Oak gall ink is one of the most durable and useful inks available from the natural world. If you are not familiar with oak galls here is a bit background information about them.
The oak galls, sometimes referred to as oak apples, are grown by the tree in response to the larvae of the gall wasp injecting chemicals into the developing leaf buds. There are about 800 species of gall wasp in North America alone. They are small creatures, less than 1cm long. Different species stimulate different types of galls. Some galls are on the leaves and some are on the branches. There are even knopper galls in Britain that are modified acorns. The larvae develop in the galls feeding on the inner gall tissue until they metamorpihize into a wasp and exit the gall. Once the process is complete the gall turns brown and often falls to the ground on a windy day. The galls are not harmful to the oak tree. Look for an exit hole on the oak gall to know the wasp has emmerged. Fall is the most likely time to find empty oak galls.
For the purposes of ink making oak galls are an excellent source of tannins, specifically gallic tannin. Tannins are occur in many woody and flowering plants. They serve as a deterrent to herbivores. The color of black and green tea is caused by the tannins in the tea leaves dispersing into the hot water.
When iron is added to any dye/ink containing tannins the ink will darken. In the case of an oak gall solution, the ink will shift from a tan or brown color to grey or black. The source of the iron can vary. I personally use Ferrous Sulfate crystals purchased from a natural dye supplier. Here is one place you can buy them. You can also make your own iron solution by steeping rusty metal items, such as old nails, in vinegar. Here is a recipe. (Note the vinegar will affect the pH of your ink.)
Because of oak galls high concentration of tannins you can use them to make a true black ink. When an oak gall solution is combined with ferrous sulfate it creates ferrous tannate. Follow this link for more details about the chemistry of oak gall ink. The resulting ink is waterproof and soaks into the paper creating a stain that is not erasable with conventional methods.
Oak gall ink was the primary ink used in Europe from the 4th to the 19th century. It is the ink that was used to write the United States Constitution and the Declaration of Independence. These links will take you to detailed descriptions of the ongoing conservation of these oak gall ink documents at the National Archives.
Oak gall ink will darken on the page as it is exposed to air. By the time it is dry it will be fully darkened. As you are applying it to the page it will initially appear grey. Some historical recipes contained logwood dye so the ink would be darker as it is applied. The logwood is not very lightfast and would eventually fade leaving only the oak gall.
Loose Instructions for Making Oak Gall Ink:
Begin by soaking oak galls in water to make an oak gall solution. To speed up the process heat the water and/or grind the oak galls into smaller particles to expose more surface area. Steep the oak galls for several weeks at room temperature or several hours over heat. Boiling will not harm the color. This will create a tan or brown tea depending on how concentrated the particular oak galls are in tannins. Before using the solution strain out the oak gall solids. The oak gall solids can be saved and used multiple times. I dry them in the oven on low heat so I can store them and use them again.
Add ferrous sulfate to the oak gall solution. Start with a small amount, perhaps a 1/4 teaspoon of crystals dissolved in a bit of hot water for 2 cups of oak gall solution. Stir it in and let is sit for an hour or so. The iron will react over time. If the solution is still brown in tone add some more iron until you get a grey or black tone. Too much iron will show up as the rusty color of unreacted iron with the black. Too much iron is hard on paper, but otherwise not a problem. Add more oak gall solution if adjustment is necessary. The ink will have a chalky texture when dry on paper. Add some gum arabic solution, either the crystals predissolved in water or the premade solution from the art supply store, to get a stable, glossy ink that does not rub off on your fingers when dry on the page.
As always experiementation is the best teacher.