Glass is one of the most common and important materials submitted for forensic trace evidence analysis
Glass is frequently encountered at crime scenes, particularly those involving motor vehicle accidents, car theft and burglaries. Glass fragments that may remain on clothing for a long time are very stable. They don't degrade like biological evidence and don't alter over time.
Different methods of production and variations in composition of the raw materials cause differences in the physical and chemical properties of glass. By using sensitive and highly discriminating techniques, even two seemingly identical glass sources may be distinguished from each other.
When glass is broken, it produces many small fragments. These small fragments can be transferred to the clothing, hair or footwear of anyone in the vicinity of the glass as it breaks. Studies have shown that any fragments transferred tend to be lost from clothing with the passage of time as a consequence of the activity of the wearer and that the majority of any fragments transferred are normally lost within a matter of a few hours under normal circumstances.
Microscopic fragments of glass can be recovered at the laboratory from items of clothing, footwear or hair combings of a suspect. Such fragments can also be recovered from implements e.g. baseball bat that may have been involved. Glass embedded in such items will indicate the forceful nature of the contact of that item with the breaking or broken glass.
As glass is typically a transparent material, its refractive index can be measured using the Glass Refractive Index Measurement (GRIM) system machines, even though the fragments are often microscopic. This, in combination with the chemical composition of the glass, its thermal history, and any surface characteristics, provides a sensitive means of distinguishing these fragments from glass fragments from other sources.
As well as undertaking the range of available instrumental tests, if fragments of glass are large enough, it may be possible to fit the glass back, jigsaw fashion, to recreate its suspected source. For example, the piece of glass from a broken headlamp left at the scene of a road traffic collision can be linked to the broken headlamp of a vehicle suspected to be involved in the incident.
Markings on the edge of pieces of broken glass can give indications of how the glass was broken, for example, by thermal or physical stress and high or low velocity impact.
Due to the versatility of glass and its wide range of applications, the transfer of glass fragments can provide an effective means of linking an item, vehicle or suspect to a wide range of different incidents, from assaults to traffic offences, such as fail to stop collisions, burglaries or thefts from vehicles.
The number of glass fragments found and the location of the fragments can be a very useful means of indicating when the contact with the source of glass occurred and the extent of the contact in question.
When and how a window was broken can also be a critical consideration in the investigation of a range of incidents. For example was the window broken before the fire? How did it break? Was the damage caused by heat or was it smashed to gain entry?
Here at Cellmark, examinations are carried out in purpose-built laboratories following UKAS accredited procedures. State-of-the-art laboratory air flow management allows examinations for trace evidence to be undertaken with absolute confidence. The refractive indices of glass fragments are determined by skilled operatives using GRIM3. Surface characteristics are compared and re-annealing of fragments can determine whether they have originated from a source of toughened glass. The chemical composition of glass fragments is determined by SEM/EDX in partnership with Oxford Materials Characterisation Service.