Tempered glass and laminated glass do not behave in the ways described above. Tempered glass breaks into small squares when it is broken. Laminated glass, used in windshields and side windows in vehicles, has a layer of plastic embedded between two sheets of glass (Innes, 2002).
Glass fragments as Evidence
Physical evidence can broken be down into two general categories. There are types of physical evidence which can be considered have “class” characteristics. Evidence with class characteristics can be said to have similar characteristics within a group but can not be linked to a particular individual. An example of evidence which has class characteristics is as blood types (Saferstein, 2004). All blood types exist in any given population. The blood type itself only indicates the percentage of people in a population from which the sample could have come from based on the ratio of occurrence in that population. It does not tell an investigator what particular individual the sample came from.
Still evidence with class characteristics can be useful. Several different kinds of evidence such as hair, blood type and glass fragments together may significantly raise the probability of linking a suspect or victim to a crime scene (Saferstein, 2004)( Crocker, 1999).
The second type of evidence is that which exhibits ‘individual’ characteristics. This type of evidence has numerous points of comparison. Each point of comparison from an ‘unknown source’ which matches a ‘known source’ sample raises the probability of the two samples having the same origin (Saferstein, 2004). In a case of a hit and run where it is possible to recover the headlight or taillight and the broken bits from the scene an investigator may be able to fit all the pieces together. This situation is where individualization of the evidence is possible.
Glass comes in many types: headlight glass, window glass, ophthalmic glass, windshield glass, light bulb glass and many more. When fragments of glass are found during an investigation they may be analyzed by testing the density and refractive index of the sample.
The density is determined by weighing the sample and then measuring the volume. A formula of: Density = mass/volume is used to find the density.
The refractive index is the number assigned to a sample of glass which reflects the bending of light as it passes through a sample. An automated system known as the Grim 2 is used to calculate the refractive index of a glass sample.
From these values the analyst can usually determine the type of glass the sample came from. Determining the type of glass gives investigators evidence which is considered to have ‘class’ characteristics.
Only in cases where the suspect fragments exactly match fragments from the crime scene is it possible to consider this type of evidence as showing ‘individual’ characteristics which point to a specific source. The likelihood that all of the fragments from both locations will be found and fit together is extremely remote (Saferstein, 2004).
Spectrography is also used to analyze the composition of glass. This technique is useful in determining the profile of a specific sample but is destructive in that is requires heating of the sample to the melting point (Saferstein, 2004)
Two men leave the local bar at 2 a.m.. A few blocks away the driver of their vehicle looses control of the vehicle and slams into an oncoming car. The occupant of the other car is killed. Both men survive the crash. By the time the police arrive both men have exited the vehicle and are found sitting on the curb next to the wreckage. Both men are injured and neither remembers who was driving. The police notice a pair of shattered eye glasses in the first man’s hand. The vehicle is processed and the glass fragments are collected from the vehicle. Each set of samples are labeled as to the location in the vehicle it was found.