Have you ever seen an apple on an oak? If you did, you might want to do a retake, because you should realise the fruit of an oak is not an apple, but the acorn. But yet sometimes you can see these small bulbs (2 to 5 cm) on branches or twigs on an oak. These are called oak apples, but are also known as oak galls. Galls can be present on pretty much all plant species and are comparable to what (benign) tumors are to animals. They are caused by parasites, fungi, bacteria, insects, … These secrete a kind of chemical that promotes rapid growth of plant tissue, and the resulting ‘apple’ is used as both habitat and food source. But not every gall you can see on an oak is an oak apple, there’s also oak marble galls, oak artichoke galls and acorn cup galls. Each have a different form that tells them apart, but there can be some confusion when it comes to comparing apples and marbles, as under some conditions and untrained eyes, they can look similar. Both are caused when a single egg of a Cynipidae gall wasp is deposited and the larva starts to hatch. So there is only one larva inside a single gall, right in the center, and there are several species who cause the same phenomenon. The most common European species to cause oak apples is Biorhiza pallida.

An oak apple, the brown areas are what’s left of the leaf buds

There’s also some folklore surrounding the peculiar gall. When an oak apple is opened on Michaelmas Day (29 September, the feast for Archangels, Michael in particular, it’s mostly a Serbian thing) and a ‘worm’ (aka the gall wasp larva) is found, the year will be pleasant but nothing extraordinary. A spider found is a bad omen, foretelling a disastrous harvest with food shortages. A fly means an average season, but if nothing at all is in there, it spells serious diseases all year round. Given that you probably can find nothing at all in the gall more likely than a spider or fly, we should all blame oak apples for everything that has happened in 2016.

Another much more practical use that oak apples were usually used in, is the production of iron gall ink. The ink is made by mixing iron salts and tannic acids. The iron usually came from iron sulfate, but nails and iron metal scraps worked just as well. For the tannins, it could come from vegetables, but most commonly used was, obviously, the oak apple. The tannic acid was released from the galls by fermentation or hydrolysis.

Mommy iron/vinegar, baby iron gall ink and daddy oak apple extract.

The ultimate colour of the iron gall ink was a purplish black, due to the oxidation of the iron. This meant that the ink had to be stored in well-sealed bottles as too much oxidation rendered the ink unusable. It also meant that after decades or centuries, small rusty halos could form around the letters written with it, which in turn affects the parchment or vellum it was used on. The ink also attached to the paper through mechanical, not chemical, bonds and could not, like with india ink, be erased by normal rubbing or washing. Only when you actually scraped the top layer off, you could remove the ink. There was the problem that iron gall ink is acidic as well, which means that it sometimes degraded the paper it was used on. This was solved by adding crushed egg shells which neutralised the solution.

Ink erosion

Despite these setbacks, iron gall ink was the standard ink used in Europe from the 9th century onto the 20th for both writing and drawing due to its ease to make, water resistance and (relative) permanency. Pliny the Elder (23 – 79 AD) supposedly had an early recipe for oak gall ink, but this is not entirely undisputed and mostly vague. The Codex Sinaiticus, the oldest of the four great uncial codices which contain the entire text of the Greek Bible, was written in iron gall ink. Many manuscripts of the Middle Ages and Renaissance used this ink as well, with lamp black or carbon black ink in addition. There were specific laws that dictated the content of iron gall ink to use in royal and legal records so that they would not go lost in time by fading letters. The reign of iron gall ink ended with the invention of other waterproof, chemically produced inks in the second half of the 20th century.

Traditional iron gall ink would clog fountain pens which use capillary action to function. The modern mixture of iron gall ink differs in a few aspects, making them usable in fountain pens as well addressing the potential problem of paper damage. Even then, the manufacturers advice to clean the pens more thoroughly than with conventional inks, with diluted vinegar or ammonia if necessary. These days it’s only used by art enthusiasts (read: hipsters) as well as some legal documents that need a permanent ink. It sees some use in birth, marriage and death certificates in the U.K. and in German civil law notary legal instruments.

Whatever that means.

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