By Pat A. Wertheim
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 one-half to one
teaspoon of powder (approximately one millilitre) 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.
Pat A. Wertheim
Director of Training
Forensic Identification Training Seminars, Ltd.