Making a secret Halloween picture
Only under adult supervision.
Equipment and reagents
- 5 mL 1% tannin solution;
- 5 mL 0.1 mol/L iron(II) sulfate solution;
- empty marker;
- cotton cylinders;
- watercolor paper;
Use a pipette to fill an empty marker with 5 mL 0.1 mol/L iron(II) sulfate solution. Draw a colorless web on a sheet of paper and develop it using a cotton cylinder soaked in 1% tannin solution.
Tannins are a class of plant-based organic compounds found in leaves, nuts, oak wood, and the bark of other trees. Iron(II) sulfate solution mixed with tannin solution forms the well-known iron gall ink. Upon exposure to air, this grey solution forms an intensely purplish-black iron(III) tannin compound that fixes firmly to the paper. Such ink dates back at least 1000 years, and was used all the way up to the 20th century. And did you know? A similar experiment is included in the “Chemistry for spies” set from the MEL Chemistry subscription!