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The left photo shows a
single print of questionable quality, while the right photo shows a cluster of three
fingerprints with the quality reduced to allow web display. The rule of
thumb that I always used was that if an impression had ridge detail,
then take it. One can always call it unsuitable for comparison
at a later date. The cluster on the right would be an
exceptional encounter at a scene, but occasionally one can be lucky
enough to encounter such prints.
Today, there are numerous methods to
develop latent fingerprints deposited on nearly any surface under
all types of conditions.
The fingerprints in both photos were developed on a smooth surface with
black powder in the left photo & grey powder in the right, and then
photographed. The image
colour for the right photo can now be reversed on the computer to provide black ridges on a contrasting
white background. While fingerprint
powder remains the most common type of scene development method, latent prints
can also be developed using an array of methods including a wide variety of
chemicals and specialized lighting techniques.
What now may be the second most
common method of development is the use of cyanoacrylate ester (CA)
fuming, or ‘crazy glue’. In a sealed container, exhibits are exposed to the glue’s fumes
with moderate heat and humidity. The
material making up the latent print absorbs the fumes, and the moisture causes
the CA chemicals to polymerize and turn white in colour. Powders can then be used to further enhance
the print if necessary and practical.
This method develops prints on a wide variety of surface types, including
surfaces on which ordinary powders could not normally be used. An adaptation of the method is to examine
the exhibits in a low level vacuum chamber.
This allows entry of the fumes to inner surfaces of exhibits such as
sealed plastic bags, containers, etc., without the potential of first
destroying exterior surface evidence.
However this method then requires the use of specialized chemicals and
lighting techniques to develop and enhance the usually ‘weak’ print that
otherwise would remain completely invisible.
Several years ago, the laser was the main speciality light source used
in this chemical process, but other light sources have now evolved so that the
laser light source is hardly ever used today.
Paper and other fibrous materials can
be examined by submersion in several different chemicals that with
curing will allow the prints to develop and become visible. Indeed, some of these methods
have produced prints known to be in excess of 80 years old.
Other
chemicals allow for the development of prints on exhibits that have been
submerged in water. The oils and fats
deposited as latent prints are not water soluble, while the salts and other
deposits may wash away. Another method
involves a good quality vacuum chamber and the deposition of metals such as
gold on the print medium. Metal
deposition is particularly useful in examining exhibits such as plastic or
smooth surfaces from very old scenes where no other method will work due to
long term exposure to the elements.
Identifiable prints have been developed on plastic bags found at outdoor
homicide scenes that were as much as five years old. |
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Fingerprints have even been developed
on a human body at the scene of a homicide. One of the oldest chemical methods, iodine
fuming, was used to develop latent finger and palm prints on the skin surface
of the victim at a homicide scene in British Columbia, Canada several years
ago. Other methods including
cyanoacrylate ester have been tested and offer promise. However, the length of time between latent print
placement and development is usually extremely critical when dealing with
fingerprints on human skin.
Once found, the print must then be
recorded before it can be identified. To do this, a protocol must be followed that
includes allowances for proper peer review, or verification. One must include the correct latent print
marking procedure, photography, possible latent print lifting, exhibit
retention/photography/continuity of possession, and then the searching and
subsequent identification of the print.
With training and experience, one can
usually tell much about a scene latent print. By the position of the print,
number of prints, and their pattern type, ridge count, and their physical
relationship to one another, one can usually identify the correct finger and
either a left or right hand when viewing a single or a cluster of prints. This narrows the search criteria when the
print is searched either manually or through a computerized fingerprint
database. Over the years, some
‘experts’ have written texts stating that they can offer an age for scene
latent prints. While physical and
mechanical conditions dictate how long a latent will usually remain on a
surface, it is not always possible to know to what extent these conditions may
have affected the scene prints. Sun and
wind on a hot summer day will quickly dry out most latent prints on an exposed
surface, and they will then quite literally blow away. Rain will wash them away, but other surfaces
may undergo less obvious environmental effects. To what degree this may have occurred at a particular scene can
never be known unless it has been monitored, something that obviously was never
done. Add to this the conditions of the
physical surface, and the amount and type of material that actually makes up
the latent print deposit. On a cold
winter’s day, a person having just removed his hand from a glove is less likely
to leave behind a latent print than that same person on a hot summer day after
riding his bike for several kilometres.
As a piece of trivia, the letter ‘R’
with a number is usually used to mark each print. This is done for a specific
reason – to determine the correct laterality of the print later in the
identification process. Most letters of
the alphabet are symmetrical in shape, such as the letters ‘A’, ‘B’, ‘C’, ‘D’,
‘E’, etc. If turned on either the
horizontal or the vertical axis, they look the same. So when viewing the image via a film negative or on the computer
screen, it is impossible to determine whether the letter is upside down, right
side to left, etc. Several letters such
as ‘F’, ‘G’, & ‘R’ are not symmetrical, and can easily be seen as being
correctly orientated, or having correct laterality. The letter ‘R’ is normally used because it is easily and quickly
written even when one is in an awkward position, such as upside down on a roof
reaching around to the back side of an eave trough – been there. done it,
and have wore out the T-shirt! |
