Shoe impressions

When a pair of shoes are manufactured, unless they are hand-made, the soles are moulded from rubber and stitched, glued or both, to the uppers.

At this point all of the soles from the same mould are identical, except for incidental manufacturing flaws such as microscopic air bubbles. It is, however, possible to identify the size of the shoe, the manufacturer and, possibly, the shoe type. This provides a useful tool to eliminate potential suspects from a crime scene.

The moment the shoe is put on and used, it starts to develop unique characteristics. Cuts and marks are made into the patterns by the surfaces and objects which are walked upon and items may become embedded into the grooves.

In addition the wearer of the shoe begins to create a wear pattern which is individual to that person, due to the weight and gait of the person.

In a very short time the shoe develops a pattern which may be as unique as a fingerprint.

In the same way that fingerprints can be deposited at a crime scene, shoe prints can be left for the CSI to find. These shoe prints may be latent (i.e. invisible to the naked eye) or patent (visible).

Latent shoe impressions may be left on surfaces such as tiles, windowsills or paper and require special techniques to make them visible. Some of these methods involve the use of powders which adhere to the patterns left by the sole, with the resultant print being recovered using vinyl lifting sheets or by photography.

An interesting technique for recovering dusty shoe impressions on tiles or carpets is to use an ElectroStatic Lifting Aparatus (ESLA). The CSI rolls a length of black, foil-backed lifting sheet across a surface which has been walked upon by the offender. They place the ESLA machine’s electrical probes between the sheet and a metal plate placed on the floor. Once switched on, a high voltage is passed between the sheet and the floor causing a static charge to develop on the sheet which draws it into close contact with the floor.

At the same time dust is attracted from the floor onto the black surface of the sheet where any shoe pattern will be visible. This pattern is delicate and the sheet must be treated very carefully. The image on the sheet is often only visible using oblique lighting techniques.

Patent shoe impressions can be deposited by shoes covered with soil, blood, oil, etc. onto clean surfaces or they might be in the form of ‘plastic’ impressions where the shoe has left a three dimensional impression in a surface such as mud, sand or snow.

In both circumstances the print is initially recorded using photography. With a ‘plastic’ impression this would normally involve oblique lighting techniques that provides the best definition of the mark. An essential element of these photographs is the inclusion of linear scales, placed at right-angles around the mark, so that it can be reproduced, at a later date, at actual size. A one-to-one photograph of the mark is essential for a comparison to be made with a suspect’s shoe.

Once photographed a ‘plastic’ impression will usually be cast using plaster. A dam is created around the mark and it is stabilised using a plasticising spray, such as hairspray. This spray is drifted across the mark and built up in gradual layers.

The casting media is then mixed and carefully poured into the dammed area to ensure the mark is preserved. Once the cast is almost dry it is uniquely marked with the exhibit reference number, date and location. This is done directly into the surface of the cast or by embedding an acetate sheet into the still wet cast with the details already written. A photographic record is made of the cast, showing its position and identification marks.

The cast is recovered with the surface still adhering to it and no effort is made to remove it. This delicate operation is left to the forensic scientist, once the cast is fully cured.