The Evidential Value of Glass Fragments

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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:

Silica (Sand) - Sand is heated by intense temperatures to form a fragile glass.
Soda Ash (Sodium Carbonate) - Added to the mixture to lower the melting temperature of the sand.

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Soda Ash comes in the form of a white powder.
Limestone (Calcium Carbonate) - Upon addition strengthens the glass.
Glass can also come in many different types some include:
Float.
Toughened.
Patterned.
Wire re-inforced.
Laminated.
Container.

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'.

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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.

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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:

Time - The longer the time after the incident the less likely it is to find glass fragments on the suspect.
Washing - If clothes are washed then the samples on outer clothing and lower clothing with probably be lost but in some cases glass can remain in the pockets and turn-ups after cleaning.
Activity - If a person is very active then a lot of glass fragments will be lost early on after the incident but if they have rested after the incident then most particles of glass should persist.
Texture - Varying materials will depend on how well glass fragments can persist upon them. If the suspect wore a thick woollen jumper it would be the case that more glass fragments were present than what would be on a shell suit.

But, there are many more influences that effect whether glass fragment would be embedded in clothing or how long they would persist these include:

Window size - Area of damage relates to number of fragments embedded by backward fragmentation rather than window size.
Glass thickness and type - The harder the glass the more difficult to break therefore the more fragments produced upon breaking.
How was the window broken (i.e. one blow or multiple blows) - Multiple blow breakage yield more fragments while a heavier weight at fast speed gives more fragments than a small weight at low speed.
The offenders position with regard to the window - The nearer the person is to the window the more glass that will embed in and on their clothes due to backward fragmentation. If the point of breaking is above the waist then it is highly likely that glass fragments will embed in the upper clothing, hair and shoe uppers
Whether the premises were entered or not - If entry was gained then it is more likely that glass will be found on clothing as the suspect enters the premises as well as the possibility of fibre transfer.
Method by which the suspects clothing was recovered - if no clean brown paper was laid before clothes were removed then some fragments may be lost.
Weather at time of incident - There is no official information but it could be assumed that wet clothing would be more retentive than dry clothing.

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:

Six pieces, from the broken area, should be collected.
If many panes or items are broken six pieces should be recovered for each item.
All recovered glass should be of the same thickness as the original source and represent of all the glass present.
All glass collected should be obtained from the source (i.e. window frame) itself and not from the ground or surrounding area, if this is not possible are collected samples should be labelled accordingly.
The glass should be labelled, using a permanent marker, whether it is the inside or the outside face of the pane.
If known, a sketch should be drawn of the height and size of the glass and whether it was broken in or out.
Upon retrieval of all control samples storage should be as follows:
Samples of glass from different sources should be package separately and given different identification numbers. The number of sources should be written on the submission form.
Samples should be placed in a polythene bag and then placed in a hard-wearing, and sealed, box from which no glass can obtrude.
To prevent injury the samples should be sealed as soon as possible.
Boxes for glass storage are available from the Forensic Science Service (FSS)

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:

Old stained-glass windows (High Evidential Value).
Older style windows (Medium Evidential Value).
New float glass windows (Low Evidential Value).

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:

Hair Combings/Shoe Uppers
Outer Clothing
Lower Clothing
Pockets/Turn-ups
Soles of shoes

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.