Latent Prints

The Skin
The human skin is composed of numerous layers: the epidermis on top, followed by the papillae, and then the dermis. The form and pattern of ridges on the surface of the skin is determined by the dermal papillae. These ridges, known as minutiae, are formed pre-birth, and stay with the individual throughout their life. Each skin ridge holds a row of pores through which sweat is released. It is a combination of these ridges and the sweat that causes a fingerprint to be left behind when the finger comes into contact with a surface.

The fingerprints left behind, which are unique to an individual, are composed of a collection of loops, whorls and arches. Loops are characterised by ridge lines that enter from one side of the pattern, curve around, and exit from the same side. Whorls are divided into four types; plain, central pocket whorl, double whorl, and accidental. Arches are characterised by ridge lines that enter the print from one side and exit the other side. There are four basic bifurcations (divides) in fingerprints; where a ridge divides, where a ridge ends, a lake, and an independent ridge.

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Fingerprints can be visible, plastic or latent. Visible prints are left in a substance such as paint or blood, clearly visible. Plastic prints are left in some kind of soft surface, such as putty or wet paint, and are also visible. However latent prints are left in bodily oils, and may require treatment to be visualised.

Enhancing Prints
It is necessary to treat latent prints in order to enhance them for collection and comparison. Many forms of print enhancement are based on the fact that latent prints contain numerous different compounds that will react to certain tests. The method used will often depend on the surface onto which the print has been left, and the environment and circumstances. Prints on a non-absorbent surface, such as glass, are usually enhanced using powders or superglue fuming. However prints on soft and porous surfaces, such as cloth, may require some kind of chemical treatment.

The application of aluminium powder is the most common method of developing latent prints. The fine powder is applied with a brush, after which it adheres to perspiration residues and body oil deposits, visualising the print.

The Magna Brush can be used to apply magnetic-sensitive powder. As the brush has no bristles, the chances of the print being damaged are greatly reduced.

Ninhydrin, or triketohydrindene hydrate, is a compound that reacts with the amino acids in the print to produce a purple colouring. This technique is particularly useful on porous surfaces such as paper, though is not useful on wetted items or silk finish surfaces.

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The iodine fuming method uses iodine crystals that vaporise by sublimation when heated. These vapours combine with components on the latent print, making it visible. The print developed will eventually fade, so should be photographed immediately once observable. The superglue fuming method used ethyl or methyl cyanoacrylate which, when fumed, produces a white deposit on the latent print. The superglue is placed in an enclosed chamber with the item and heated, causing the superglue to adhere to the print.

When lifting an enhanced print, tape should be applied to the entire print and pressure applied before being carefully lifted. The lift should be smooth and without pauses.