Glass as evidence
Glass as evidence refers to the use of glass fragments found at crime scenes to establish connections to potential sources or incidents. Forensic scientists analyze these fragments, which can range from tiny pieces embedded in clothing to larger shards, utilizing various physical and chemical comparison methods. Two key techniques are density determination and refractive index measurement. In density testing, glass fragments are compared in liquid columns to see if they float at the same level, indicating similar density. For assessing refractive index, fragments are immersed in oils to find a match point, which helps determine their identity.
Advanced technologies, such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and micro X-ray fluorescence spectrometry (µ-XRF), enhance the analysis by allowing scientists to perform detailed elemental and isotopic analysis. Additionally, forensic experts may examine glass fracture patterns to discern the direction and sequence of impacts, providing further insights into the circumstances surrounding a crime. Overall, glass fragments can serve as important evidence, although they may not definitively link a suspect to a particular source.
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Subject Terms
Glass as evidence
DEFINITION: Hard, brittle substance, typically consisting of a mixture of silicates, that is neither liquid nor solid but instead exists in an amorphous state.
SIGNIFICANCE: When law-enforcement investigators find fragments of broken glass at crime scenes, forensic scientists can often link the fragments to their sources through comparisons of the physical and chemical characteristics of the glass.
When glass breaks, fragments of the material shower other objects and people in the vicinity, and often tiny pieces of glass become embedded in clothing, shoes, and other objects. When glass fragments are found at a crime scene, forensic scientists can collect them and compare them with the various sources of glass at the scene. Such analysis consists mainly of the comparison of density and refractive index. Although these comparisons can link the fragments to a particular kind of source, they are not sufficient to link the fragments to one source to the exclusion of all other sources.
For density determination, two thin glass columns are filled with a liquid mixture. The glass fragment of interest is placed in one column, and a similarly sized piece of possible source glass is placed in the second. Each piece of glass will float at the level where the density of the liquid is equal to the density of the glass. Therefore, the two fragments will float at the same level if they have the same density. If the fragments cannot be distinguished by density, the refractive index can be determined.
For refractive index determinations, the glass fragment is immersed in an oil of known refractive index and viewed through a microscope. A bright halo, known as the Becke line, is observed around the fragment. As the microscope stage is lowered, the Becke line will move into the medium (oil or glass) that has the higher refractive index. This procedure is repeated with the glass immersed in different refractive index oils until the match point is determined. At that point, the Becke line disappears, indicating that the oil and the glass have the same refractive index. The procedure is repeated for a piece of the possible source glass, and then the refractive indices of the two pieces of glass are compared. Automated methods now exist for refractive index determinations in which the oil is heated using a microscope equipped with a hot stage. The refractive index of the oil changes on heating, although the refractive index of the glass is unaffected. Heating is stopped when the match point is reached and the refractive index of the glass can be determined.
Two technologies commonly used in forensic glass analysis are laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and micro X-ray fluorescence spectrometry (µ-XRF). LA-ICP-MS, which allows investigators to perform highly sensitive elemental and isotopic analysis directly on solid samples, typically yields the most reliable quantitative data. One of the most frequently used technologies in forensic glass analysis, µ-XRF allows investigators to distinguish between different samples by comparing their element intensity ratios.
In some investigations, forensic scientists also analyze glass fracture patterns to determine the direction and sequence of impacts. With low impact, a projectile bounces off a pane of glass and falls on the side of impact. With high impact, the projectile creates a hole and passes through the glass. The hole is smaller in diameter on the entrance side than it is on the exit side. Cracks also radiate from each point of impact. Because radial cracks from secondary and subsequent points of impact always terminate at preexisting cracks, the sequence of impacts can be determined.
Bibliography
Caddy, Brian, ed. Forensic Examination of Glass and Paint: Analysis and Interpretation. New York: Taylor & Francis, 2001.
Green, David A. "Glass Analysis." American Society of Trace Evidence Examiners, 2024, www.asteetrace.org/glass. Accessed 15 Aug. 2024.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.
Trejos, Tatiana. "Forensic Glass Examinations - A Review Focused on Elemental Spectrochemical Analysis." WIRE's Forensic Science, vol. 5, no. 2, 19 Dec. 2022, doi.org/10.1002/wfs2.1476. Accessed 15 Aug. 2024.