Activity: Fingerprinting

Zoe with her file of suspect and evidence fingerprints

On SCOPE's Forensics episode, you saw Zoe dust for fingerprints at home. Here's how she did it. Watch the clip.

What you need

• A mug or something that holds fingerprints nicely
• A fine powder that is a different colour to the surface of the fingerprint (talcum or cocoa powder)
• Sticky tape
• Paper
• Soft pencil (2B)

What to do (evidence prints)

1. Lightly dust the surface containing the fingerprint with the powder.
2. The powder should stick best to the print, so gently blow on the surface to get rid of the excess.
3. By now you should be able to see the print.
4. Stick some sticky tape over the powdered fingerprint and then lift it off.
5. Now stick the tape on some paper the opposite colour to the powder (so it shows up nicely)

What to do (suspects' prints)

1. Colour a small section of paper with a soft graphite pencil
2. Press the suspects finger onto the graphite patch firmly
3. Lift up their finger and place sticky tape over the coloured area. Press down on the tape.
4. Peel off the tape and stick the print onto some white paper next to their name.
5. Repeat for their other fingers.

What's happening?

The tips of your fingers are covered in tiny ridges, which help us to grip things. These ridges normally have a small amount of sweat and oils on them. Whoever touched the mug, some of their sweat and oils are left behind as a fingerprint.

Depending on how much oil occurs naturally on your hair or skin, rubbing your finger through your hand or against your nose may have made your fingerprint more visible.

If you look at the ridges of your fingerprint through a magnifying glass, the ridges on your fingerprints will end, or have small gaps in them, or even split into two separate ridges. Forensic scientists use these points to match fingerprints. For them to be used in court, they normally need to be able to find a certain number of these points to match a fingerprint from crime scene to a suspect.

This method works because every single person on earth has their own unique fingerprints - even twins!