Casting

Casting is the process used to replicate three-dimensional prints or marks. It is widely used to obtain the exact replicate of toolmarks, tire tracks, shoeprints, and sometimes teeth. Casting is of paramount importance in forensic sciences as it allows a crime scene investigator to collect an identical copy of a mark or print from a scene, which can then be compared to a seized tool, shoe, or tire in order to establish a link between a suspect and a crime scene.

Casting can only be accomplished on three-dimensional marks or traces. In the case of tool-marks, for example, casting can be used to obtain the perfect copy of the mark of a screwdriver used to force open a door during a burglary. With a shoeprint, it allows for the shoeprint of a thief that was left in the soil outside the window of the apartment he or she exited to be preserved as evidence. A vehicle used to flee the scene of a murder could leave tire tracks in the snow, which can be recorded and saved for later comparison with a suspicious vehicle. Casting is also used to record dental characteristics of a body and compare these characteristics with known dental records in order to make a proper identification.

The choice of casting material depends on the mark to be copied and the surface on which it is found. For most toolmarks, a dental cast polymer is used. It consists of two pastes mixed together right before the cast is taken. Once mixed together, the paste is applied onto the mark and allowed to dry before being removed. With tire tracks or shoeprints, usually a plaster, such as plaster of Paris, is used. This kind of casting material does not provide as many details as the dental polymer, but can cover a bigger surface and will dry very well over surfaces such as soil. On snow, the use of plaster is not ideal, and molten sulfur offers a much better cast. Sulfur is heated until it liquefied and then poured onto the trace. As soon as the sulfur touches the cold snow, it immediately hardens and takes the shape of the print.

SEE ALSO Crime scene investigation; Microscope, comparison.