Contributed by David A. Green
Glass can be found in most localities. It is produced in a wide variety of forms and compositions, and these affect the properties for this material. It can occur as evidence when it is broken during the commission of a crime. Broken glass fragments ranging in size from large pieces to tiny shards may be transferred to and retained by nearby persons or objects. The mere presence of fragment of glass on the clothing or alleged burglar in a case involving entry through a broken window may be significant evidence if fragments are detected. The significance of such evidence will be enhanced if the fragments are determined to be indistinguishable in all measure properties from the broken window.
The raw materials for glass manufacturing are first mixed together to form a batch and then melted in a furnace to produce a liquid. Most modern, commercially produced glass is manufactured in a nonstop process wherein raw materials are fed continuously into one end of a melting tank and liquid glass is drawn from the other end. The composition of the glass changes gradually as more raw materials are added (Arbab 2005).
Although modern glass manufacturing is a highly automated process that produces glass with large scale uniformity, minor variations in the properties of the resulting glass remain. Each of the raw materials used to produce glass contains impurities that are uncontrolled by the manufacturers and consequently vary in amount and composition over time. The mixing of raw materials during batching is incomplete, and the batch will unmix during transport and delivery to the furnace. Some mixing occurs as the molten glass flows through the furnace, but it is not sufficient to make an absolutely uniform product. The refractory material lining the glass furnace is gradually eroded into the glass melt over the lifetime of the furnace. These factors result in glass products with small but measurable variation in their chemical, optical and physical properties both within and between production runs (Koon et al. 2002).
There are several measurable characteristics that may be available for laboratory analysis and comparison between and known and questioned sample. The examiner may attempt to determine if two pieces of glass are a physical match. Only when two or more broken glass fragments physically fit together can it be said that they were once part of the same object.
An examiner may screen through glass samples by comparing color, fluorescence, thickness surface features and curvature of the glass samples. All of these techniques are non-destructive and may produce results sufficient to reach an exclusion without conducting any additional examinations.
An examiner can also measure the optical properties exhibited by a glass fragment. Refractive index is the most commonly measured property in the forensic examination of glass fragments because: (1) precise refractive indices can be measured rapidly on the small fragments typically found in casework (2) It can aid in the characterization of glass and (3) it provides good discrimination potential (Koons et al. 2002).
While there are several methods that can measure this property, the forensic community primarily uses an automated method using a phase-contrast microscope, hot stage and monochromatic light source. ASTM E1967-11a Standard Test Method for the Automated Determination of Refractive Index of Glass Samples Using the Oil Immersion Method and a Phase Contrast Microscope has been published as a standard for this method. A second, non-automated technique that is utilized by the forensic community is published by the Association of Official Analytical Chemists (AOAC) Method 973.65 which uses a monochromator and hot stage to allow for variation of temperature and wavelength simultaneously.
As previously stated, concentration of trace elements during the manufacturing process may allow for very good discrimination between two glass samples. Many methods have been used for elemental analysis of glass which include scanning electron microscopy-energy dispersive spectrometry and X-Ray Fluorescence Spectroscopy. ASTM E2926-13 Standard Test Method for Forensic Comparison of Glass Using Micro X-ray Fluorescence (u-XRF) Spectrometry has recently been published.
The most discriminating analytical method for glass fragment comparisons can be accomplished utilizing inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS). Currently there are two ASTM standards with regard to ICP-MS analysis, ASTM E2330-12 Standard Test Method for Determination of Concentrations of Elements in Glass Samples Using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for Forensic Comparisons and ASTM E2927-13 Standard Test Method for Determination of Trace Elements in Soda-Lime Glass Samples Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry for Forensic Comparisons.
When no differences within the limits of the analytical techniques of the assessed properties can be identified, the possibility that the glass fragments originated from the same source cannot be eliminated.
In addition to above listed ASTM standards for glass analysis, the Scientific Working Group for Material Analysis (SWGMAT) has produced guideline documents for glass analysis, which include, Elemental Analysis of Glass, Glass Density Determination, Glass Fractures, Glass Refractive Index Determination, and Initial Examinations of Glass, all of which can be found at the FBI online publication Forensic Science Communications.
1. Arbab, M., Shelestak, L. J., and Harris, C. S. Value-added flat-glass products for the building, transportation markets, part 1, American Ceramic Society Bulletin (2005) 84:30–35.
2. Bottrell, M. Forensic Glass Comparison: Background Information Used in Data Interpretation, Forensic Science Communication [Online]. (April 2009). Available: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review/2009_04_review01.htm/.
3. Koons, R. D., Buscaglia, J., Bottrell, M., and Miller, E. T. Forensic glass comparisons. In: Forensic Science Handbook. Vol. I, 2nd ed. Richard Saferstein, Ed., Prentice Hall, Upper Saddle River, New Jersey, 2002, pp. 161–213.
4. Scientific Working Group for Materials Analysis (SWGMAT). Elemental analysis of glass, Forensic Science Communications [Online]. (January 2005a). Available:
5. Scientific Working Group for Materials Analysis (SWGMAT). Glass density determination, Forensic Science Communications [Online]. (January 2005b). Available:
6. Scientific Working Group for Materials Analysis (SWGMAT). Glass fractures, Forensic Science Communications [Online]. (January 2005c). Available:
7. Scientific Working Group for Materials Analysis (SWGMAT). Glass refractive index determination, Forensic Science Communications [Online]. (January 2005d). Available:
8. Scientific Working Group for Materials Analysis (SWGMAT). Initial examination of glass, Forensic Science Communications. Available: