As a type of evidence glass can be very useful contact trace material in a wide range of offences including burglaries and robberies, hit-and-run accidents, murders, assaults, ram-raids, criminal damage and thefts of and from motor vehicles. All of these offer the potential for glass fragments to be transferred from anything made of glass which breaks, to whoever or whatever was responsible. Variation in manufacture of glass allows considerable discrimination even with tiny fragments.
Research has indicated that 60% of cases involving glass provided some positive evidence and that in 40% of these cases this evidence was strong. Depending on circumstances, the findings can also refute the allegation that a person was involved in a crime.
For example, when a pane of glass is broken, minute glass fragments can be showered onto the hair, clothing and footwear of people in close proximity – at least 1.5 and possibly up to 3 metres away. The number of fragments transferred decreases rapidly with distance from the breaking pane. Aside from backwards projection of fragments towards the ‘breaker’, fragments can also be acquired, for example, by climbing through a broken window or treading on pieces of broken glass.
Fragments of glass recovered from hair and clothing are generally in the range 0.25 – 1 millimeter (1mm = 0.039″). Most glass is lost fairly rapidly – depending on the activity of the wearer and the texture of their clothing. For example, a woollen jumper will tend to retain glass fragments for far longer than a leather jacket, although fragments can be trapped in pockets, in crevices on shoe uppers and remain embedded in shoe soles for long periods of time.
The results of surveys of the numbers and types of glass fragments found on the clothing of people selected at random shows that it is unusual to find more than a few fragments of glass from the same source on someone’s clothing purely by chance.
Discriminating between different glasses
By and large forensic scientists are interested only in freshly broken glass (clean and sharp-edged fragments) recovered from hair combings or the surfaces of garments, i.e. glass which could have been broken and/or acquired recently. These will be compared and contrasted with reference samples of broken glass, e.g. from the scene, by way of refractive index measurements (a measure of the degree to which the glass bends light) and chemical analysis tests. Refractive Index (RI) is a very accurate test which can distinguish between a large number of different glasses. If the recovered fragments differ in refractive index from a reference glass sample, then they could not have originated from the same piece of glass. Chemical analysis provides detailed information about the chemical constituents of the glass i.e. from the sand and other materials used in its manufacture. Chemical analysis, using, for example, a scanning electron microscope, can be used to distinguish between glass samples which have the same refractive index but different chemical composition.
In addition, the surfaces of glass fragments will be microscopically examined for evidence of the method of manufacture and the type of object from which they came, for example, flat glass and patterned glass (both from window panes) or curved glass (drinking glass or bottle). Further tests may be performed to see if the source was of toughened glass (forming small cubes when broken and typically found in some car windows and in door and window glazing).
Interpreting what’s found
If the recovered fragments match the reference sample both in RI and chemical composition it is necessary then to discover how rare or otherwise the glass might be. To do this the scientist may consult a computer database containing the combined results of the RI measurements and the chemical analysts results for each and every reference glass sample examined by his lab. This will show how many times a particular type of glass has been encountered, but this is not necessarily a reliable indication of how common it might be. This is because most glass samples submitted to forensic laboratories are from broken windows, although research has shown that glass found on clothing by chance is more likely to have originated from a container (such as a glass or a bottle). Aside from this, there tend to be local pockets of specific types of glass, e.g. from buildings all built at the same time. All this means that information held on the database might be skewed and should be treated with caution.
Aside from databases, the forensic scientist will also need to consider for example, the number and distribution of the glass fragments he has recovered and whether this fits in with the alleged circumstances.
The finding of numerous fragments of glass on a suspect detained within a few hours of windows being broken may be very strong evidence, for example if the glass is unusual and/or there is more than one type of matching glass. On the other hand, less weight would be given to the presence of one or two fragments of a common type of glass found on clothing seized several days after an incident.
Finally, it is often suggested that fragments might be transferred between people who come into contact after the crime has been committed, for example when they travelled side-by-side in a police vehicle. Whilst in theory this is quite conceivable, the results of tests show that only one or two fragments at most are likely to be transferred in this way and only a few remain on the seat occupied by the contaminated person.