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The Evidential Value of Glass Fragments.

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The Evidential Value of Glass Fragments 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. 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'. 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) ...read more.


> 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. ...read more.


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. ...read more.

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