The Lowtech Approach To Fingerprints

Latent print residue that is deposited on the surface of nonabsorbent materials generally stays there and is not absorbed into the surface. This is adsorption, rather than absorption. The properties of the residue are different from the surface properties. This constitutes the basis for creating color differences through the selective absorption of powders and dyes.

Powders used for developing latent prints are of two general types: carbon and powdered iron. Fingerprint powder in the form of carbon black is the oldest and most widely used developing agent. The powder is applied using a brush that is twirled between the forefinger and thumb so that the bristle only very lightly contacts the surface with the print residue. The difference in surface characteristics usually is such that the print residue adsorbs the powder more readily than does the surrounding substrate. The result is that the print becomes more visible. The photograph shows a fingerprint brush, powder, and a developed print.

Carbon-based powders come in various textures in white, silver, gray, and black. Fluorescing powders of various colors are also available. The fluorescing powders are used with lasers and forensic light sources, which will be discussed in a following section.

Powdered iron filings are magnetic; they will stick to a magnet. By using a magnetic wand and powdered iron filings, a fingerprint "brush" can be created. When the magnetic wand is brushed across the surface, small amounts of the iron filings will adhere to the fingerprint residue. The advantage over the carbon powder method is easier cleanup (the magnetic wand can be used to pick up loose iron particles) and less interference by substrates. The powdered iron filings are marketed as magna powder. The wands have pull-up centers that demagnetize the wands to allow return of the magna powder to the container. The magna powder and wand are shown in the photograph.

Carbon Powder Fingerprinting
Fingerprint brush, powder, and print (Courtesy of the author)
Magna powder and wand (Courtesy of the author)

Magna powder wands also come in large sizes so that large surface areas can be checked quickly. The large wands are useful for developing test impressions of footwear and tires using a special technique that involves first spraying the sole of the footwear or the tread of the tire with silicone.

Although the use of fingerprint powder or magna powder usually results in a greater degree of adsorbance by the fingerprint residue, there is often some adsorbance by the surface the fingerprint is on. This can be such a problem that the technique fails to provide sufficient contrast to be able to clearly distinguish the fingerprint. One simple way to overcome this is for the examiner to exhale very close to the fingerprint so as to have the moisture from the breath adhere to the fingerprint residue. This in turn results in more powder adhering to the residue. This may also be accomplished in a somewhat more sophisticated way by using a small steam iron to create the moisture.

In the early 1980s labs found that superglue (cyanoacrylate ester) adheres to fingerprint residue, particularly in a high-humidity environment. The superglue deposits on the friction ridges, turning them white. The examiner can then apply fingerprint powder or magna powder to

0 0

Post a comment