Footwear sole impressions and shoeprints
There are different types of marks left by people as they walk, run, or stand. Sometimes these are described as ‘footprints’ however that is an erroneous term. Correctly they should be named as ‘footwear sole impressions’, ‘shoeprints’, or ‘footmarks’. Footwear sole impressions and shoeprints are the same thing and are caused by the passage of a person’s footwear: Footmarks are left by bare feet.
Caution should be applied to the thinking that the size of a footwear sole impression will indicate the size of the foot; that is not necessarily the case; the indication will be the size of the shoe worn on the foot, not the foot itself.
Sometimes shoes have unique markings that will assist in distinguishing the objects from others. Many shoes have very different markings that will indicate the class characteristics of the shoe (for example, the size and make). To move towards being able to match a particular shoe to an event, further discriminating information is required. This may be apparent in such things as wear marks or damage.
Collecting 3D impressions
In soil footmarks, footwear sole impressions and tyre impressions can be recorded by the making of casts effectively collecting a 3D mirror image of the indented impression.
There is a need to ensure that inadvertent material loss from casts is reduced by proper handling, packaging, storage, and transportation.
In mediums such as snow a special wax has been successfully used to harden the snow before a cast is taken.
Collecting 2D impressions
Two-dimensional shoeprints in dust can be lifted by –
In order that adhesive lifters will take-up dust it is necessary that the chemical and physical forces are greater on the adhesive than on the substrate, this of course means that there is a risk that the impression will be altered during the lifting process.
Electrostatic plates can be used to lift dust, thus if a person were to stand on a surface and either disturb dust with their shoes or leave dust behind these plates can be used to capture a mirror image of what is visible.
Electrostatic lifting (‘ESL’) is useful only on surfaces that are resistant to electric flow and that are positively charged because surface charge will cause an imbalance meaning that the metallic sheet and the surface will both exhibit attractive forces.
In ESL a metal sheet which has been coated with an insulating layer is electronically charged with static electricity attracting dust to the surface. The dust particles are held by a weak electric charge. Simply put, the plates work on the principle that dust is attracted to a negative charge once it has taken on a positive charge.
Gelatin lifting is recommended when ESL is not possible or when the nature of the impression cannot be protected. A study found that ESL is equal to or better than gelatine lifting on many substrates except for wet shoeprints and is superior on non-smooth, porous and fibrous substrates.
Photography is also a useful medium for recording such marks especially when they are not indentations but rather are transient in nature, for example left in water, oil, or blood. Specialist training is required for this type of photography due to the necessity to faithfully replicate the impression that has been made.
© Hadyn R Green 2012