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Galls and Copperas

Ink kept from oak trees and old nails

The Recipe

  1. Crush 30g of dried oak galls coarsely — a pestle is enough, they do not need to be powdered, only opened.
  2. Steep in 250ml rainwater, covered, for five to seven days at room temperature, swirling once a day. The liquid turns from pale amber to a deep tea-brown.
  3. Strain through cloth into a clean jar. Keep the spent galls; they still have some tannin left for a second, weaker steep if needed.
  4. Dissolve 12g of ferrous sulfate (copperas) in 50ml warm water separately, off to one side.
  5. Add the iron solution to the gall liquid slowly, a little at a time, stirring. It will darken almost immediately from brown toward blue-black — this reaction is the ink.
  6. Stir in 15g gum arabic, pre-dissolved in a little warm water, to bind the pigment and slow it settling.
  7. Let stand, covered, for 24 hours. Skim any scum. Bottle. It will darken further over the following week as it oxidises on the page.
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The Gall

Oak galls form where a gall wasp lays an egg in a leaf bud; the tree's reaction to the larva builds the gall around it. The ones worth collecting are the hard, round "oak apple" type from Quercus robur, not the soft spring galls, which are low in tannin and rot before they dry properly.

Ripe galls are picked in late summer once the wasp has already emerged — a small round exit hole confirms this and also means the gall is not still growing. A gall taken too early, before the exit hole, has the wasp still inside and higher moisture; it will mould in storage more often than not. Spent galls (post-exit) keep for years in a dry tin.

The Iron

Copperas is ferrous sulfate, historically a by-product of pyrite weathering, sold cheaply for centuries as a dye mordant and only later as a garden treatment. It is harder to buy plain now than it was; garden-centre "sulphate of iron" lawn conditioner is usually the same compound but check the label for added nitrogen, which will affect the reaction.

An old iron nail left to rust in a jar of vinegar for two weeks makes a workable substitute solution, weaker and slower but genuinely usable — this was, in effect, the household method before copperas was bought ready-made.

Notes on Failure

Batch 14, too little ironThe ink stayed brown rather than blue-black and never fully darkened on the page. Traced to a copperas jar left open for a month — it had oxidised to a form that no longer reacts properly with the tannin.
Batch 9, mould in the steepLeft the galls steeping in a warm room for nine days instead of seven; a white film appeared on the surface by day eight. Discarded rather than risk it — mould byproducts are not something you want on a page meant to last.
Batch 21, too little gumInk separated in the bottle within a week, pigment settling to a sludge at the bottom that would not redisperse by shaking. Gum arabic was old stock, likely under-strength; replaced with fresh and the same recipe held fine.