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Introduction
Shoe and tire impressions
in dirt, sand or snow can be recovered by a combination of photographic
techniques and taking a three-dimensional cast of the impression.
Dental stone, also called casting powder, a simple, inexpensive
casting material, can record sufficient detail for comparison
purposes. It is always recommended that photographs using oblique
lighting from a number of sides be done before casting material
is placed into the impression. When dealing with impressions
in snow or sand, it is recommended to use Snow Print Wax™
to "fix" the impression so it will not be destroyed or damaged
when the casting material is poured in.
Preparation
Before mixing the casting
material, it is recommended to photograph the impression. After
sufficient photographs are taken but before mixing the casting
material, it may be necessary to remove leaves, twigs, or other
loose debris which may have fallen into the impression. However,
in no instance should an attempt be made to remove debris if
that debris is part of the impression or if there is any possibility
of destroying part of the impression by removing it.
The first step is to place
a form around the impression. This will contain the casting
material and also help give the cast extra thickness. Two sizes
of forms are available. The shoe impression frame is 7 1/4"
wide and expands from 10" to 18". The tire impression frame
is 12" wide and expands from 15" to 28".
Mixing Instructions
Casting powder may be mixed
in a bowl with a spatula or other stirring device. The mixing
ratio is approximately 12 oz. of water to two pounds of casting
powder. This amount should cast an average shoe impression.
For tire track impressions, larger quantities will be needed.
An easier method for shoe
print impressions, which can be prepared ahead of time is to
measure approximately two pounds of casting powder into an 9"
x 12" zip-top plastic baggie. Measure water into a 12 oz. plastic
bottle. Clearly label each baggie and bottle as to its contents
and the date it was prepared. A few baggies and bottles can
be carried in a crime scene response vehicle until needed. When
needed, add about two-thirds of the water into the baggie and
mix it by kneading the bag. If the powder seems too thick, it
needs more water. Add a little water and knead it again. Repeat
until the desired consistency is reached. If the whole 12 oz.
bottle is added initially, it may be determined that a little
less water was needed as the mixture is now too thin. It is
better to have to add more water than to have to add more casting
powder, which may not be available at the scene.
Application
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
form by pouring it onto a flat stick or spoon held close to
the surface of the ground and slightly to the side of the impression
so as not to wash away portions of the impression. The mixture
must be allowed to flow evenly over the impression. In approximately
30 minutes, the cast may be checked. If the cast is firm, carefully
lift it from the surface. If soil or debris from the impression
area is adhering to the cast, no attempt should be made to clean
it. The cast should be allowed to air dry for 24 to 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.
Snow Print Wax™
For information on Snow
Print Wax™ for use in casting impressions in snow, please
see our Snow Print Wax™ Technical Note in this section.
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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