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Introduction
On occasion, it
is necessary to obtain an exemplar of a known shoe from
a suspect. While some options include inking and rolling
the shoe, similar to taking a rolled impression of a
fingerprint, BIO-FOAM® Impression Foam offers a
method of obtaining a three-dimensional exemplar of
the known footwear (see photograph to the right). Using
the BIO-FOAM® Impression Kit is easy, takes only
seconds, and does not get the footwear dirty. Each BIO-FOAM®
Impression Kit comes in a cardboard container which
is suitable for storage and will protect the impression
while it is in the evidence storage room. The foam is
blue in color and measures 14" x 6" x 2 1/4".
Application
Press the footwear
into the BIO-FOAM® Impression material about 1-2
inches. Gently remove the shoe. Liberally coat the BIO-FOAM®
Impression with hair spray. Wait until the hair spray
dries and prepare a quantity of dental stone (see next
paragraph). Pour in the casting powder and allow it
to cure. Remove the cast from the BIO-FOAM® Impression
and allow it to dry for about 48 hours. Once the cast
is completely dried, rinse it with running water and
gently scrub it with a bottle brush to remove any excess
foam.
When the mixture
approaches the consistency of thin pancake batter it
is ready and should be poured before it becomes thicker.
Pour the mixture into the foam by pouring it onto a
flat stick or spoon held close to the surface of the
foam. The mixture must be allowed to flow evenly over
the impression. In approximately 30 minutes, the cast
may be checked. Once the cast is firm, carefully remove
it from the BIO-FOAM® Impression material. The cast
should be allowed to air dry for 48 hours.
The cast should
never be placed in an airtight container or wrapped
in plastic. Paper bags can be used but they do not afford
the best protection. The bag can easily tear and the
cast can fall out or otherwise be damaged. Dried casts
should be stored in cardboard boxes. Cast boxes are
available with the dimensions of 17 1/4" x 11" x 2 1/4".
They are sold in quantities of 25.
Photographic
Scales
The camera used
should be placed on a tripod, which will allow the film
plane to be parallel with the surface containing the
evidence impression. Some impression evidence examiners
prefer black and white photographs over color photographs,
as they feel the range of grays with black and white
film shows the details better than color films. Illumination
of the impression should be done at an oblique angle
from a number of directions. In other words, hold the
strobe at an oblique angle to the left of the print
and take a picture. Take additional exposures with the
strobe held at an oblique angle to the right, to the
top and to the bottom of the impressions. If added details
can be seen by holding the light in other directions,
take additional pictures.
With photography
of this nature, scales should be included in the photograph.
The Bureau photomacrographic scale set was designed
by the Federal Bureau of Investigation for this particular
purpose. The "L" shaped scale and straight scale are
white with black marks on one side and black with white
marks on the other. Choose the side that will show the
best contrast with the surface and place it close to,
but not overlapping, the evidence impression. A larger,
3-Part, folding scale which is handy for tire track
impressions is also available. The advantage of an "L"
shaped scale is that if the scale divisions on each
leg of the scale are not equal in the photograph or
if the circles with crosshairs are not absolutely round,
you will know that the camera was not parallel to the
impression. If the angle of error is not severe, it
can be corrected in the darkroom. Tilt the paper carrier
to bring the "L" shaped scale back to parallel perspective
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