Iron oak ink is probably my favorite medieval ink. You can write with it, draw with it, outline with it, and dye with it. It is also very very permanent, because it chemically bonds with the strata. The black hours manuscripts that everyone loves? Dyed with Iron gall ink.
The color of the ink comes from the interaction of some type of tannic acid and Iron sulfate, historically called green Copperas. Dont let the name fool you, there is no copper involved. Upon contact, the light tan tannin solution turns black.
Oak galls, which are formed by the gall wasp's reproduction cycle, seem to be the favorite source of tannin. In that case, it's gallo-tannic acid. But you can use literally any part of the oak tree to make a permanent ink. St. Edmund's College at Oxford has a probably-16th-century recipe for acorn ink. Which I have tried and used with varying levels of success, including the year I won the Æthelmearc Arts & Sciences Champs.
The first question at that point should be: why the variable results? Followed by: how do we control the outcome?
One of the main adjustments we modern people need to make when working like our medieval forebearers is to let go of the idea of standardized materials. While we can explain the chemical reactions that take place with a detail that folks 800 years ago would be amazed at, they were for the most part much better at troubleshooting. Because they had to be. Modern tools and materials are fairly easy to bend to our will. Period materials come with a lot more restrictions in many cases, and we need to work with them.
(This is my personal hobby horse, and it comes up frequently. Medieval artisans had a level of material knowledge and mastery that blows my mind.)
I can't figure out how to type chemical equations on here. Working on it. But the reaction we are wanting is for the Copperas or Iron sulfate to react with the tannin. Once everything is balanced out, we can see that this requires twice as many Copperas molecules as tannin molecules. Great. How do we do that?
Copperas can be manufactured at home by mixing vinegar with anything Iron and letting it get gunky in a glass jar for a few months. LEAVE THE LID OFF THE JAR. This is an exothermic reaction, and it will explode if left under pressure.
In the most general terms, the limiting ingredient in amy chemical reaction is the one that runs out or gets used up first. In this case it's the vinegar. There comes a point where all the vinegar has been turned to Copperas, and the reaction will stop. This also means that the Copperas you make at home is going to be pretty standard in strength. Yay! Let that evaporate, and we have 100% by weight Iron sulfate. Go us!
Say we want to use 2oz. Copperas to make our ink. That means we need 1oz. of tannin.
On average, oak galls are approximately 50% tannin by weight. This is an overall average of the average of six or seven species of oak. If we know what type of oak our galls are from, we can get more specific. The range is from about 40% to about 80% tannin by weight, so there can be a significant difference in the amount required.
If we are going with the 50% tannin by weight average, we need 2oz. of oak galls to get 1oz. of tannins. Tannins are generally extracted by soaking the galls in water for several days, sometimes with heat.
My preference is for 1 pint of water for this basic recipe, so we put 2oz. crushed oak galls in 1 pint water and let it leach tannins for 5 to 7 days. We can then decide to strain the gall pieces out of what is basically yucky tea, or leave them in and strain at the end. Then add the 2oz. copperas, and gum Arabic as a binder. My personal preference is 1.5oz.
If the gum Arabic doesn't want to dissolve, low heat will encourage it.
With huge enthusiasm, we test our ink....
...and it looks dirty. Even after it dries, it's a pale drab gray. What went wrong?
NOTHING! We just have an incomplete reaction. And we can be reasonably sure the copperas is full strength, so we need to add what? Tannin.
It can be a fussy process, and we want to add only a little at a time, but the ink will get progressively darker as more of the existing copperas molecules bind with the newly introduced tannins.
And our end product is:
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