Fingerprints are a vital clue for crime-fighting. Sometimes fingerprints can be seen with the naked eye, but often they are invisible. This week, we will make some fingerprints visible, using the same basic method as many forensic scientists.
You will need
Some aluminium foil
Superglue
A clean jar, with a lid.
Magnifying glass
An adult
CAUTION: Read the safety instructions on the super glue and follow them carefully. Fumes from superglue can be toxic. Do this activity in a well-ventilated area. Avoid getting super glue on your skin, as it can stick instantly. Have an adult assist you with this activity.
What to do
Tear off a small piece of aluminium foil.
Make three fingerprint on the foil:
Make one fingerprint by just putting your fingertip down on the foil.
Run your fingertip through your hair, then make another fingerprint on the foil.
Rub your finger against the side of your nose, then make a third fingerprint.
Place the foil in the jar, facing upwards. I found it was easier if I laid the jar on its side.
Use a second piece of foil to make a cup, about the size of a thimble. I found the easiest way was to take a small piece, wrap it around my fingertip, then flatten the end.
Place the cup in the jar.
Put 5-10 drops of superglue into the cup.
Seal the jar.
Leave the jar for two hours.
You should find the fingerprints on the foil became visible.
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 on them. When you touched the aluminium foil, some of this sweat was left behind as a fingerprint.
You have probably noticed that super-glue has a particularly strong smell. This is caused by chemicals called cyanoacrylates evaporating from the superglue. When you put the superglue in the jar, the cyanoacrylates evaporated from the glue and filled the jar.
Cyanoacrylates will react with chemicals in human sweat to produce a white sticky material. (That's why superglue sticks finger together so well.) As the cyanoacrylates in your jar reacted with the fingerprints on the foil, the chemical they produced gathered on the fingerprint, making them visible.
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, you will see that at some points 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.
A fingerprint. Can you find some identifying points that could match this fingerprint to a suspect? I found over 30. Roll your mouse over it to see the ones I could spot. (This is a negative image of one of the fingerprints I made on the foil)
This activity was adapted from the Forensic Science KAOS Kit, available through www.csiroshop.com You can learn lots more about forensic science in the November/December issue of Scientriffic.
Applications
Police departments have used fingerprints to identify people for over 100 years. They were first officially introduced by Scotland Yard in 1901. Today, the FBI has fingerprint records of over 250 million people.
Many police departments use superglue to make fingerprints visible. Unlike dusting for fingerprints (where fine dust is used to make fingerprints visible), superglue can be used after the sweat has dried, because the chemicals that react with cyanoacrylates are still left behind. Superglue is most effective on non-porous surfaces, such as glass, plastic or metal.
Humans aren't the only species with fingerprints. Many primates, such as gorillas, chimpanzees and some monkeys have fingerprints. Some monkeys even have fingerprint-like ridges on their tails. Koalas have them as well
