Tough Fingerprints

As you learned in Project Dusting Fingerprints, some surfaces are better than others for lifting fingerprints. On certain surfaces, even dusting will not make a latent print visible. However, forensic scientists have discovered new ways to make fingerprints visible on difficult surfaces. Try the following activity to learn how to collect tough fingerprints.

Materials

- jar with a lid or other closable container
- two 3-inch (7.5 cm) squares of aluminum foil
- super glue (or glue that contains cyanoacrylate)
- timer

Procedure

1. Open the jar and set it on its side.
2. Put extra oil on your fingertip by rubbing it alongside your nose or in your hair.
3. Press the oiled fingertip on one square of aluminum foil, and then put the foil in the jar.
4. Squeeze a small amount of Super Glue onto the other foil square. Put the second piece of foil next to the foil in the jar.
5. Close the lid of the jar and wait about 30 minutes.
6. Observe the first piece of aluminum foil, the one your finger touched. What do you see?
CAUTION: Be careful not to get the glue on your hands.

More Fun Stuff to Do

Try this procedure with other materials. Repeat the experiment, substituting the aluminum foil with squares of cardboard, wood, cloth, and other materials that did not work very well in Project Dusting Finger Prints.

Explanation

A fingerprint made on aluminum foil cannot be detected using normal procedures, such as dusting, and has to be treated with chemicals. One such chemical is cyanoacrylate, a chemical used in Super Glue and similar products. In this activity, cyanoacrylate in the glue rises into the air. Trapped in the closed jar, this chemical sticks to the invisible oils in the fingerprint and hardens on them. As more cyanoacrylate particles harden on the fingerprint, it becomes visible and can then be easily identified.

There are other chemicals that can be used to make latent fingerprints visible on certain surfaces. Ninhydrin is a chemical that reacts with sweat and oils in the fingerprint to produce a purple image. It is ideal for lifting fingerprints from paper and some other porous surfaces, like bare wood and plaster. Iodine vapor can bring out fingerprints made on rough, absorbent, light-colored surfaces, such as paper and cloth. The iodine vapor reacts with the oil in the fingerprint and turns the fingerprint brown.

Detective Science in Action

Fingerprints first played a major role in solving a murder in 1902. Joseph Reibel was found dead in his apartment in Paris, France. The detective assigned to the case was Alphonse Bertillon, one of the first detectives to collect and record the physical characteristics of criminals, including their fingerprints. At the crime scene, Bertillon found broken glass from a cabinet and blood on the carpet. It was obvious the intruder had cut himself and unknowingly left behind fingerprints on several pieces of the glass. By comparing the fingerprints on the glass to those of several known criminals, Bertillon was able to match them to the fingerprints on the record card of Henri-Leon Scheffer, a convicted swindler. Scheffer was picked up by the police, and burdened with guilt and remorse, he confesses to crime.