Forensic Science - Powders Used For Finding Invisible Fingerprints

What kind of powder is used when CSI's embark on the search for fingerprints at a crime scene? In this article, I will attempt to explain the various types of fingerprint powders that investigators use to find prints left behind by an offender.

What do fingerprint powders do? Fingerprint powders stick to moisture and oils of the residue in a latent, or invisible, print and thereby reveal the pattern of the friction gyri, or ridges. Fingerprint powders come in a variety of types and colors. Criminal scientists use the color that gives the highest amount of contrast with respect to the background surface. Black powder, which is made from charcoal or carbon black, and gray powder, which is made from titanium or aluminum powder, are most commonly used.

Powders that interact with magnets are also used. These powders too come in different colors. These powders are used with a special magnetic tool that resembles a brush. It is not a real brush because it contains no bristles, and thus does not come in contact with the fingerprint thus destroying potential evidence via smearing or damaging the fingerprint. It is more like a tiny feather duster that loosely holds the powder by a weak magnetic force. This so-called brush leaves behind magnetic powder when it is passed over the fingerprint, and the fingerprint all of a sudden shows up.

Other special types of powders are fluorescent. After they are applied, the fingerprint glows under a laser light source.

After the powdering process is done, the fingerprint is either documented via photograph or lifted. Lifting is a procedure done by gently laying the sticky side of a strip of transparent tape such as Scotch packaging tape over the fingerprint. As the tape is removed, the fingerprint pattern sticks to the sticky surface of the tape. The tape is then put on a card for later study and comparison. Because smears can damage fingerprint evidence, it is essential that hands like a surgeon are used when performed the lifting procedure.

When you watch your favorite CSI program, you will be familiar with this method of using magnetic powder dust to discover invisible fingerprints. Whenever you hear a CSI technician "dusts (an object) for fingerprints," you will know what they are talking about.

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