Iron gall ink is made by soaking crushed oak galls in water for 3-5 days, boiling the liquid down, adding iron sulfate, and binding it with gum arabic at 10% by weight; it writes pale and darkens to black as it oxidises. Carbon ink is soot suspended in a gum binder and does not fade. Berry and plant inks fade within a season, so use iron gall or carbon for any record meant to last.
Part of Organic Chemistry
Writing inks from oak galls and iron, from soot, and from plant juices, and what makes each last or fade.
Two Chemistries: A Reaction Ink And A Suspension Ink
Iron gall ink is a chemical reaction. Tannin from oak galls meets iron(II) from iron sulfate to form a colourless complex that, once exposed to air, oxidises to a dark iron-tannate. That is why the carrier says it writes pale and darkens over hours. It bonds into the paper and, as the carrier notes, resists water and lasts centuries. The corpus covers it in Iron Gall Ink and Iron Gall Ink (education).
Carbon ink is a physical mixture of fine soot in a binder. It does not react with the paper; the carbon sits on the fibres and cannot fade because carbon is inert. See Carbon Ink and Carbon Black Ink. General making guidance is in Ink Making; plant dyes as colours are in Colored Inks.
What You Need
- Oak galls, the round growths caused by wasp larvae, crushed.
- Iron sulfate (green vitriol). The carrier gives two sources: iron dissolved in sulfuric acid, or the water from soaking rusty iron in vinegar.
- Gum arabic, the acacia tree sap used as binder.
- Water, a pot, a strainer and bottles.
- For carbon ink: soot from a clean flame, gum binder and a small amount of water.
Method: Iron Gall Ink, Following The Carrier
Step 1: Crush the galls and soak them in water for 3-5 days.
Step 2: Boil the liquid down to concentrate it.
Step 3: Add the iron sulfate.
Step 4: Add gum arabic at 10% by weight as binder.
Step 5: Strain and bottle.
The ink will look pale on the pen and dark on the paper after some hours.
Worked Example: Gum Arabic In A Batch
The carrier’s binder rate is 10% by weight. If the 10% is taken on 200 g of ink liquid, the gum is 200 × 0.10 = 20 g, and the finished ink is 220 g with 20 ÷ 220 = 9.1% gum. If the 10% is meant on the finished batch, the batch is 200 ÷ 0.90 = 222.2 g and the gum is 22.2 g. The carrier does not say which it means, so decide in advance and write it in the batch record.
Common Mistakes
- Dissolving iron in sulfuric acid in a closed bottle. The reaction gives off hydrogen gas, which is flammable; do it outdoors and open. This is the general behaviour of iron with dilute acid.
- Expecting an immediate black. It darkens slowly by oxidation.
- Judging a plant ink by day-one colour. Berry inks fade within a season.
- Using the strongest acid-iron ink on thin paper. Iron gall ink is acidic and can eat through paper over very long periods; this is a general archival observation, not a corpus figure.
Safety
Sulfuric acid and hot galls
Concentrated sulfuric acid destroys tissue; if used to dissolve iron it must be diluted by adding the acid to water, never water to acid, and the hydrogen released can ignite (see Sulfuric Acid). Rusty-iron-in-vinegar is the gentler route. Keep soot out of the lungs: wear a cloth over the nose when scraping it.
What’s Next
- Learn next: Fuel Chemistry — the next branch of the organic workshop
- Combine with: Mordanting — the same iron and tannin chemistry applied to cloth