Iron Gall Ink
Difficulty: Easy | Time: 20 minutes | Visual Impact: High
Historical Context
Iron gall ink was the dominant writing ink in the Western world for over a thousand years. The Magna Carta (1215), the Gutenberg Bible, Leonardo da Vinci’s notebooks, Bach’s musical manuscripts, and the original US Declaration of Independence were all written with iron gall ink. It was made by combining oak galls (the tannin-rich growths caused by wasp larvae on oak trees) with vitriol (iron sulfate) - a recipe unchanged for centuries.
The chemistry was not understood until the 19th century. Early chemists recognized that “the combination of a metallic salt with a vegetable acid” produced the black color, and that the initial pale gray-blue ink darkened as it dried and oxidized. Ferrous ions (Fe²⁺) in the iron sulfate coordinate with the polyphenol groups of the tannin; air then oxidizes Fe²⁺ to Fe³⁺, forming the intensely dark, insoluble iron(III)-tannate complex.
There’s a dark irony: the same chemistry that makes the ink permanent also destroys the paper. The acidic ink slowly hydrolyzes cellulose fibers over centuries, eating through the very documents it was used to record. Conservators today work to halt this process in archives worldwide.
Materials
- Tannic acid - 1g dissolved in 50mL water
- Ferrous sulfate - 0.5g dissolved in 20mL water
- Gum arabic powder - a pinch (optional, improves flow and prevents feathering)
- Dip pen, toothpick, or small brush
- White paper
Procedure
- Dissolve 1g tannic acid in 50mL water - produces a pale yellow solution.
- In a separate container, dissolve 0.5g ferrous sulfate in 20mL water - pale green.
- Pour the ferrous sulfate solution into the tannic acid solution and stir.
- The ink turns blue-gray immediately as iron(II) tannate forms.
- If using gum arabic, add a small pinch and stir until dissolved.
- Write with a dip pen or brush. The ink starts blue-gray.
- Watch the writing darken over the next 5-10 minutes as the iron oxidizes in air to give the permanent blue-black color.
Bonus - Natural Tannin Sources: Replace the tannic acid with strongly brewed black tea or red wine. Both contain enough tannin to make functional ink - you can see and compare the different shades.
Reactions
Initial complexation (blue-gray, soluble):
\[\ce{Fe^{2+}_{(aq)} + tannin -> [Fe^{II}\text{-tannate}]_{(aq)}}\]
Air oxidation gives the permanent black (insoluble) form:
\[\ce{[Fe^{II}\text{-tannate}] + \tfrac{1}{4}O_2 + \tfrac{1}{2}H_2O -> [Fe^{III}\text{-tannate}]_{(s)} + OH^-}\]
The Science
Tannins are polyphenols - large molecules with many phenolic -OH groups capable of chelating metal ions. Iron(II) from ferrous sulfate coordinates to these groups, forming a soluble blue-gray chelate. When exposed to air, Fe²⁺ is oxidized to Fe³⁺, which forms a much stronger chelate that is insoluble and deeply colored. The writing literally “develops” as it dries and oxidizes - a natural color reaction happening in real time.
This is the same oxidation-state difference behind Prussian blue synthesis and the rust on iron - the transition between Fe²⁺ and Fe³⁺ compounds produces dramatically different colors.
The tannic acid also precipitates proteins, which is why strong tea (also rich in tannins) makes your mouth feel dry - it’s cross-linking the proteins in your saliva. The same interaction binds the ink permanently to paper fibers.
Explore Further
Compare oxidation states: Set aside a small sample of the fresh blue-gray ink. Over several hours, compare it to ink written on paper. The paper sample darkens; the sealed sample may not. What does this tell you about what the air is doing?
Try natural sources: Brew very strong black tea or steep crushed oak bark. Add iron sulfate and compare to the tannic acid version. Are natural tannin sources as effective? Why might they differ?
pH and color: What happens if you add a few drops of baking soda solution (basic) to the finished ink? Try it - the color should shift.