Interpretation of Glass Fragments
Written by Mukesh Sharma, Shailendra Jha, and V N Mathur   

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Forensic Interpretation of Glass Fragments

TRACES OF GLASS can often become a source of forensic evidence. Glass fragments are regularly encountered at crime scenes, particularly those involving motor-vehicle accidents, car theft, and burglaries. Windows are a common point of entry into buildings for burglars, and large quantities of broken glass may be produced. Such glass fragments may remain at the scene for a long time (depending on the type of crime) and do not degrade like biological evidence.
 
Physical evidence can be divided into two categories: Class characteristics are based on similar characteristics that are common within a group, but cannot be linked to a particular individual. Still, evidence with class characteristics can be useful. Recovering several kinds of evidence—such as hair, fiber, and glass fragments—may significantly increase the probability of linking a suspect or victim to a crime scene. Individual characteristics, when present, may allow the forensic examiner to link the evidence to a specific source. This type of evidence has numerous points of comparison. Each point of comparison from an “unknown source” sample that matches a “known source” sample raises the probability of the two samples having the same origin. In a case of a hit-and-run accident, where it is possible to recover the broken bits of the headlight or taillight from the scene, an investigator may be able to fit all of the pieces together. This is where individualization of the evidence is possible.
 
Glass fragments may be recovered at the crime scene in many forms: headlight glass, window glass, windshield glass, light bulb glass, or bottle glass. In addition, fragments may also be transferred to anyone present during the glass breakage, or even to someone who makes contact with the offender.
 
The forensic scientist’s role in analyzing glass fragments is to clearly and unambiguously determine the origin of the sample. Although the bulk composition of glass used for domestic windows, vehicle windows, and headlights is usually very similar within a glass type, many varied trace elements may be used to discriminate between them—even when dealing with very small fragments.
 
In this article, the purpose of fragment analysis is to evaluate the glass associated with the suspect, and to determine if that glass is or is not from the same source as the fragments found at the crime scene.
 
Methodology
 
First, glass fragments must be recovered from the suspect. This is most frequently achieved through shaking or brushing the garments. The resulting debris is observed under an optical microscope, and glass fragments can be manually separated. The physicochemical properties of the recovered fragments are then determined, often through glass refractive index measurements and other instrumental methods of elemental analysis.
 
Forensic glass examination has a well-established analytical pathway. First, glass fragments are recovered, observed, and identified by microscopy. Then, refractive index measurement of a proportion of these fragments is performed, and these measurements are compared to measurements taken from a control sample. If they match, or where otherwise appropriate, further elemental analysis of these fragments takes place.
 
A wide variety of techniques exist for the elemental analysis of glass. Most at present can be classified as: scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDX), electron microprobe analysis (EMPA), inductively coupled plasma mass spectrometry (ICP-MS), or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The fundamental steps adopted for the examination of glass as trace evidence in the forensic laboratory are summarized in the flow-chart on this page (Figure 1).
 
Refractive index measurement
 
Most forensic laboratories’ approaches to glass refractive index analysis are almost entirely uniform. By far, the majority use the GRIM

Figure 1—Procedure for analysis of glass fragments used in forensic laboratories.

Figure 2—Overlapped spectrum of three exhibits of glass. The green line in the spectrum shows the control exhibit C-1 while the violet- and red-line spectra are questioned exhibits E-1 and E-2, respectively.

Table 1—Details of glass fragments recovered.

Table 2—Glass-fragment measurement and results using GRIM3.

(or Glass Refractive Index Machine) produced by Foster + Freeman. In the cases described here, the GRIM3 model was used for analysis of the exhibits.
 
GRIM3 utilizes the oil immersion/temperature variation method for glass refractive index determination. The system utilizes a standard laboratory microscope with phase-contrast optics and a Mettler hot stage for temperature control. By varying temperature to alter the refractive index of calibrated oil, the refractive index of an immersed fragment of glass can be determined at the point of null refraction—that is, the point at which the refractive indices of glass and the immersion oil match. Using the GRIM3, repeat measurements produce results with a typical standard deviation of 0.00002RI over a 5-hour period and 0.00003RI over a 5-day period.
 
