I’ve been doing more research on medieval ink recipes. It’s all a bit complex, and it was clear to me that there were many different recipes all of which would work to a greater or lesser extent. There were different proportions of oak galls, iron sulphate, gum arabic; some used vinegar, some wine, some both.
To add to the fun, there were also ink recipes that used carbon black, but they appeared a bit earlier than oak gall ones and were out of date by the 13/14th centuries.
There has also been a lot of discussion amongst amateurs, and perhaps experts too, about why some ink ends up brown and some black. Lacking any analytical equipment, it had occurred to me that the black ones were lampblack and the brown ones were oak gall ink.
Now if only all the combative people would catch up with modern research which I have below the fold…

It turns out I was wrong, because brown inks come about from adding too much iron sulphate to the ink. In fact the recipes examined by researchers have a ratio of about 5.5:1 of ferrous sulphate to tannin (see Neevel 1995), meaning there was about 50% more iron present than necessary. Over time the excess iron ends up as rust, iron oxide, which is brown. Therefore the presence of brown writing is understood as a result of the poor ratios of the recipes of the time, but they were not to know that.
The precise chemistry of it all is complex and apparently not properly elucidated until the 1990’s.
(Plant and other natural product chemistry is usually horribly complex, which is one of the reasons biotech is taking a bit longer to get anywhere)

Even worse, this excess of Fe ions means that, depending on the conditions, the Fe atom can become an Fe 2+ or 3+ ie. Lose electrons, and take part in reactions which degrade the paper that it is on. So conservationists have to try and neutralise the excess iron ions and that would be a post by itself I think. The moral of this story is don’t add too much iron sulphate to your ink if you want the documents to last a long time and the ink still be black.
This is of course not helped by the fact that English oak galls have the lowest about of tannins, the best being those from Spain or Aleppo.

How it works is:
That the Fe2+ in solution from the iron sulphate (FeSO4) becomes Fe3+ in contact with air and gallotannins, which are molecules made of a benzene ring with carbon and oxygen links off from it, the substance of interest being pyro-gallate, formed from the hydrolysis (that is, loss of water) of gallotannic acid. The point being that there is a molecule of the sugar glucose with 5 of these gallotannic acid groups attached, which all get split off on their own when you add the iron sulphate and heat it. The use of wine, vinegar or hydrochloric acid also encourages this, as does fermenting the mixture for a couple of weeks, because of a mould which breaks down the glucose sugars releasing the gallotannins. (Botti, Mantovani and Ruggiero)

The simplest method of avoiding any problem with your ink over the next few hundred years is simply to not use too much iron sulphate. If you try taking the authentic recipes and divide the amount of FeSO4 by 5 that should be close enough. It’ll be interesting though to see how much of a difference it makes; if in doubt add some vinegar to the solution when preparing it to help maximise the yield of the gallotannic acid.
I shall have to experiment on this myself this season. My inks so far have been surprisingly brown, although maybe I did add too much iron sulphate. The problem being that I am not preparing ink in industrial quantities – one oak gall will do for a couple of days, so getting the proportions right is hard.

I am still looking for decent recipes for red and green ink. Red was commonly used for marking the start or important points in a text, and was apparently commonly red lead or sometimes vermillion (cinnabar, mercury sulphide). Green was sometimes used and made with verdigris, copper carbonate, the question is, how? Grind it small, add gum arabic? I need to find a recipe to be sure.

Calcium Phytate in the Treatment of Corrosion Caused by Iron Gall Inks: Effects on Paper
Restaurator. Volume 26, Issue 1, Pages 44–62, ISSN (Print) 0034-5806, DOI: 10.1515/REST.2005.44, January 2008

Neevel, Johan G.
Phytate: A Potential Conservation Agent for the Treatment of Ink Corrosion caused by Iron Gall Inks, Restaurator 16 (1995): 143-160.