Key concepts
Forensic science
Chemistry
Biology

Introduction
Do you know that you leave fingerprints on everything you touch? If your hands are very dirty, this is obvious because you can actually see them. But even if your hands seem clean, your fingerprints will stay behind on the surfaces you touch—they are just invisible! Do you want proof? Then make them visible in this activity and collect your own fingerprints!

Background
Detecting invisible fingerprints is an important task in forensic science, a branch of science that helps criminal investigations by collecting and analyzing evidence from crime scenes. Each fingerprint pattern is unique to a specific person, and therefore a very reliable way of identifying a suspect. There are different types of fingerprints that can be left behind: an imprint in a soft surface, such as wax or soap; a patent fingerprint, visible to the naked eye, such as those resulting from dirty hands; and latent fingerprints, which are invisible.

These invisible latent fingerprints are made of water, fatty acids, amino acids and triglycerides—in other words, they result from the oil and sweat that your skin produces naturally. To make them visible, you have to find a way to detect one of these substances present in the invisible fingerprint. The easiest method is called dusting, in which you use a very fine powder that can stick to the oil in the fingerprint. Once the fingerprint becomes visible, you can lift it from the surface with clear tape and transfer it to another surface to then take into the laboratory to analyze further. Other methods include using chemicals that react with the amino acids or water in the fingerprint; the chemical reaction results in a colored fingerprint, which you can then analyze easily.

Many factors determine the quality of a fingerprint on a surface. One of the most important factors is the surface texture. Fingerprints are most easily detected on smooth, nontextured dry surfaces. The rougher or more porous the material the more difficult it will be to get good fingerprint evidence. Another factor is the skin condition of your fingertips. If they are very sweaty and oily, you are more likely to leave behind prints than if they are dry and clean. Test it yourself, and collect your own fingerprint evidence like a real crime scene investigator in this activity!


Materials

  • A glass or smooth metal surface (where you have permission to test for finger prints!)
  • Raw cacao powder if your test surface is light; baby powder if it is dark
  • White paper if your test surface is light; black paper if it is dark
  • A small bowl
  • Clear tape
  • A fine brush with soft bristles, such as a makeup brush
  • Dust cloth
  • Water
  • Soap
  • Hand lotion


Preparation

  • Pour a little of the cacao or baby powder into a small bowl.
  • Wipe the smooth glass or metal surface that you are using for your experiment very thoroughly with the dust cloth.


Procedure

  • Choose one finger of one of your hands and press it firmly onto the clean, smooth surface. Remember where you put it on the surface. Can you see your fingerprint?
  • Dip your brush carefully into the cacao or baby powder. (If your fingerprint is on a dark surface, use the baby powder; if on a light one, cacao.) Only the tips of the bristles should be covered with powder. Then, tap the brush to remove any excess powder.
  • Gently sweep the brush carefully over the area where you put your fingerprint. Make sure to not apply too much pressure so that you do not wipe the print away. If necessary, add more powder to the brush, but be careful not to add too much. What happens when you apply the powder to the surface? Can you see a fingerprint developing?
  • When you are done, gently blow off excess powder from the surface and check your result. Did you get a visible fingerprint? How well can you see the print? Can you make out the fingerprint pattern?
  • Use a piece of clear tape and carefully press the tape down onto the surface on top of the developed fingerprint. Peel the tape off and transfer the fingerprint onto a piece of paper. (If you used dark cacao, use white paper; for baby powder, use black.) Did you successfully transfer your fingerprint? Does it still look the same on the paper compared with your glass or metal surface?
  • Wash your hands thoroughly with warm water and soap. Then, repeat the steps with the same finger you chose before. Did you get a fingerprint again? How does it look compared with the first one? Can you still see lots of details?
  • Finally, apply some hand lotion and repeat the fingerprint steps again. How does this fingerprint compare with the previous ones? Is it easier or more difficult to detect?
  • Once you have transferred all your fingerprints onto a piece of paper, compare your results. Do they all look the same? Which one can you see the best, the least? Can you explain your results?
  • Extra: In this activity you tested a nice, smooth glass or metal surface. Do you think other surface textures or materials will result in fingerprints as well? There is only one way to find out! Test other materials such as paper, textiles or wood. How do fingerprints look on these surfaces?
  • Extra: Now that you are like a professional in collecting fingerprints from surfaces, try to find them in your house! Where is the best place to look for them? Can you find your own or some from your family members and make them visible? Be sure you have permission to dust on surfaces before you test them.


Observations and results
Were you able to collect some of your own fingerprints? On a smooth surface such as glass or metal, fingerprints tend to stick very well. With your unwashed hands, you should have been able to make your fingerprint visible with either cacao or baby powder. Just a little powder applied with a brush should be enough to reveal your fingerprint. If you apply too much powder, however, the fine details of your print might have gotten lost. When you press too hard onto the surface with the brush, the fingerprint will be wiped away—so you have to be careful when treating the surface with the powder.

Your freshly washed hands have much less skin oil and sweat because they have been washed away with the soap and water. This results in a much less pronounced fingerprint. You might have had difficulties in collecting this fingerprint or may not have found one at all. On the other hand, if you apply hand lotion, which contains lots of oil and fat, this will make your fingertips much stickier, leading to a much more pronounced print. You should have seen a big fat fingerprint once you applied the powder to the surface where had you pressed your finger. If you compare all the prints you collected, the one with hand lotion should be most visible whereas the impression with your washed hands should be barely visible.

If you did the extra activities, you might have noticed porous or rough surfaces or materials such as paper or textiles are not very good for collecting fingerprints. Considering your results, where would you look for fingerprints in your home? Did you find some?

Cleanup
Clean out your brush, wipe your fingerprinted surfaces with the dust cloth and dispose of the used baby or cacao powder in the trash.

More to explore
How Does Fingerprint Powder Work?, from Scientific American
How Do Different Surfaces Affect the Preservation of Finger Prints?, from University of California, Santa Barbara, ScienceLine
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

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