As a crime scene examiner you are on many occasions going to find glass when approaching a crime scene.
It may be a burglary where a person has broken a window to gain entry or egress, a car taken without owner’s consent or even a murder scene where a victim has been bludgeoned by a glass bottle. In these cases glass may be the deciding factor in whether a conviction can occur or whether a suspect can be linked to a crime scene.
But what exactly is glass? Glass is a multi-form material, depending on the way it was manufactured and its chemical composition, and is produced from three main components:
But, how can glass be used to link a suspect to a crime scene? Firstly we look at Locards principle that states ‘when A comes into contact with B something from A is transferred to B and vice versa’. This is termed two way transfer as it links the person to the scene but also links the scene to the person, an example of this would be a burglary by which a fibre from the criminals clothing has been left on a window sill (links person to scene) but also some glass from the broken window is found on the shoe uppers of the criminal (linking the scene to the person).
When glass is broken there are two types of breaks: Radial and Concentric.
The radial crack is the first thing to occur upon impact and as the crack radiates through the glass concentric cracks appear in the opposite direction of the radial crack creating a phenomenon known as backward fragmentation by which small fragments of glass are thrown back towards the window breaker. When this occurs some fragments may become embedded in the clothing, hair, shoe uppers, shoe soles or even in the pockets/turn-ups of the window breaker. These glass fragment that may be embedded upon the suspect are retrieved and analysed in the forensic laboratory to discover if the glass present on the suspect is of the same refractive index, density, thickness and colour as that of the glass at the scene.
Refractive index is a property that regulates how a beam of light bends when it passes through a medium, in this case glass. To express the refractive index as an equation we look at the following diagram:
From the diagram we see that 1 is the angle of incidence while 2 is the angle of refraction and by using Snell’s Law we can equate the refractive index to be:
RI = (sin i / sin r) = (Vair / Vglass)
Before any analysis can occur the glass must be recovered in the appropriate manner and in accordance with the scenes of crime handbook. The time of the incident with regard to the amount of glass possibly left on the suspect for recovery is a very important factor to consider as within a couple of hours most of the pieces will probably have been lost, while within one hour it is possible all glass in the hair will have gone. The retention factors said to affect glass recovery are:
But, there are many more influences that effect whether glass fragment would be embedded in clothing or how long they would persist these include:
Having realised all these factors that affect glass retention great care should be taken upon retrieval of any samples. Control samples should be retrieved so that we have a sample of the original glass for analytical comparison. Before recovering the control glass it must be taken into account that there may be a possibility that a blood stain or footwear mark is present. In the event of blood being present this should be swabbed or scraped and under no circumstances should a footwear lift be attempted, if possible the whole item should be recovered. When recovering control glass samples:
Now that all control samples are recovered, retrieval of suspect glass samples can begin. When retrieving from a suspect all recovery techniques should be performed over a piece of large brown paper in case any loose fragments fall to the ground. To start head hair combings should be taken using a fine, clean tooth comb (available from FSS) and upon retrieval should be placed in a paper wrap, with the comb, and sealed in a tamper evident bag. The sample should then be stored in a cool, dry environment.
After head hair combings clothing should be recovered. The clothes should be removed over the brown paper and kept in the dry store. If it is required to determine the direction of the breaking force and/or perform a physical fit, all glass should be recovered and labelled in or out and packaged in a sturdy box.
With all samples recovered it is now possible to begin the analysis.
When analysing glass the methods that can be used include chemical analysis, annealing (toughened glass only) and the most popular process used is Glass Refractive Index Measurement (GRIM). It should be taken into account that refractive index is influenced by the chemical components present and the cooling process from manufacture, in this case it could be possible to find pieces of glass from the same pane that may have different refractive index values. GRIM is an example of phase contrast microscopy in which a piece of glass is immersed in silicone oil. Using a microscope, upon immersion in silicone oil (of known refractive index), and a light source the glass is examined and on the edges of the glass a corona effect is observed.
As the temperature of the silicone oil is higher than the match temperature of the glass the sample is visible, but as the temperature is lowered using a hot stage the glass sample become less visible until it reaches the match temperature where the glass disappears from view. The value for match temperature is then, using a calibration line calculated by known standards, converted into a refractive index measurement. All the glass has now been collected and analysed.
If the control glass sample has been analysed to give the same refractive index value as that of the suspects sample it can be said that the glass both came from the same sources which therefore links that person to the scene via one way transfer but if a fibre or bloodspot left by the suspect also yields the same result this corroborates the result of the glass analysis and links the scene to the suspect as well. This would be regarded as high evidential value in a court of law.
The evidential value of glass can vary considerably with regard to the type of glass present and the location of the embedded glass with regards to the suspect. The different types of glass factor is important because you are less likely to find old stained-glass window fragments than plain new float glass windows:
It must be take into account that, for example, a block of council flats will be glazed all at once using the same manufacturer and glazier therefore many flats will have glass of almost exactly the same refractive index. While, the position of the embedded glass fragment can be listed in evidential value, highest to lowest, as follows:
Therefore from looking at the position of the embedded glass, the type of glass and the factors affecting the retention of glass fragments it can be stated that the evidential value of glass is dependent on a number of factors which greatly affect its value as evidence in a court of law. Glass should be regarded as high value supportive evidence but never as conclusive evidence.