Zinc dust

Reveal fingerprint with zinc powder

Difficulty:
Danger:
Duration:
5 minutes
Experiment's video preview

Reagents

Safety

  • Put on protective eyewear.
  • Conduct the experiment on the plastic tray.
  • Be careful not to blow or brush against the metal powder.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

The fingerprint is very poorly seen; the entire glass remained covered with a layer of powder.

The reason may be the presence of moisture or other contaminants on the glass. Thoroughly clean the glass from all contamination. This can be done with alcohol or alcohol-containing liquid, acetone, nail polish remover or just water with a detergent. After cleaning, dry the glass, and then redo the experiment.

The finger print turned out blurry and indistinct.

It is easy to improve the quality of the print. First rub the pulp of your finger against your forehead and then gently but firmly press it against a clean dry glass. Then repeat the procedure with zinc powder.

Also, try to use adhesive tape as gently as possible: it is very easy to blur the print with it!

Step-by-step instructions

  1. Leave a fingerprint on a glass slide, preferably in the center. Remember the spot where you left the fingerprint. Now put your protective gloves on.

  2. Pour out some zinc powder Zn onto the spot where you left the fingerprint.

  3. Carefully shake the glass slide to distribute zinc on its surface.

  4. Holding the glass slide over a piece of filter or regular paper, turn it upside down. Tap from the other side of the glass slide.

  5. Turn the glass slide over again.

  6. Carefully stick an adhesive tape over the fingerprint.

  7. Take the tape off.

  8. Attach the tape to a piece of white paper.

  9. You've successfully lifted a fingerprint! Examine the fingerprint and add it to your records.
Graphical step-by-step instruction

Expected result

Small particles of zinc powder stick to fats remaining on the glass slide. It reveals the fingerprint. Using of a scotch tape, one can even transfer the fingerprint on a paper surface.

Disposal

Dispose of solid waste together with household garbage.

Scientific description

Why does zinc reveal a fingerprint?

The surface of the fine zinc particles readily adheres to the oily residue of the fingerprint, creating the dark ridges you see when you transfer the print onto the white paper.

Learn more

To better understand why the zinc powder reveals a fingerprint, it is important to understand the “wetting” properties of the oily residue in the fingerprint. The term “wetting” refers to the ability of a liquid to maintain contact with a solid surface, resulting from the intermolecular interactions when the two are brought together. It is quite easy to see which substances wet a surface well. Spread a thin layer of liquid onto a solid surface. If the liquid collects into small droplets, it means it cannot wet to the surface; that is, it cannot adhere or stick to it. Alternatively, if the liquid forms a thin layer, it means it is wetting, or adhering to the the surface. In the case of this experiment, the metallic zinc is wetted with the oily residue in the fingerprint. However, it cannot be wetted with water.

detective_zinc_wetting

Zinc powder in a drop of water (on the left) and oil (on the right). It is clearly visible that the oil wets or adheres with the zinc as opposed to the water.

Our fingerprints contain a significant amount of skin oils. Therefore, the zinc powder sticks to a fingerprint but not to the surface around it. Excess zinc powder can easily be shaken off onto a clean dry surface. Avoid pouring zinc over a wet fingerprint–it wouldn't provide a sharp picture - because the zinc powder doesn’t wet with water. Other powders that possess similar properties to zinc, are for instance, aluminum (Al), zinc oxide (ZnO), and graphite (C). In the following video you will see how a fingerprint is developed using an aluminum powder:

How to enhance the quality of a fingerprint?

Criminalists have a saying: “Only about 20% of the population leave clear fingerprints, and the others are too greedy.” Meaning, some people do not have enough oily residue on their fingertips to leave a nice, clear fingerprint. In order to obtain a good quality fingerprint, simply increase the oil content on your fingertip. To do so, rub your forehead with your finger intensely for 2-3 seconds.

Learn more

Although our fingertips contain no oil glands, why do we find oil on them? Believe it or not, we regularly touch our skin and hair reflexively with our fingertips. These touches are enough to cover our fingerprints with a significant amount of oil; enough to provide a good fingerprint!

