Dusting Fingerprints

How can detectives identify fingerprints left at the scene of a crime? They must remove the prints and transport them back to the crime lab, where they can compare them to other fingerprints on file. One way to locate fingerprints is by a technique called dusting. The fingerprints are coated with powder, then lifted and taken to the lab for identification. Try the following activity to learn how to dust and lift fingerprints.

Materials

- drinking glass
- cocoa
- small paintbrush
- transparent tape
- 5 sheet of light-colored construction paper

Procedure

Note: You get a better fingerprint when more body oils are present. To do this, rub each finger alongside your nose or through your hair before making each fingerprint.

1. Press one finger at a time on the side of the drinking glass.
2. Sprinkle cocoa on the glass to coat the fingerprints.
3. Very gently brush the powdered area with the small paintbrush. When you’ve brushed off the loose powder, you should see the fingerprints.
4. Lift each print from the glass by placing the sticky side of a piece of transparent tape on the dusted finger print and then carefully lifting the tape from the glass. The dusted fingerprint should stick to the tape.
5. Place each piece of tape on a separate sheet of light-colored construction paper.

More Fun Stuff to Do

Try to use other surface to dust for fingerprints. Put fingerprints on a variety of surface, such as wood, cardboard, aluminum foil, smooth paper, rough surfaces with talcum powder and light surfaces with cocoa.

NOTE: Be careful to avoid breathing in the talcum powder, as it can irritate breathing passages. Place prints made with talcum powder on dark-colored construction paper and prints made with cocoa on light-colored construction paper. Which surfaces allow you to make fingerprints that can be identified using the dusting technique?

Explanation

If a criminal’s hands are covered with dirt, paint, or other substances, his or her fingerprints may be easily seen. These are called visible prints. Other prints may be harder to see and may need treatment with chemicals or dust to make them visible.
The best surface for dusting latent prints is one that is smooth, flat, and firm. Fingerprinting with powders does not work well on a surface that is rough or too flexible.

There are several powders that forensic scientists use to dust for latent fingerprints. Two common powders are (1) vegetable black, a fine carbon powder similar to the pencil scrapings that you used in Project Taking Fingerprints, and the choice for light-colored surfaces, and (2) aluminum powder, a fine white powder used on dark-colored surfaces. Forensic scientists use a fingerprinting powder that contrasts with the color of the surface on which the fingerprint is located. This makes the dusted fingerprints more visible and easier to photograph for evidence.

Forensic scientists also add other chemicals, such as lead, cadmium, copper, and mercury, to fingerprint powders to make them stick better to latent fingerprints. Fluorescent and phosphorescent chemicals can also be added to fingerprint powder to make dusted fingerprints glow in the dark.