January 30, 2011
I have been asked to examine forensic evidence as it relates to glass in criminal investigations. I will examine how investigators identify, collect (handle), evaluate, and compare and contrast glass as it pertains to a criminal investigation.
First, we will examine the history of glass. Glass is a product of inorganic materials that solidified, but did not crystallize. Glass is mainly composed of silicon dioxide (SiO2), and is extremely prevalent in everyday life. Usually, windows are the most fragile elements of a building or a vehicle, and thereby are broken by thieves or criminals in order to penetrate the premises or the vehicle. For example, when glass breaks at the scene of a crime, small particles of glass are projected not only forward, but also backward, onto the perpetrator and into the immediate environment. These particles can later be retrieved and used to establish a link between a suspect and a crime scene. Glass can be classified either by chemical composition or by use. There are four main chemical compositions of glass: soda-lime, lead, borosilicate, and special glass. While glass is mainly composed of silicon dioxide (SiO2), it also contains modifiers that are used to vary the quality and properties of the glass.
Second, in identifying glass it is characterized according to its physical and chemical characteristics. When investigating glass, the first examination is visual. The investigator observes its color, its thickness (if the fragments are big enough), its patterns, and its fluorescing (light-emitting) properties. Pieces of the glass can often be reassembled, revealing patterns that can be compared to crime scene samples. Demonstration of origin by assembly is the only way the common origin between two fragments of glass can be clearly established. The refractive index of the glass fragments is then measured. This is typically achieved by immersion of the fragment in oil and observing the lines of refraction at different temperatures. Finally, elemental composition of the glass is determined.
Third, when glass is being collected investigators take careful steps in preserving the evidence in order to analysis it and maintain its continuity. For example, if an individual breaks a window or has contact with broken glass, fragments can be transferred to their clothing or person. Also, clothing, hair, tools or instruments can be searched for the presence of glass fragments. The forensic scientists can analysis the glass fragments to determine whether they are similar to the fragments found at the crime scene. Take this case scenario, in the case of a house breaking, broken glass on the suspects clothing would be compared with glass from the broken window at the crime scene, referred to as the control sample.
The glass can be collected and examined in the following ways:
Scanning Electron Microscope (SEM)
Physical matching remains the most definitive means of establishing a common origin between any samples. If the forensic scientist can fit the two samples of glass together to form one item, then they can determine that the samples have only originated from that one item and nowhere else. The forensic scientist will also look at the physical properties of the recovered glass such as its color and thickness. One of the characteristics of glass is the degree to which light is refracted, or how it bends, as it enters the glass. This is measured using an instrument called GRIM (Glass Refractive Index Measurement). Each type of glass will refract the light differently and will have its own refractive index. If two pieces of glass have the same refractive index, the forensic scientist can conclude that the glass originated from the same source. The Scanning Electron Microscope is used to analysis the elemental composition of the glass.
Fourth, when evaluating glass its interpretation