Interestingly, it is now possible to tell how much time has passed since a fingerprint was left on an object, based on the distribution of oils within a fingerprint. Current technology can be used to determine the degree of aging of the oily residue. For example, the palmitic acid (C16H32O2) distribution profile provides information about fingerprint age, but only within the first 96 hours. Unfortunately, this technology is not yet used today because it requires expensive equipment and special training for the personnel.

detectives_iodine_palmitic Palmitic acid, C16H32O2.

Follow up

Fingerprints on wood

You can use the same method to reveal fingerprints on a smooth wooden surface. To remove the excess zinc powder use a soft brush. Do not blow off the zinc powder!

Most importantly – remember that this should only be done on a surface that you do not intend to eat of. Also, don’t forget to wipe off all the powder after the experiment. For this purpose, use a slightly wet paper towel.

Graphite is a real finding for a detective

Instead of zinc powder use graphite. Graphite powder is easy to obtain just by crushing the lead of an ordinary pencil.

The true detective

This kit is called “Chemistry for detectives” and so it’s time to start an investigation.

Take (do this in gloves so as not to leave your own finger prints) a few glasses and ask your friends and family members to leave their fingerprints on some of them.

With the use of table starch powder and a soft brush find fingerprints on the glasses. Ask all the participants to pass a fingerprint analysis – leave samples of their fingerprints. You can make a “database” using the instructions to this experiment or by using graphite powder instead of zinc. Try to reveal by the outlines, who of your family members left their prints on which glass. Compare the fingerprints carefully! And remember, they are unique to every person!

That's interesting!

What are argents?

The word “argent” originated from the Latin argentum, meaning “silver.” In dactyloscopy, argents are special powders with metallic luster that reveal clear pattern when deposited on fingerprints. Mainly, they are powders of zinc, aluminum, and bronze. An advantage of using these powders is their ability to reveal fingerprints both on dark and light-colored surfaces, including metals.

Interestingly, metals may be used to find fingerprints even on very sophisticated surfaces, such as fabrics. This is where advanced technologies come into play: metals here are not used in form of powders, but are instead deposited on a surface from vaporous state, in a special vacuum chamber. A sequential deposition of gold and zinc over an analyzed area reveals a golden pattern on a silvery background. Before this method has been invented, it was impossible to obtain a clear image of a fingerprint on fabric.

Powders in dactyloscopy

detectives_expert photo by Etan J. Tal

From the very beginning through today, dactyloscopy methods are based on powdering fingerprints with various substances. It would be impossible to list all the formulations used for such purpose. The main requirement to such powders is their ability to adhere to a fingerprint stronger than to an adjacent surface itself. Numerous compounds meet this condition. Though, let’s focus on the main principles of using powders in dactyloscopy. The application technique is essentially a thin-layer powder deposition on a surface, followed by removal of excessive powder. After that, a fingerprint pattern is transferred with an adhesive tape onto another surface to store the obtained information in a database.

Depending on the color of a surface with fingerprints, various colored powders would qualify. For instance, in order to find traces on a dark background, light-colored powders are used: white lead (2PbCO3*Pb(OH)2), bronze, zinc oxide (ZnO), etc. And for light-colored surfaces – on the contrary: graphite (C), iron oxide (Fe2O3), aluminum (Al), and zinc (Zn) powders. There is continuous search for new powders to use in criminalistics. Today, the leading role is taken by fluorescent powders: they glow under ultraviolet illumination. Their main advantage is economical factor: a minimal possible amount of such powder is used to reveal fingerprint traces.

detectives_three_prints image is a derivative of photo «Fingerprintonpaper» by Katpatuka and photo «Paluchy» by JohnRambo PL

Besides classification into dark, light-colored, and fluorescent, powders in dactyloscopy are divided into magnetic and nonmagnetic. The latter have been used since the very early development of dactyloscopy and are still applied today. They are deposited with a soft wide brush, and then excessive amount is removed. Nonmagnetic powders include zinc, graphite, aluminum, white lead, and zinc oxide. With technical progress, magnetic powders found more applications. They are deposited with a magnetic brush, so as excessive amount of powder is instantly drawn back to the brush. Today, magnetic particles (iron (Fe) and iron oxide (Fe2O3)) serve as a base for various powders development in dactyloscopy, including fluorescent powders. The ease of deposition and low consumption during their use make magnetic powders the most promising for use in forensic science and criminalistics. Here is the procedure of developing fingerprints with this type of powder: