Black Powder Processing
This article is the first in a comprehensive series of articles on latent print development techniques written by Pat A. Wertheim, Director of Training for Forensic Identification Training Seminars, Ltd. Watch for future articles in each issue of Minutiæ and save these issues in a binder to compile a manual of fingerprint development techniques.
Although old-fashioned black powder is the workhorse of fingerprint development techniques for crime scene use and is also an important method in the laboratory, maximizing the effectiveness of powder requires far more sophistication than simply dipping a brush into the jar of powder and painting it onto a surface.
More control can be exercised over black powder by working out of a shallow dish. The perfect disposable dish can be made by cutting or tearing a blank inked fingerprint card from any edge into the center of the card. Overlap the two edges of the cut by about an inch (two or three centimeters) and tape the card back together to make the dish. A large laboratory weighing dish may be used, or any other shallow dish or bowl.
Place 1/2 to one teaspoon of powder (approx. 1 milliliter) into the dish. Use a quality fiberglass brush (my personal favorite has always been the Zephyr® Brush.) Nylon brushes tend to be too stiff and carry too much powder, camel hair brushes are too stiff and cover too small an area, carbon fiber brushes shed loose fibers too easily, and feather dusters tend to carry too little powder (although there are times when less powder is better). Fiberglass brushes work better after a break-in period. But you should never touch the bristles of a brush or allow other residue to contaminate its fibers.
Gently grind the powder in the center of the dish using a rotating motion by twirling the brush handle slowly between the thumb and index finger. This action accomplishes two tasks: it breaks up the little balls or clods of powder that result from settling during shipment (these little clods of powder can streak a surface), and it loads the brush fibers with powder.
Before powdering a surface, tap the excess powder from the brush back into the dish. When powdering a vertical surface, place the dish against the surface to catch excess powder falling during the powdering process.
There are two schools of thought on the best method for applying powder: painting and twirling. In the painting method, the brush is swept gently back and forth across the surface. In the twirling method, the brush is twirled between the thumb and index finger. The two methods may be combined and the brush twirled while moving back and forth across the surface.
When twirling a quality fiberglass brush, the fibers flare slightly to form a rounded or spherical brushing surface. Only the very center part of this area of the brush should be allowed to gently touch the surface being powdered. When the first hint of a latent fingerprint begins to develop, care should be taken to neither overdevelop nor erase the latent. As much as possible, the brush fibers should be in motion with the flow of the ridges rather than against the ridge flow.
For large surface areas such as sliding glass doors or automobile exteriors, I personally prefer to paint the surface until the first traces of a latent print begin to show. Then I twirl the brush very lightly until the latent has sufficient detail to allow for an identification. A latent should always be photographed or lifted as soon as it is identifiable. Attempting to improve an identifiable latent print prior to photographing or lifting frequently results in destruction rather than improvement.
With all of the modern technology available, many people lose sight of the fact that old-fashioned black powder, correctly used, is still one of the most effective methods of fingerprint development. Work from a shallow dish with a quality brush and proper technique, and many good latent prints will result.