World of Forensic Science | 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. Often, windows are the most fragile elements of a building or a vehicle, and are thus broken by thieves or criminals in order to penetrate the premises or the vehicle. 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. Soda-lime glass is obtained by adding a certain amount of soda (Na₂CO₃) and lime (CaO). It is this glass that constitutes most windows and bottles. Borosilicate glass is made by the addition of boron oxide and is much more resistant to heat. Different colors of glass are achieved by introducing small amounts of additives. For example, chromium (Cr) is used to give a green tint, cobalt (Co) for a blue tint.

Almost all types of glass are commercially available. Window glass is probably the most common type of glass, and is usually found as a flat, transparent piece composed of soda-lime glass. This type of glass does not resist high temperatures, quick temperature changes, or corrosive substances. Most of flat glass is now prepared using the floating process. This consists of laying the molten glass onto a bath of molten tin in an inert atmosphere in order to achieve a perfectly flat surface. Tempered glass is another type of glass that is much stronger than regular glass. This particular strength is achieved by introducing extra forces on both sides of the glass through rapid cooling and heating during the manufacturing process. This glass will shatter in very small pieces when it breaks. It is used on side and rear windows of cars. Laminated glass is a glass composed of multiple sheets of glass bonded together with a plastic film such as polyvinyl butyral.

When a criminal breaks glass during a criminal act, some small particles are projected onto his/her clothing, hair, or shoes. If the suspect is apprehended within a relatively short time span after the crime, these small particles of glass can be found on the hair, clothing, shoes, or inside pockets. At the crime scene, the crime scene investigator usually collects some of the broken glass as evidence for further comparison with any glass fragments found on a suspect. The comparison process might lead to the exclusion of a common origin between the glass from the suspect and the glass from the crime scene. Conversely, it might also show that the characteristics are similar and the two samples cannot be differentiated, thus supporting the hypothesis that the two samples of glass come from the same origin. It is important to apprehend the suspect shortly after the glass was broken, because the number of glass fragments on the clothing or shoes of the suspects diminishes very quickly after the activity. About 90% of glass fragments are shed from clothing within 24 hours.

Glass 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.

The interpretation of glass is complicated by the fact that the characteristics exhibited by a large piece of glass (such as a window) might vary from one end to the other. Thus, the analyst needs to determine the extent of the intravariability (variations of characteristics within a same sample) before it can be compared to a different sample. If the variation exhibited between the two samples is greater than the variation exhibited within one sample, then the two samples can be excluded as having a common origin. On the contrary, if the two samples cannot be differentiated, then this supports the hypothesis that they have a common source. However, it does not indicate that they have the exact same common source. Again, the characteristics exhibited by the samples might be very common and found in many other pieces of glass. Thus, the analyst usually expresses his/her findings using statistics.

SEE ALSO Criminalistics; Minerals; Monochromatic light.