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Transcript
Lifting Fingerprints with Powders and Chemicals
Lawrence Chow
Lifting fingerprints from a crime scene is crucial in determining a possible suspect. There are three types of
prints at a crime scene: plastic prints, visible prints, and true latent or invisible prints. Plastic prints are ridge
impressions left on a soft material such as wax, putty, soap, or dust. While visible prints are made by fingers
touching a surface after the ridges have been in contact with a colored material such as blood, grease, ink, or
paint. Finally latent prints are impressions caused by the transfer of body perspiration or oils present on finger
ridges to the surface of an object. Locating a plastic or visible print at the crime scene is usually a simple task
because it is usually visible to the eye. However, finding latent or invisible prints is much harder and requires
the utilization of techniques that will help visualize the print. There are several different methods for
visualizing a latent print; the one of choice depends on the surface that is to be examined. Prints on hard and
nonabsorbent surfaces (such as mirror, tile, glass, and painted wood) are developed with the application of powder. While soft and
porous surfaces (such as cloth, paper, and cardboard) are made visible by the treatment of a chemical.
Fingerprint powders are available in a variety of colors and compositions. The powder of choice is the one that shows the best
contrast with the surface being examined. Experienced examiners find that black and gray powders are satisfactory for most latent
print work. Therefore, the black powder, composed of black carbon or charcoal, is applied to white or light-colored surfaces. The
gray powder, composed basically of an aluminum dust, is used on dark-colored surfaces; it is also applied to mirrors and metal
surfaces that are polished to a mirror-like finish, because these surfaces will photograph black. These powders are applied lightly to
a nonabsorbent surface with a fiberglass or camel's-hair brush, and will stick to perspiration residues and/or deposits of body oils
left on the surface. Another type of powder is the magnetic-sensitive powder that can be spread over a surface with a magnet in
the form of a Magna Brush. It also decreases the chance of damaging or destroying the print because the Magna Brush does not
have bristles that touch the surface. These powders also come in a variety of black and gray colors. Fluorescent powders can also
be used to develop latent prints, which will fluoresce under ultraviolet light.
Chemicals are also used to visualize latent prints. The oldest method is iodine fuming which is based on the fact that iodine has
the property to sublime. Iodine is a solid crystal that, when heated, is transformed into a vapor without passing through a liquid
phase. The suspect material is usually placed in an enclosed cabinet with iodine crystals. While heated, the vapors will combine
with the constituents of the latent print and make it visible. However once the process is stopped, the print will begin to fade. It
must therefore be photographed or sprayed with a 1% solution of starch in water, which will turn the print blue and make it last for
several weeks to several months. Another chemical used is silver nitrate, which after being applied is developed with the exposure
of ultraviolet light. This event is made possible because of the chemical reaction between sodium chloride (salt, left by perspiration
after evaporation) and silver nitrate which forms the colorless silver chloride. After a 3% solution is brushed onto the evidential
object and exposed to ultraviolet light, a silver print will be shown as a reddish-brown or black color. Ninhydrin (triketohydrindene
hydrate) is dependent on the chemical reaction to form a purple-blue color with amino acids present in trace amounts in
perspiration. The 0.6% solution of ninhydrin powder with a suitable solvent, like acetone or ethyl alcohol, is usually sprayed onto
the porous surface with an aerosol can. Prints begin to appear within an hour or two, and can be hastened if heated in an oven or
on a hotplate at a temperature of 80� to 100� C. Another chemical treatment for fingerprint development is Super Glue fuming,
which produces a white-appearing latent print.
http://www.detectopoint.com
Super Glue is approximately 98 to 99 percent cyanoacrylate ester, and it's this chemical that actually interacts with and visualizes
a latent fingerprint. Cyanoacrylate ester fumes can be created when Super Glue is place on absorbent cotton treated with sodium
hydroxide or by heating the glue. The fumes and the suspect material are contained with in an enclosed chamber for up to 6 hours.
With these various methods, the one chosen depends on the surface to be worked on. Powders should be selected when the
surface is smooth, while chemicals for soft and porous surfaces. When attempting to utilize all of the chemical methods of
development, one should use iodine fuming first, ninhydrin second, then silver nitrate, and finally super glue fuming if it applies.
This is the procedure for optimum visualization because iodine fuming is not permanent, and if ninhydrin fails, silver nitrate can be
used but will wash away all the fatty oils and proteins from the surface. Hence, silver nitrate is used last if super glue fuming is not
used.