What’s an oak gall?
In the picture above you see an autumn oak leaf, discarded by its tree having extracted all the sugars it can beforehand. What look like cankers on the underside of the leaf are galls. Two are pretty obvious: the large ball is an oak marble gall; then there is a rash of common spangle galls; and if you look even closer, you’ll see a tiny patch of silk-button galls – smaller than the dew drops next to them.
These galls enclose the larvae of various species of wasp, laid as eggs by females. When the larvae hatch, they trigger a chemical response in the leaf, changing its normal growth processes to create the gall. The galls protect the growing larvae until they burrow their way out of the gall. If you see a mature, brown marble gall hanging from an oak tree, the chances are you’ll see the tiny holes made by exiting wasps tunnelling out, ready to start their own lives as adults – looking for an oak leaf to lay their eggs on.
How many species of gall wasp are there?
Quite a few: our two native oaks can support about 70 species, of which those shown on the leaf in the picture are some of the most common. Others include the large knobbly galls formed by the larvae of the oak knopper gall wasp, and the artichoke gall, which looks like a smaller version of the vegetable. The RHS has a good identification guide: https://www.rhs.org.uk/biodiversity/oak-gall-wasps
Do they do any harm to the tree?
No: they enjoy a long-evolved partnership with the oak, which is unaffected. Part of the natural cycle of the oak means that it grows a new set of leaves every year – sometimes twice a year. The relationship is just one of those wonderful natural adaptations. If you want a more exotic, grislier tale of wasp-life, check out fig-wasps – a positively gruesome life cycle!
What is oak gall ink?
The larger galls: marble, knopper and oak apple, concentrate tannins. Tannin is a natural poison which does not harm the wasp larva; and it’s also concentrated in oak bark. Tannin is a fantastic preservative and dye – oak bark is used in the traditional process of tanning leather. Medieval monks also used to make a very dark black permanent ink from it. You can do the same (there are many recipes online). Essentially, you simmer the galls to produce a liquor; make a ferrous sulphate solution from soaking wire wool in vinegar for the few days; add the two to make the ink and reduce to the preferred consistency. You can add gum Arabic as a binder.
One of the great works of Early Medieval writing is a single-volume bible produced at the monastery of Wearmouth-Jarrow in the first decade of the 7th century AD and gifted to the Pope. It’s called the Codex Amiatinus (a long story) – but its lines were written in oak gall ink, and they are as clear today as they were thirteen hundred years ago.
