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What is casting – in forensic science
admin to Forensic Science  



A technique to preserve and replicate impression evidence made in soft material such as soil or snow. Casts can be made of tire prints, shoe prints, and occasionally toolmarks and bite marks. When done properly, casting techniques produce excellent positive replicates of the impression, but they are not exact duplicates. For example, plaster of paris casts tend to shrink and will be smaller than the original impression. Thus, it is always preferable to transfer the entire impression to the laboratory. However, in many cases, the impression evidence is in materials such as snow or mud that cannot be preserved or removed for transport. Similarly, bite marks in human flesh or perishable food will not last, and casts are the best way to record the impressions for detailed study.

A key component of casting is a detailed series of photographs taken of the impression from many angles, using different lighting and clear indication of scale. This documentation is essential for studying the original impression in place and for comparing it to the cast made. For impressions in soil such as tire prints and shoeprints, dental stone is the preferred material for casting, although plaster of paris and sulfur have also been used. After photography is complete, the next step is to insert or construct a retaining frame around the impression that will contain the casting liquid. Before the liquid is poured, the impression is usually sprayed with a lacquer material to prevent the weight of the casting material from pressing down and altering features. The cast is poured and allowed to dry for about half an hour before it is lifted out. Twenty-four to 48 hours is needed for the cast to dry completely, and it must be carefully protected during transport to prevent damage or alteration. Foot and tire impressions in snow are a special challenge requiring a different approach since casting media gives off heat as it sets (an exothermic reaction), which would cause the snow to melt and alter the impression. Specialty products such as Snow Print Wax are used in such cases.

Bite marks are another type of impression amenable to casting. For bite marks in skin, an additional consideration is the possible presence of saliva, so the area is swabbed before the casting. The cast is made of silicon or other rubber casting material, usually after the area is photographed and traced to provide additional information and documentation. For bite marks in food, the type of food must be considered; the approach to casting a bite mark in a chocolate bar (water soluble and easily melted) is different from casting a bite mark in an apple, for example.