LATENT FINGERPRINTS - Chemists - Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)

Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)

9. Chemists




Glass microscope slide or other small glass object

Glass jar with cover (big enough to hold the microscope slide)

Paper towel

1 to 2 drops of superglue

Saucepan of water

Kitchen stove

Oven mitts

Jar of iodine crystals from “Extracting Iodine” project (page 187) Business card, note card, or other small white card Hairspray

When scientists provide help for law enforcement, that is called forensic science. In this project, you will act as crime lab chemists to develop latent fingerprints—that is, fingerprints that you normally cannot see and that can’t be made visible by “dusting for prints.”

You will use two different chemical techniques in this project. One is good for developing fingerprints on smooth surfaces such as glass or plastic. The other works for surfaces like paper.

The first technique uses superglue, known to chemists as cyanoacrylate. Superglue is a liquid that polymerizes (hardens) when it encounters water vapor. But a greasy fingerprint makes it harden faster, and that is the key to this technique.


Start out by making a nice fingerprint on a glass microscope slide (a conveniently small piece of glass—you can substitute a small jar or other glass object).

Rub your finger on your nose or forehead where the oils from your skin are most pronounced, and carefully press your finger on the glass so as not to smudge the fingerprint.

Place the microscope slide in a glass jar. (I used a binder clip to make it stand up, but that is not really necessary.) Next, add a piece of paper towel, moistened with a bit of water. This will provide the water vapor for the glue to harden. Last, add a drop or two of superglue, being careful not to drop it on the slide or the paper but just on the bottom of the jar. You don’t want to glue anything down accidentally.

Cover the jar tightly.

Now you want to boil the cyanoacrylate. It boils at a fairly low temperature compared to water, so the easy way to do this is to boil some water, then take the pan off the stove and put the jar into the almost-boiling water.

Look into the jar at where your fingerprint was on the glass slide. It will not take long before it starts to appear, as the cyanoacrylate vapor meets the greasy fingerprint and starts to harden, leaving a white fingerprint visible on the glass.


If you leave it in the jar too long, the white ridges of the fingerprint will continue to grow more polymer until the ridges merge together and the fingerprint is ruined. So when it looks like a good sharp image of a fingerprint, open the jar and remove the glass slide. The print will look something like the picture below.

Since the superglue hardens into a white print, it would not help very much on a white piece of paper. Luckily, there is another technique for developing latent fingerprints that works very well on paper.

For this you will use the jar of iodine crystals we made in the project “Extracting Iodine” (page 187). By this time, there will probably not be any visible crystals in that jar, as they will have evaporated and then recondensed on the walls of the jar in a thin brown film. Don’t worry, the iodine is still there and will work nicely for this project.


Rub your finger on your nose or forehead again and press it carefully but firmly on a white card, such as the back of a business card or a 3-by-5 note card. If the card is too big to fit in the jar, cut it to size. Then simply drop it into the jar of iodine and close the lid again. In less than a minute, you will start to see a faint tan fingerprint appear. Wait a few more minutes, and the tan will develop into a nice dark brown that clearly shows the ridges of your fingerprint.


Make sure you close the jar of iodine so it doesn’t evaporate away. You can use it again many times before you have to make a new batch of iodine.


The iodine print will gradually fade away if left in the open air. To preserve it, you can spray it with a little hairspray. The hairspray leaves a thin coating of plastic that holds the iodine in place.

An important part of a scientist’s work goes a step beyond the scientific method. Discovering a new bit of knowledge is of little use to anyone but the discoverer unless it is published. And then the publication can lead to other scientists retesting the hypothesis to make sure the original work was done properly and without errors. It can also lead to alternative hypotheses to explain the data and the experimental results, leading to new hypotheses and tests to see what is really going on.

In chemistry, this general scientific approach to learning new information is used all the time. So chemists are scientists. But so are mechanics, judges, animal trainers, and parents. Anyone who uses the scientific method can be a scientist.