Iron gall ink

Make inks from iron sulfate and tannin!

10 minutes
Experiment's video preview



  • 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

My secret message is not revealing. What to do?

Put the marker again into tannin solution and simply wait a little bit more: probably, it hasn’t yet absorbed the solution. Proceed to the next step.

Step-by-step instructions

We will use an empty marker to write a secret message with tannin.


Write your secret message.


Apply the iron(II) salt FeSO4 solution to the paper with your inscription. The tannin reacts with the iron ions, forming an intensely-colored compound.


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.

Scientific description

What is tannin?

Tannins are a compound class of plant origin. They can be found in tea leaves, nuts, oak, and other tree bark. Tannins boast a brown-yellow color, astringent taste, and discreet, pleasant aroma (you may carefully smell the solution). The main ingredient of the tannin solution in our kit is tannic acid, which is one of the most easily-available tannins.

Why does the secret message appear?

Tannin molecules are able to 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 tannin solution with 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

Tannins may bind not only to iron (II) Fe2+ ions but also to other metal ions: for instance, copper (II) Cu2+.

Initial copper sulfate and tannin solutions are slightly colored. Thus, you may write your message with CuSO4 solution, following up with an application of tannin-soaked cotton pellet to reveal your message.

Otherwise, you may do everything in opposite order: write your message with tannin solution and then make it visible with cotton pellet wetted in copper sulfate CuSO4 solution.

That's interesting!

Astringent protection

Tannins can 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, they discovered that mixing certain leaf extracts with iron salts produces permanent inks that are much more water-resistant. These revolutionary (at the time) 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 an 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!