Casts

Footprints, shoe prints, and tire tracks found at the scene of a crime can be important evidence to connect a suspect with a crime. A forensic scientist makes a cast (a form made by pouring plaster of Paris into a mold) of this type of impression. Try the following activity and learn how to make a cast of a shoe print.

Materials

- tape
- 2 pieces of 1-by-10-inch (2.5-by25-cm) cardboard
- 2 pieces of 1-by-5-inch (2.5-by-12.5-cm) cardboard
- petroleum jelly
- shoes
- soft soil in an outdoor area
- plaster of Paris
- water
- paper cup
- craft stick
- timer
- newspaper
- small paintbrush

Procedure

NOTE: This is an outdoor activity

1. Tape the four pieces of cardboard together to make a 5-by-10-inch (12.5-by-25-cm) rectangular frame. Grease the inside of the cardboard frame with petroleum jelly.
2. Wearing the shoes, press the sole of one show into soft soil so that you leave a shoe print in the soil.
3. Surround the shoe print with the cardboard frame.
4. Mix plaster of Paris with water in the paper cup according to the instructions on the box of plaster. Stir with the craft stick.
NOTE: Be sure to mix the plaster in throwaway container. Do not pour plaster down the sink, as it can clog the drain.
5. Pour the liquid plaster over the print. Allow it to dry for 1 hour.
6. Lift the plaster cast out of the ground. The soil will stick to the plaster. Bring the plaster cast indoors and place it on newspaper. Allow the plaster to dry overnight.
7. Brush the loose soil from the cast with the paintbrush.
8. Observe the cast. What can you tell about the shoe that made the print?

More Fun Stuff to Do

Make shoe-print casts of several helpers. Can you identify which shoes made each print? Make a cast of the track made by a tire in mud. What can you tell about the tire by looking at the cast?
Explanation

Casts of impressions, such as shoe prints, tire prints, or even bare footprints, taken at the scene of a crime can connect a person or vehicle with the crime.

When a shoe makes an impression in soft material, such as soil, it leaves behind a negative mold of itself, including any special marks or a logo or tread pattern, may be visible. By carefully filling the mold with the plaster of Paris, investigators can make an exact reproduction of surface of the sole of the shoe.

This reproduction is then sent to the crime lab where forensic scientists analyze it and compare it to known objects. For example, the pattern on the sole of a running shoe can be used to identify the brand of shoe, or the wear pattern may indicate that the wearer walks with a limp or drags one foot. Similar marking on a tire cast can tell forensic scientists what of tire made the mark, and may even tell what type of car the tire were on.