Common cutting instruments include knives, bolt croppers and drills, with screwdrivers and jemmies being common levering tools. Common instruments encountered fall into two categories cutting instruments and levering instruments. Instruments and tools used during a crime will often leave marks behind at the scene, which may prove beneficial in establishing links between a particular object and the scene. If a tyre impression is discovered at a scene the impression corresponding to the opposite tyre should also be searched for, as the distance between these may provide further information regarding the vehicle in question. The enhancement and collection of these is similar to that of footwear impressions. The size of the shoe, which may easily be obtained by examining the recovered impression, may prove useful, though not as a positive identifier.Īs vehicles may be present at crime scenes, before, during or after the crime, tyre impressions may be discovered at the scene, usually left behind in soil. As a shoe is worn certain details fade in different places, depending on the weight and walk of the wearer, and specific damages may be caused. Though these patterns are identical for the same brand and type of shoe, a certain degree of individuality may be imparted from the manufacturing process or general wear.
In some locations such patterns have been stored in databases for comparison purposes. Almost all items of footwear will bear an undersole with distinctive patterns, which manufacturers are increasingly designing to be specific to them. If a three-dimensional impression is to be obtained, it should, if possible, be produced using the same methods and mediums as the original impression.Įven if no other samples are available for comparison, a recovered shoe impression may yield a vast amount of information. Alternatively the undersole is oiled and pressed onto plain white paper, which is then dusted with fine black powder similar to that used to develop latent prints. By applying a film of light oil to the undersole of a shoe and pressing it into a sheet of oil-impregnated foam rubber, a test impression can be produced. This is boiled to produce a hot casting compound which, upon contact with the cold snow, solidifies to produce a detailed cast.Īny footwear impressions collected from the crime scene may be useless unless there are suspect samples available for comparison. Alternatively flour sulphur may be used to cast snow prints. The impression can then be cast as normal. This is applied to the impression numerous times at intervals of one to two minutes and then left to dry.
In this instance aerosol products exist, such as Snow Impression Wax. As dental stone emits heat as it sets, it is evidently not suitable for casting impressions in snow. The electrostatic charges cause particles of the impressions to jump onto the black underside, recovering the dust impression. An electrostatic lifter passes a voltage across a thin layer of conductive film, which is composed of a lower layer of black insulating plastic with an upper layer of aluminium foil. Impressions in dust are obviously extremely delicate, though can be carefully recovered using electrostatic treatment. Once set, it can be removed and taken for examination and comparison purposes.
The plaster is mixed with an appropriate amount of water and gently poured into the impression. One of the more common methods of recovering three-dimensional impressions is to create a cast of the impression, usually using plaster of Paris, dental stone, or a similar casting material.
The light source should be positioned to give a low angle of incident light, creating shadows to provide a contrast. The application of alternative light sources can enhance two-dimensional footwear impressions. However paper and similar porous surfaces will simply absorb such chemicals, rendering the impression useless. Certain chemicals and dyes may enhance impression on surfaces such as glass or tile. The gentle application of a fine powder may develop footprints on flat surfaces. Two-dimensional impressions can often be treated in a similar way as fingerprints. Numerous techniques are available for the enhancement and recovery of footwear impressions, though non-destructive methods should always be employed first if possible. Such an impression may be two-dimensional, the print left behind on a flat surface in some deposited material, or three-dimensional, formed in a soft surface such as soil. Whenever an individual takes a step, a footwear impression may potentially be left behind on the surface. Such evidence encountered includes footwear impressions, tyre marks, and markings created by tools and similar instruments. Impression evidence includes any markings produced when one object comes into contact with another, leaving behind some kind of indentation or print.
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