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Plaster Casts

In this activity, students make a plaster cast of their hand or foot (Part 1 of this activity) that they can keep and use in a forensic observation game (Part 2). 

Wherever we go, we leave a trace.

Our traces can provide important evidence for legal cases. Footprints, tire tracks, tool marks, and any other evidence where one object leaves its mark in another through physical contact is called impression evidence.

2D impressions (like a dirty footprint on the floor) can be lifted using techniques similar to fingerprinting. (See activities Fingerprint Fun and Fingerprint Fuming for more details). 

3D impressions, however, are discovered in soft surfaces like dirt, sand, and mud, and are preserved by casting. The casting process is similar to how a dentist might make a model of your teeth: a substance is poured into the impression, hardened and then removed, creating a 3D model of the object that made the print. Investigators sometimes even use dental stone as a material for casting.

Objectives

  • Create a cast of impression evidence (handprint or footprint) and analyze it.

Materials

  • Part 1
    Per Student:
    aluminum-foil roasting pan or pie tray
    popsicle stick
    1 yogurt container

    Per Class:
    sand (enough for 1 inch in each foil tray)
    water
    1 juice pitcher
    1 package plaster of Paris

  • Part 2
    Per Student:
    paper
    washable ink pad
    ink roller
    felt pen

Key Questions

  • Why do forensic scientists need to cast impressions?
  • If you were to commit a crime, what would you do to make sure you left no impressions behind? Is it possible?
  • If you were an investigator coming to the scene of a crime, what would you do to ensure that potential impression evidence would not be disturbed? Where would you look for evidence?

What To Do

Part 1: Making the casts

Preparation
Measure plaster of Paris and water according to the manufacturer’s instructions. Don’t combine until just before you’re ready to use it!

Activity

  1. Fill each pan with approximately 2–3 cm (1 inch) of sand.
  2. Add water and mix using the Popsicle stick, so that the sand is damp.
  3. Pick a body part that is most likely to leave an impression at a crime scene (e.g., hand, foot, shoe) and firmly press it into the sand. (Hands leave the best outlines.)
  4. Mix together the plaster of Paris and water in the juice pitcher and quickly pour around 300ml of the mixture into each yogurt container. (Pre-marking the side to indicate the required level is helpful).

Hint: This step should be done by the teacher to ensure it happens quickly, before the plaster of Paris begins to harden. Ensure students work quickly from this point for best results.

  1. Pour the mixture inside the outline of the hand- or footprint and spread the plaster around with a Popsicle stick so that it is evenly distributed.
  2. Wait 45 minutes or so for the plaster of Paris to harden.
  3. When the cast is dry, gently dig away the sand from around and beneath the cast. Carefully retrieve the cast from the tray. Hint: The plaster may be fairly hard at this point but it can still break or crumble. Handle the casts with care.

Part 2: Play a game using clues from the casts

  1. Cast owners secretly put a symbol or letter on the back of their cast so that only they can identify it.
  2. Make a print of each cast by rolling the ink over its surface and pressing it to a piece of paper. Cast owners write their name at the top of the paper.
  3. Give each cast a number and place them all face up so everyone can see. Place all the prints with students’ names visible on another table.
  4. Using their powers of observation (without touching the casts), students try to match the casts to the prints