Iron gall ink

Make your own inks from iron sulfate and tannin!

Scientific name: The reaction between tannin and iron(II) sulfate leads to dark color of the writing. The secret message becomes visible!


Iron gall ink - Make your own inks from iron sulfate and tannin!

by MEL Science

Iron Gall Ink: A Quick and Easy Method

by Letter locking

Gall Invisible Ink (reveal: Copper II Sulfate or Iron II Sulfate)

by Letter locking


Put protective gloves on. Conduct all the experiments on the tray.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Perform this experiment

Reaction formula

FeSO4 + tannin → Fe"["tannin"]"SO4

Step-by-step instruction

WARNING! This experiment is dangerous! You should NOT perform this at home. ONLY carry out this experiment, if you are a trained chemist, and you understand local safety and legal requirements, which are required to perform such experiments

  1. Fill 1/4 of an empty plastic vial with 1% tannin solution (about 2 mL).
  2. Dip the brush into the tannin solution.
  3. Write a secret message on yellow paper between the lines or on any blank space.
  4. Let the paper dry. It will take 2–3 minutes.
  5. Moisten a cotton pad with 0.1M iron (II) sulfate solution FeSO4. You will need 15-20 drops.
  6. To reveal the message, rub the paper surface with the cotton pad.
  7. Wash the brush with water for use in future experiments.

Scientific background

What is tannin?

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

Why does the secret message appear?

Tannin molecules are able to bind with metal ions: for example, iron Fe or copper Cu (see the “Follow-up” section to learn how to reveal the secret message written with copper sulfate CuSO4). The reaction forms complex molecules exhibiting bold dark color. Hence, mixing tannin solution with iron sulfate FeSO4 solution (containing iron (II) Fe2+ ions) provides dark coloring and renders the secret message visible.


Published on 17 January 2016

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst