"Fingerprinting" Return to list of pracs

Aim: This activity is used to demonstrate finger printing detective work.

Simple Fingerprinting:

Equipment: The equipment includes...  

• spatula;
• sheet of newspaper;
• carbon powder;
• soft brush;
• glossy paper, filter paper, black plastic, cloth.

Method: Lay a sheet of newspaper on a table. Put one of the sample paper or cloth on the newspaper and press your finger into it to make a fingerprint. Sprinkle a very small amount of carbon powder onto the reverse side where the fingerprint is. Gently brush away any carbon powder. Try some of the other samples.

Background knowledge: Filter paper and cloth will not create a good fingerprint. Fingerprinting is only good on some surfaces. You can investigate this with your class. A similar thing can be done using white powder on the same surfaces. No two people have the same fingerprint.

What makes fingerprints? Sweat glands release water, salt and urea on the surfaces you touch. The sebaceous glands release oil onto the surface of your skin also. Small amounts remain behind when you touch things. Fingerprints are sorted into 3 types of prints: arch, loop and whorl. Fine dark powder is used to show up fingerprints on light, smooth surfaces and fine white powder is used to show up fingerprints on dark, smooth surfaces.

Fingerprinting from stick surfaces:

Equipment: The equipment includes...  

• stick tape;
• gentian violet;
• 250mL beaker;
• tweezers;
• glass rod;
• scissors.

Method: Cut a piece of sticky tape and stick it to a glass rod (like a flag exposing some of the sticky side to the tape). Press your finger into the sticky tape near the end. Carefully pour 100mL of gentian violet into the beaker and then lower the sticky tape into the beaker solution. After 15 minutes lift the glass rod from the beaker and run the sticky tape under water thoroughly to remove the stickiness. The fingerprint should now be visible.

Background knowledge: Gentian violet, mixed with phenol and alcohol, can be used to show up fingerprints on stick surfaces. The phenol, which is an acid, takes away the stickiness and the fingerprint is stained a deep purple colour by the gentian violet.

Fingerprinting from rough surfaces:

Equipment: The equipment includes...  

• screw jar mid containing iodine crystals;
• filter paper;
• clear tape;
• tweezers.

Method: Press your finger on to a piece of filter paper. Put the filter paper into the jar of iodine crystals and screw the top on. When you can see the fingerprint clearly, remove the paper using the tweezers. Cover the fingerprint on both sides with clear tape. Repeat this procedure with another piece of filter paper but this time do not cover it with clear tape.

Background knowledge: Iodine crystals are used to show up fingerprints on rough, absorbent surfaces. These crystals give off iodine vapour which dissolves on the tiny drops of oil in the fingerprint. The fingerprint then shows up more clearly.