Iron gall ink

Make inks from iron sulfate and tannin!

10 minutes
Iron gall ink


  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
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

I accidentally used the black marker. What should I do?

No problem! Continue the experiment.

I measured 20 drops of FeSO4 into a plastic cup, but the solution isn’t covering the bottom of the cup.

That’s perfectly fine! You just need to lower the tip of the marker into the solution of iron sulfate so that the cartridge can absorb the solution.

How long should I hold the tip of the marker to the filter paper?

You just need to touch the filter paper a few times for a couple seconds. This should blot away any excess solution.

My secret message isn’t appearing. What should I do?

Try to recall whether the solution was dripping from the other end of the marker when you first filled the cartridge. If it wasn’t, then add iron sulfate solution to the cartridge until the solution starts dripping from the other end.

If you did everything else correctly, is it possible you didn’t dip the tip of the marker into the puddle of iron sulfate in the cup? In this case, lower the tip of the marker into the iron sulfate solution and wait 2 minutes.

Can I conduct this experiment with ordinary paper?

There is enough paper in the set to repeat the experiment several times. If you really want to use ordinary paper, you can certainly try! Document any differences in your outcomes, and let us know the results of your observations!

How long will the marker last?

Good question! Try figuring out the answer experimentally – test the marker once a day and note when it stops working. Why might the marker stop working? Usually it stops working when it dries out. Just dip the tip of the marker into some more iron sulfate solution. You can even add a few drops of iron sulfate solution directly into the cartridge. Have fun!

Step-by-step instructions

Fill a marker with iron sulfate FeSO4 solution. This compound is non-volatile and rather pale, and thus is well-suited to the role of invisible ink. As long as you know how to make it visible, at least.


Iron sulfate FeSO4 solution doesn't have a very intense color. And the small amount of it on the paper is practically invisible.


Tannin solutions are also relatively pale, but apply some to the paper with your FeSO4 inscription.


The tannin combines with Fe2+ from the FeSO4 to form a strongly-colored compound, allowing you to see the message clearly.


Expected result

The reaction between tannin and iron(II) sulfate gives the text a dark coloring. The secret message becomes visible!


Dispose of solid waste together with household garbage. Pour solutions down the sink and wash with an excess of water.

Scientific description

This compound of tannin and Fe2+ is actually so good at leaving traces on paper that it has been used as an actual ink for centuries. It is most well known as "iron gall ink." The "iron" part is FeSO4, a chemical known to humankind since time immemorial. And the "gall" part refers to so-called "oak galls"—outgrowths of oak trees  provoked by a certain species of wasp. These galls are a rich source of tannins. In fact, tannins are found not only in oak galls, but also in oak bark and leaves, as well as in other plants. One particular plant that contains a lot of tannins is Camellia sinensis, the plant that plain old tea is made from! If you run out of the tannin provided in this set, you can probably guess how to develop your secret messages nonetheless.

What is tannin?

Tannins are a compound class of plant origin. They can be found in tea leaves, nuts, oak, and some tree bark. Tannins are brownish-yellow and have an astringent taste and discreet, pleasant aroma. If you’d like, you can try carefully smelling the solution. Do you know the safe, proper way to smell chemical substances? Hold the container in one hand approximately 20–25 cm (9–10 inches) from your face. Use your other hand to gently “fan” or “waft” the air above the bottle towards your face. The main ingredient of the tannin solution in the set is tannic acid, which is one of the most easily-available tannins.

Why does the secret message appear?

Tannin molecules can bind with metal ions such as iron Fe or copper Cu (see the “Follow-up” section to learn how to reveal a secret message written with copper sulfate CuSO4). The reaction forms complex molecules with a bold, dark color. Hence, mixing a tannin solution with an iron sulfate FeSO4 solution (containing iron(II) Fe2+ ions) darkens the mixture significantly and renders the secret message visible

Follow up

Copper sulfate and tannin

In addition to iron(II) Fe2+ ions, tannins can also bind to other metal ions such as copper(II) Cu2+.

Solutions of tannin and copper sulfate CuSO4 are both almost colorless. Try writing a message with CuSO4 solution, then using a tannin-soaked absorbent to reveal your message.

You can also do everything in reverse: write your message with tannin solution and then make it visible using an absorbent wetted in copper sulfate CuSO4 solution.

That's interesting!

Astringent protection

Tannins can be found in many plants, often protecting them from the encroaching outside world. Tannins can bind to protein molecules, thereby inhibiting the activity of various microbes and preventing them from attacking their host plant. Moreover, most animals and insects dislike the astringent taste of tannins and avoid plants where tannins abound.

Because of their ability to bind with proteins, tannins are used as hardening agents for leather and even as a topical anti-inflammatory medicine.

Moreover, tannins’ restorative powers are utilized in the medical industry to treat bleeding, bowel dysfunction, and diarrhea. Tannins can act as an antidote against mercury or lead salt poisoning: they bind firmly to the cations of heavy metals while staying soluble in water, which allows them to pass through and out of the body.

In medicine, we can also observe what is perhaps tannins’ most unusual application: surgeons tan their hands prior to prolonged surgeries. This concept is pretty similar to that of tanning leather goods. This treatment closes the pores in the skin, leaving the surface of surgeons’ hands sterile for much longer than usual. Obviously, this makes surgical procedures much safer: even a tiny rupture in protective gloves can cost someone their life if a wound is infected from a non-sterile hand.

Permanent inks

Writing turned out to be a tipping point in the growth and development of human civilization. For a long time, people used carbon and various soot-based inks to put their thoughts on paper (or parchment).

In the Middle Ages, people discovered that mixing certain leaf extracts with iron salts produces permanent inks that are much more water-resistant. These then-revolutionary inks bind so strongly to paper thanks to the fact that their Fe2+ cations are oxidized into Fe3+ in the open air. This reaction doesn’t change the ink’s color, but makes it water-resistant and sets it more permanently on paper. As it turns out, in our experiment we recreate an ink-preparation procedure as it was practiced in the Middle Ages and up to the 20th century!