What Oak Galls Are, and Why They Make Ink

An oak gall is one of the woodland's quietest marvels. A gall wasp lays a single egg in the soft tissue of a leaf, bud or acorn cup, and the oak answers by growing a nursery around the developing larva — a marble gall, a knopper, an oak apple, a spangle. Inside that structure the tree packs tannins, and it is those tannins, chiefly gallotannic acid, that make ink. Crush a gall, steep it in hot water and introduce iron, and you get ferric tannate: a rich brown-black that has written a good share of Europe's surviving charters, parish registers and letters. The ink in many a medieval document began life as a lump like the one in your palm.

Making your own is a straightforward kitchen-and-campfire sort of chemistry, and it gives you a reason to walk the same few oak trees through the seasons, watching what the wood does.

Gathering Galls with a Light Hand

Late summer into early winter is the season. Knopper galls sit like wrinkled acorns on acorn cups; marble galls are smooth and round, often on the underside of leaves or on young shoots; oak apples are larger, spongy and green-flushed before they brown. Look on the tree first, then on the ground after a windy night — fallen galls are perfectly good and save you reaching into branches.

Remember that every gall is somebody's home. Take no more than one in ten from any single tree, and far fewer from young oaks, which have less to spare. Carry a cloth or paper bag rather than a sealed plastic one, so the galls can breathe and do not go mouldy. If you keep them in a jar indoors, you may find small wasps emerging weeks later. Tip them out beside an oak in spring and they will get on with their lives.

What You Will Need

• 15 to 20 dried oak galls, cracked into pieces
• 500 ml water — rainwater or filtered is better than very hard tap water
• A source of iron: a handful of clean rusty nails, or a little ferrous sulphate solution from a chemist
• Gum arabic, about two teaspoons
• A stainless steel or enamelled pan — avoid aluminium
• Muslin or a fine sieve, glass jars, a wooden spoon, a funnel
• Old clothes and gloves, because this stains permanently

The Brew: Simmering and Straining

Crush the galls roughly with a hammer or pestle — cracked pieces, not powder. Put them in the pan with the water and bring to a gentle simmer, lid ajar, for 30 to 45 minutes, until the liquid has reduced by roughly half and looks like weak tea. Keep it below a rolling boil; a hard boil muddies the colour. Let it cool, then strain through muslin into a clean glass jar, squeezing the pulp to get the last of the liquor.

Now the transformation. Add iron a little at a time: a few drops of ferrous sulphate solution, or two rusty nails left in the jar overnight, stirred now and then. The change is quick and startling — muddy brown to inky blue-black. Go slowly, because too much iron makes an ink that turns brittle and eats paper over decades. Then stir in the gum arabic, roughly a teaspoon per 100 ml, to thicken the ink and help it cling to a nib. Leave it to stand for a day or two and skim off any film that forms.

Testing, Bottling and Troubleshooting

Test on scrap paper with a dip pen or a fine brush. Good ink flows without flooding, sits brown-black on the page and darkens as it oxidises over the next few hours. If it is too thick, add a splash of water; too thin, a little more gum; too pale or grey, a few more drops of iron. Acidity is normal and part of what makes the ink bite into the paper, so leave it be.

Store in a tightly lidded glass bottle in a cool, dark cupboard, labelled with the date and the tree you took the galls from. Made with clean water and clean jars, it will keep for years. If a mould film appears, strain it through muslin and start a fresh bottle. Keep it away from textiles and off good tables; it does not come out.

Writing, Sketching and Looking After the Wood

This ink is a pleasure for sketching oaks, hedgerows and field margins with a dip pen or a brush, and it thins beautifully with water for soft washes. Iron-rich ink corrodes steel nibs if left to dry, so rinse your pen and brushes as soon as you finish, and a glass or gold nib will last indefinitely.

Conservation is the other half of the craft. A good foraging trip should leave the wood exactly as you found it, or better. Take few galls, from few trees, mostly from the ground. Leave seedlings and saplings alone, and never strip a branch to fill a bag. Note which oaks crop well and return to them next year rather than ranging wider. Keep a small record — date, tree, gall type, how the ink turned out — and a pantry project becomes genuine local knowledge. The wasps, the oak, the fungi in its bark and the birds in its canopy all run on the same tree. Take a little, leave plenty, and the inkpot stays full for years to come.