Topic
Crime scene investigators make
shoe impressions with plaster of paris and dental stone.
Introduction
The most common method of
entering and leaving an area where a crime is committed is by walking.
Criminalists often gain information by examining the surfaces on which a
perpetrator walked. Footwear or bare feet can leave superficial prints on firm
surfaces such as flooring.
However, walking across a soft
surface like damp soil leaves impressions or indentations.
Footwear impressions can
provide crucial clues in an investigation. Prints or impressions can show the
points of entry and exit at the crime scene.
In addition, shoes carry
materials from one environment to another. This material can provide
information about where this person has been.
An impression can tell a lot
about the shoe that made it. Every shoe is unique, with its own pattern of wear
and tear. Therefore an impression of footwear can be compared to a particular
pair of shoes to help connect those shoes to the crime scene. Impressions can
also tell something about the height and weight of the wearer. Heavy
individuals make deeper impressions than light ones. In addition, a tall person
has a longer stride than a short person.
There are several techniques
for making casts of shoe impressions. In this experiment, you will compare the
effectiveness of two materials in making casts: plaster of Paris and dental
stone.
Time Required
45 minutes on day 1
45 minutes on day 2
Materials
a)
2 pounds (lbs)(0.91
kilograms [kg]) of dental stone in a large
b)
Ziploc™ bag
c)
3 lbs (1.36 kg) of
plaster of paris in a large ziplock bag
d)
3 to 5 cups of water in
a bottle
e)
5 to 10 twigs or craft
(Popsicle™) sticks soaked in water
f)
3 or 4 long strips of
cardboard (about 4 inches [in.] by 24 in.
g)
[10.2 centimeters (cm)
by 61 cm])
h)
large spoon
i)
talcum powder
j)
magnifying glass
k)
scrub brush
l)
paper towels
m)
access to water
n)
science notebook
Procedure, Day 1
1.
Follow your teacher to an outdoor area where there are some shoe impressions.
2.
Closely examine two footprints. Carefully remove any rocks, twigs, or other
debris from the footprints.
3. Sprinkle a little talcum powder over these
two footprints to help hold the soil in place and secure the impressions.
4.
Build a retaining wall around each impression using strips of cardboard
(see Figure 1).
(This wall will prevent the plaster of paris or dental stone
from escaping when it is poured into the impression.)
5.
Make a cast of one footprint using dental stone. To do so:
a. Pour about 1 cup of water into the Ziploc™
bag of dental stone.
b. Knead the bag of dental
stone for 2 to 3 minutes (min) to mix thoroughly.
c. Examine the consistency of
the dental stone. Ideally, it should be similar to pancake batter. If it is too
thick, add a little more water. (Be careful; if you add too much water, your
mixture will be too thin.)
d. The dental stone becomes
firm very quickly, so work rapidly to pour the mixture. Avoid pouring the
material directly into the impression, which can distort it. Instead, hold the
spoon just above the impression and slowly pour the mixture into the spoon so
that it falls into the impression. When the impression is about half full,
remove the spoon and pour the remaining mixture directly. The poured cast
should be 2 to 3 in. (about 5 to 7.5 cm) thick.
e. Use a stick or pencil to
etch your name and the date in the upper surface of dental stone.
f. Let the cast sit for 30 min to harden.
During this time, carry out step 6 below.
g. After 30 min or more, insert
the handle of the spoon into the ground about 1/2 in. (1.2 cm) from the
impression and gently pry up the cast.
h. The cast may have soil or
grass on it; do not remove it at this time.
i. Take the cast indoors and allow it to harden
for 24 hours.
6. Make a cast of the other shoe impression
using plaster of paris. To do so:
a. Pour about 1 cup of water
into the Ziploc™ bag of plaster of paris.
b. Knead the bag of plaster of
paris and water for 2 to 3 min to mix thoroughly.
c. Examine the consistency of
the mixture. Ideally, it should be similar to pancake batter. If it is too thick,
add a little more water. (Be careful; if you add too much water, your mixture
will be too thin.)
d. Pour about 1/2 in. (1.2 cm)
of plaster of paris into the impression, using the spoon as you did earlier to
avoid damaging the impression. Add some reinforcing materials such as twigs or
craft (Popsicle™) sticks that have been soaked in water (see Figure 2). (Dry
sticks will absorb water from the plaster of paris and make it brittle.)
Quickly finish pouring the plaster of paris into the impression to a thickness
of at least 3 in.
e. Follow directions e through
i in step 5.
Procedure, Day 2
1.
Examine each shoe cast. In your science notebook, note any significant
differences in the casts.
2.
Using a scrub brush, gently clean each cast with water. Pat dry each cast
with paper towels.
3.
Examine each shoe cast again. In your notebook, note whether or not
washing damaged either cast.
4. Closely examine each shoe cast with a
magnifying glass. Search for nicks, cuts, or scrapes in the casts that indicate identifying markings on the shoes. Record these in
your science notebook.
Analysis
1.
Why might an investigator make casts of shoe impressions?
2. in your opinion, which cast was easier to
pour, the dental stone or plaster of paris? Explain your answer.
3.
Which material held up best to washing? Explain your answer.
4. Which material provided the most detail when
viewed under the magnifying glass? Explain your answer.
5.
Describe any nicks, scratches, wear patterns, or other unique marks found
in either cast.
6.
What information might a forensic investigator acquire from examining a
shoe cast?
7.
Complete a Venn diagram (see Figure 3) comparing the two casting materials.
