emissions) are the dominant cause of the rapid warming since the middle 1900s (IPCC, 2013)." (2015) 9

Science Academies

International academies: joint statement.

"Climate change is real. There will always be uncertainty in understanding a system as complex as the world’s climate. However there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001)." (2005, 11 international science academies) 1 0

U.S. National Academy of Sciences

"Scientists have known for some time, from multiple lines of evidence, that humans are changing Earth’s climate, primarily through greenhouse gas emissions." 1 1

U.S. Government Agencies

U.s. global change research program.

"Earth’s climate is now changing faster than at any point in the history of modern civilization, primarily as a result of human activities." (2018, 13 U.S. government departments and agencies) 12

Intergovernmental Bodies

Intergovernmental panel on climate change.

“It is unequivocal that the increase of CO 2 , methane, and nitrous oxide in the atmosphere over the industrial era is the result of human activities and that human influence is the principal driver of many changes observed across the atmosphere, ocean, cryosphere, and biosphere. “Since systematic scientific assessments began in the 1970s, the influence of human activity on the warming of the climate system has evolved from theory to established fact.” 1 3-17

Other Resources

List of worldwide scientific organizations.

The following page lists the nearly 200 worldwide scientific organizations that hold the position that climate change has been caused by human action. http://www.opr.ca.gov/facts/list-of-scientific-organizations.html

U.S. Agencies

The following page contains information on what federal agencies are doing to adapt to climate change. https://www.c2es.org/site/assets/uploads/2012/02/climate-change-adaptation-what-federal-agencies-are-doing.pdf

Technically, a “consensus” is a general agreement of opinion, but the scientific method steers us away from this to an objective framework. In science, facts or observations are explained by a hypothesis (a statement of a possible explanation for some natural phenomenon), which can then be tested and retested until it is refuted (or disproved).

As scientists gather more observations, they will build off one explanation and add details to complete the picture. Eventually, a group of hypotheses might be integrated and generalized into a scientific theory, a scientifically acceptable general principle or body of principles offered to explain phenomena.

1. K. Myers, et al, "Consensus revisited: quantifying scientific agreement on climate change and climate expertise among Earth scientists 10 years later", Environmental Research Letters Vol.16 No. 10, 104030 (20 October 2021); DOI:10.1088/1748-9326/ac2774 M. Lynas, et al, "Greater than 99% consensus on human caused climate change in the peer-reviewed scientific literature", Environmental Research Letters Vol.16 No. 11, 114005 (19 October 2021); DOI:10.1088/1748-9326/ac2966 J. Cook et al., "Consensus on consensus: a synthesis of consensus estimates on human-caused global warming", Environmental Research Letters Vol. 11 No. 4, (13 April 2016); DOI:10.1088/1748-9326/11/4/048002 J. Cook et al., "Quantifying the consensus on anthropogenic global warming in the scientific literature", Environmental Research Letters Vol. 8 No. 2, (15 May 2013); DOI:10.1088/1748-9326/8/2/024024 W. R. L. Anderegg, “Expert Credibility in Climate Change”, Proceedings of the National Academy of Sciences Vol. 107 No. 27, 12107-12109 (21 June 2010); DOI: 10.1073/pnas.1003187107 P. T. Doran & M. K. Zimmerman, "Examining the Scientific Consensus on Climate Change", Eos Transactions American Geophysical Union Vol. 90 Issue 3 (2009), 22; DOI: 10.1029/2009EO030002 N. Oreskes, “Beyond the Ivory Tower: The Scientific Consensus on Climate Change”, Science Vol. 306 no. 5702, p. 1686 (3 December 2004); DOI: 10.1126/science.1103618

2. Statement on climate change from 18 scientific associations (2009)

3. AAAS Board Statement on Climate Change (2014)

4. ACS Public Policy Statement: Climate Change (2016-2019)

5. Society Must Address the Growing Climate Crisis Now (2019)

6. Global Climate Change and Human Health (2019)

7. Climate Change: An Information Statement of the American Meteorological Society (2019)

8. American Physical Society (2021)

9. GSA Position Statement on Climate Change (2015)

10. Joint science academies' statement: Global response to climate change (2005)

11. Climate at the National Academies

12. Fourth National Climate Assessment: Volume II (2018)

13. IPCC Fifth Assessment Report, Summary for Policymakers, SPM 1.1 (2014)

14. IPCC Fifth Assessment Report, Summary for Policymakers, SPM 1 (2014)

15. IPCC Sixth Assessment Report, Working Group 1 (2021)

16. IPCC Sixth Assessment Report, Working Group 2 (2022)

17. IPCC Sixth Assessment Report, Working Group 3 (2022)

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