Due to its fragile nature and abundance, glass is one of the major types of trace evidence encountered in crime scenes such as burglaries, car accidents, hit and runs, assaults, drive by shootings and bombings.
Trace evidence such as glass usually exhibit class characteristics and therefore the value of evidence depends on the discrimination power of the techniques used for its comparison. The value of evidence can also be enhanced by cross transfer of evidence between victim and suspect, multiple transfer of evidence and rarity of the characteristics of the evidence.
Glass is an inorganic product of fusion, cooled to a rigid condition. It is essentially composed by heating a mix of sand (silicon dioxide, SiO2), limestone (calcium carbonate), and soda (sodium carbonate), along with various impurities. Different types of glass are produced in different ways, and the way in which glass is made causes it to behave variously.
Due to the nature of glass and how it behaves, it may be possible to determine whether two pieces of glass originated from the same source. The density and refractive index of glass can both be used to distinguish between fragments. The density is determined by weighing samples and measuring the volume, using the formula density = mass / volume. The density of two shards of glass can also be determined by suspending the glass in liquid to work out its density.
Pieces of broken glass can be physically reconstructed to form the original object or pane. By studying this glass, the investigator can determine which side of the glass the impact came from. When a projectile hits the surface of a glass pane, the glass bends slightly before breaking. This causes radial fractures to occur on the side of the glass opposite the point of impact. Tiny shards of glass will often be found on the clothes of the perpetrator, or anyone else standing in the vicinity when the glass was smashed. Whereas most glass will fall inwards with the impact, fragments will spray in the opposite direction, known as backscatter.