Impression evidence includes any markings produced when one object comes into contact with another, leaving behind some kind of indentation or print. Common such evidence encountered includes footwear impressions, tire marks, and markings created by tools and similar instruments.
Whenever an individual takes a step, a footwear impression may potentially be left behind on the surface. 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. Numerous techniques are available for the enhancement and recovery of footwear impressions, though non-destructive methods should always be employed first.
Two-dimensional impressions can often be treated in a similar way as fingerprints. The gentle application of a fine powder may develop footprints on flat surfaces. Certain chemicals and dyes may enhance impression on surfaces such as glass or tile. However paper and similar porous surfaces will simply absorb such chemicals, rendering the impression useless. The application of alternative light sources can enhance two-dimensional footwear impressions. The light source should be positioned to give a low angle of incident light, creating shadows to provide a contrast.
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 plaster is mixed with an appropriate amount of water and gently poured into the impression. Once set, it can be removed and taken for examination and comparison purposes.
Impressions in dust are obviously extremely delicate, though can be carefully recovered using electrostatic treatment. 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. The electrostatic charges cause particles of the impressions to jump onto the black underside, recovering the dust impression. As dental stone emits heat as it sets, it is evidently not suitable for casting impressions in snow. In this instance aerosol products exist, such as Snow Impression Wax. This is applied to the impression numerous times at intervals of one to two minutes and then left to dry. The impression can then be cast as normal. Alternatively flour sulphur may be used to cast snow prints. This is boiled to produce a hot casting compound which, upon contact with the cold snow, solidifies to produce a detailed cast.
Any footwear impressions collected from the crime scene may be useless unless there are suspect samples available for comparison. 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. 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. 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.
Even if no other samples are available for comparison, a recovered shoe impression may yield a vast amount of information. Almost all footwear bears undersoles with distinctive patterns, which manufacturers are increasingly designing to be specific to them. In some locations such patterns have been stored in databases for comparison purposes. Though these patterns are the same for the same brand and type of shoe, a certain degree of individuality may be imparted from the manufacturing process or general wear. 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. The size of the shoe, which may easily be obtained by examining the recovered impression, may prove useful, though not as a positive identifier.
As vehicles are commonly present at crime scenes, before, during or after the crime, tire impressions will often be discovered at the scene, usually left behind in soil. The enhancement and collection of these is similar to that of footwear impressions. If a tire impression is discovered at a scene the opposite impression should also be searched for, as the distance between these may provide further information on the vehicle in question.
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. Common instruments encountered fall into two categories; cutting instruments and levering instruments. Common cutting instruments include knives, bolt croppers and drills, with screwdrivers and jemmies being common levering tools. Such instruments will often suffer severe damage when used, giving them characteristic features which may leave behind a distinctive impression at the scene. A cast can be made of the impression at the scene, usually using a type of silicon rubber. This can then be used in comparison with other impressions or instruments to establish a match and determine which tool was used. The cast itself will be a negative of the original mark, and so should not be directly compared with the suspected tool. Instead the suspected instrument can be used to make a number of test marks in a similar medium.