The details in Table 1 highlight three different studies, each made with the help of the GRIM3. Measurement details and results from these three cases are listed in Table 2.
 
XRF measurements
 
X-ray fluorescence (XRF) is a spectroscopic technique that is also useful in qualitative and quantitative estimation of elements in forensic samples. There are numerous types of instruments available for the measurement of XRF, but most of these are based either on wavelength dispersive spectrometers (WDX) or on energy dispersive technique X-ray fluorescence (EDX) technology. In the case described here, measurements were done by using the Lab Center XRF-1800 sequential X-ray fluorescence spectrometer from Shimadzu Corporation.
 
This case is based on an accident that occurred between a car and truck. The police had sent many glass fragments collected from the scene of occurrence to the state forensic laboratory in Jaipur, India, along with control samples from the suspect vehicles. Preliminary identification of glass fragments was made using the Foster + Freeman GRIM3. The fragments that matched best under refractive index analysis were taken for XRF analysis. Two of the samples were marked as Exhibit E-1 and Exhibit E-2, and one control sample was marked as Exhibit C-1. The investigating agency was interested to know whether the suspect’s vehicle had hit the victim’s vehicle, resulting in a fatality. In Figure 2, we demonstrated the quantitative observation of two questioned glass fragments and one control fragment.
 
It is clear from Figure 2 that the violet color and green color lines show the same peak with similar intensity. One can conclude that Exhibit E-1 has the same elements with same percentages as Exhibit C-1, the control sample.
 
Discussions
 
Vehicle windows, architectural windows, containers, headlamps, and mirror glass are some of the major sources of evidence in a large number of crime scenes. Glass examiners compare samples of glass found on suspects or found at the scene of a crime with a suspected source of known origin by measuring the physical and optical properties of color, thickness, density, and refractive index. In most cases, these have been good methods for differentiation between glasses. As quality control in the glass industry improves, physical and optical properties of glasses are more tightly controlled, making glass discrimination more difficult for forensic scientists. In addition, there is a danger in overstating the significance of a “match” when the comparison is made with a technique of low discriminating power.
 
Conclusion
 
XRF, in a forensic context, can be almost as important as the analysis itself. With the help of the flow chart of analysis of glass fragment evidence, the analyst can usually determine the type of glass the sample came from. Determining the type of glass gives investigators evidence that is considered to have class characteristics. Only in cases where the suspect fragments exactly match fragments from the crime scene is it possible to consider this type of evidence as showing individual characteristics that point to a specific source. Spectroscopy like XRF/micro-XRF, SEM-EDX is also used to analyze the elemental composition of glass. This technique is useful in determining the profile of a specific sample, and this non-destructive technique has the added benefit of preserving the sample.
 
About the Authors
 
Dr. Mukesh Sharma (M.Sc., Ph.D.), is the Senior Scientific Officer of the Physics Division of the State Forensic Science Laboratory in Jaipur, Rajasthan, India. He has published more than 54 research articles in international and national journals and conferences. His fields of research are trace-evidence analysis, forensic physics, and cyber forensics.
 
Dr. Shailendra Jha (M.Sc., Ph.D.) is the Assistant Director of the Physics Division of the State Forensic Science Laboratory in Jaipur, Rajasthan, India. He has been serving the forensic community for 27 years. He is an expert in field trace-evidence analysis, forensic physics, cyber crime, voice analysis, video authentication, and mobile forensics. He has been awarded twice the best paper awards in the All India Forensic Science Conference (2008 and 2009).
 
Mr. V N Mathur (M.Sc., DFB, extensive training in DNA profiling) has served as the Director of the State Forensic Science Laboratory in Jaipur, Rajasthan, India since March 2009. He is the most experienced scientist in the field of forensic biology, serology, and DNA profiling in Rajasthan. In 2009, he was awarded the DFS Meritous Award for Forensic Science—2008 from the Directorate of Forensic Science, Ministry of Home Affairs, New Delhi. He has been serving the forensic community for 38 years.
 
To reach any of the authors listed here, send an e-mail to This e-mail address is being protected from spam bots, you need JavaScript enabled to view it .
 
 

 

 
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