The Science of Fingerprints: Classification and UsesUnited States. Federal Bureau of Investigation
Science
The Science of Fingerprints: Classification and Uses
United States. Federal Bureau of Investigation
Fingerprints
The problem confronting the fingerprint examiner in treating fingers
which are desiccated or dried and shriveled is that of distending and
softening the skin. Desiccated fingers are generally found to have the
outer layer of skin intact and the ridge detail fairly clear. However,
due to the shrinking, numerous wrinkles will be present, and as the
drying process continues the skin and flesh harden until the fingers
become almost as hard as stone.
It is sometimes possible to distend or swell the flesh by utilizing a
1- to 3-percent solution of sodium hydroxide or potassium hydroxide,
sometimes referred to as caustic potash. As a matter of caution, this
process should be tried with one finger before using it for the
remaining fingers. This point of caution is made because of the
reaction of the potassium or sodium hydroxide, which is actually one
of destruction. While absorption and swelling of the flesh occur, the
disintegrating action of the fluid may result in total destruction of
the flesh.
The finger to be distended is cut from the hand at the second joint
and placed in the hydroxide. When it has resumed its normal size by
the absorption of the solution, it is inked and printed. There is no
set time for this process. The procedure may require a few hours or as
much as several days until suitable results are obtained.
After the finger has been in the solution for about 30 minutes, it
should be removed and examined in order to note the extent of the
swelling and the reaction of the flesh to the solution. If no material
change is noted, the finger is returned to the solution. A close watch
is maintained and the finger is examined from time to time.
The solution may cause thin layers of skin to peel from the finger.
Should this occur, the loose skin is carefully scraped off and the
finger rinsed in water for a few minutes. It is then returned to the
hydroxide for continuation of the process.
If, during the course of an inspection, it is seen that the flesh is
becoming too soft, the finger should be placed in a 1- to 3-percent
solution of formaldehyde or alcohol for several minutes in order to
harden it.
If, after several hours in the hydroxide, the finger has not reached
its normal size, it should be placed in water for an hour or two. This
has a tendency to hasten the swelling. When the finger is removed, it
will be noted that a film has coated the surface. This coating is
carefully scraped off and the finger is replaced in the hydroxide
solution for an hour or so, again scraped if coated, soaked in clean
water, etc. This process of alternating from solution to water,
scraping, and replacing in hydroxide is continued until desirable
results are obtained. The finger is then inked and printed.
The above process will so saturate the finger with solution that it
may be too wet to print properly. Accordingly, the finger may be
dipped into acetone for several seconds, removed, and be permitted to
dry, after which it is inked and printed.
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