Gall-iron ink once used widely
Today, I waited in a parking lot for my daughter to come out from her workplace. I noticed that every pin oak around the building was afflicted with galls. Galls are lumps that form on tree branches when the larvae of insects invade. It is thought that the insects emit a chemical that reacts with the tree's natural growth hormones and growth is grossly overstimulated in the area.
If I had cut a gall open, I probably would have found some of the larvae of the invading insect. If it is the horned oak gall wasp, the number of larvae inside the gall might have ranged from 1 to 160, depending on the size of the gall (Source: Biology and management of the horned oak gall wasp on pin oak.)
The galls usually don't kill pin oaks, but they can cause the branches to die back. I observed this condition on several of the pin oaks around the parking lot. And even if the tree doesn't die, it's greatly disfigured. See the image of a gall-laden tree at this post: Galls on a young pin oak tree.
In the book American Forest Trees (published by Hardwood Record, Chicago, 1913, pp.303-304), Henry H. Gibson records an interesting historic fact about galls in a chapter about pin oaks:
Oak apples or galls are the round excrescences formed on the limbs by gallflies and their eggs. They seem particularly fond of this species and specimens are often seen which are literally covered with them. The worms which live inside seem to flourish particularly well on the food they imbibe from pin oak.
The primitive school teachers three or four generations ago turned these oak galls to account. They are rich in tannin and were employed in manufacturing the local ink supply. The teachers were the ink makers as well as the pen cutters when the pens were whittled from quills
The process of making the ink was simple. The galls were soaked in a kettle of water and nails. The iron acted on the tannin and produced the desired blackness, but if special luster was desired, it was furnished by adding the fruit of the wild green brier Smilax rotundifolia which grew abundantly in the woods. It was well that steel pens were not then in use for the schoolmaster's oak ink would have eaten up such a pen in a single day.
Iron-gall ink was the primary writing ink used from 12th through the 19th centuries in the Western world. Popular with artists, architects and mapmakers, iron-gall ink is found in abundance in paper-based collections at the Library of Congress.
The tannin in these inks seems to hold up well, but the iron is a problem for conservationists because it wants to corrode.
I doubt that oak galls have been put to a useful purpose since the days they were used for ink-making!