The one technique that developed identifiable latent prints most often was
glue fuming in conjunction with regular magnetic fingerprint powder. Similar
to iodine/silver transfer, this method involves heating glue and directing
the fumes onto the skin, then applying fingerprint powder to reveal the latent
prints.
To test this technique further, researchers glue fumed several areas of skin
containing sebaceous latent prints 2 hours after depositing the prints. Sixteen
hours later, they applied various fingerprint powders to those areas. Using
a fluorescent powder specially formulated for this testing, they developed
a latent print of value for identification purposes. Initially, the researchers
believed that the special fluorescent powder provided the key to obtaining
usable prints, but additional tests proved that the type of powder did not
matter as much as the amount of time allowed for glue fuming.
Glue Fuming Device
As they continued their research, the scientists realized that they needed
an improved method for spreading glue fumes over the skin. The earlier method
used–forming an airtight plastic tent over a small area of skin or over
an entire body–did not always work. It was impossible to distribute glue
fumes evenly over the skin and extremely difficult to confine all of the
fumes to the tent. In addition, when they removed the plastic tent at the
end of the fuming process, the fumes often forced the researchers out of
the work area. To alleviate these problems, one of the researchers, the police
specialist from the Knoxville Department, developed a portable glue fuming
chamber.
The glue fuming chamber contains a built-in heat source and a small electric
fan. Glue is poured into a small disposable preheated aluminum pan and placed
in the chamber. After approximately 5 minutes, the fan is turned on and the
glue fumes flow out through a plastic hose attached to the top of the chamber.
When set at maximum, the amount of fumes forced through the hose approximates
the exhaust from an automobile on a cold day. This device enables the user
to control the amount and time of the glue fuming much more easily than the
tent method.
Using the new device, the scientists tested squares of skin to determine
the optimal fuming time. They tried fuming in increments from 5 seconds up
to 2 minutes. They obtained identifiable latent prints most often when glue
fumes had been applied to the skin for 10 to 15 seconds.
Powders
In the early testing, it seemed that particular types and brands of fingerprint
powders provided the best results. As the research progressed, however, it
became apparent that this was not the case. More than 30 brands and several
types of powders and applicators were tested. In the end, researchers determined
that powder selection is less critical than ensuring that the glue fuming
process is performed correctly.
Both fluorescent powders and regular magnetic powders produce identifiable
prints. With non-magnetic fluorescent powders, the best results are obtained
by applying the powder with a feather duster rather than a conventional brush,
which generally holds more powder. Too much fluorescent powder tends to overwhelm
the latent print and the background. While fluorescent powders work, they
do have some drawbacks. They generally cost more than regular magnetic powders,
are more difficult to see, and require special light sources, filters, and
additional photographic knowledge.
In comparison, regular black magnetic powders produce useful prints and cost
much less. They also do not require special photographic skills. Indeed,
technology does not need to be complex or costly in order to be effective.
Field Conditions
Developing latent prints under ideal laboratory conditions proved that prints
could be obtained from human skin, but the researchers wanted to make sure
that practitioners in the field could obtain similar results. In real life,
homicide victims might not be found immediately, bodies might be exposed
to the elements or other harsh conditions, or they might be taken to the
morgue and refrigerated before they can be examined for prints.
To ensure that the process would work, the researchers simulated field conditions
by testing cadavers that had been exposed to the elements for several days,
as well as refrigerated corpses. They replicated potential time delays that
could occur in the field by waiting for approximately 12 hours between the
glue fuming (which could be done at the crime scene) and the application
of fingerprint powders (perhaps conducted later at the morgue). The results
showed that by following proper procedures, investigators could develop
identifiable latent prints even under harsh conditions.