Things don’t always go the way you expect. I’ve been
working on a side by side environmental stability test of fingerprints
developed on plastic bags using: AI Red, Carbon Black and 2-Ethylcyanoacrylate
(CA). Most of the results are as expected. AI Red has greater environmental
stability than the other two systems under test. But I expected this. I have samples
left over from the first study Enhancing
Contrast of Fingerprints on Plastic Tape (Journal
of Forensic Science, November 2003, Vol. 48, No. 6) that are still as
clear as the day they were made. AI Red dyes everything but the print. Under
good storage it should last as long as the printed patter on the shirt left
alone in the back of a closet.
What I didn’t expect is that some of the CA and
Carbon Black prints got worse in the freezer. Some of the CA prints fell off
the baggies in the freezer and all of the black prints got lighter. I guess, I take
for granted that cold is good. In Forced Condensation of Cyanoacrylate with Temperature Control of
the Evidence Surface to Modify Polymer Formation and Improve Fingerprint
Visualization
(Journal of Forensic Identification, July/August 2012, Vol. 62) we showed that
chilling the evidence properly improves CA development. And the mechanism does
indeed produce more easily visualized prints.
However,
what the current study is showing is that if the CA prints are left at extremely
cold temperatures the CA cracks and in some cases loses its bond. Now there are
a few possibly explanations that are currently being discussed. For one thing
the expansion coefficient of the PE bags is 2:1 vs the CA. For another the
temperature is substantially lower than most freezers (-19°C). This is below
the recommended working temperatures for CA and well below the temperatures
used in the IAI paper. But I suppose the take away on CA uses is that while
chilling improves CA print development and visualization, freezing in extreme cold
will is deleterious.
Now for the Carbon Black prints, the fact that they
lightened in the freezer didn’t actually change the value of any of the prints
under test. Either the print lost moisture or the change is lipid viscosity or
fluid volume knocked some particles free. But in no case, did the samples lose
any detail. Even prints where the naked eye could no longer see the ridges, the
camera still captured them just fine.
In the near future I will discuss the incubator
findings.
Peace
Charles
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