How to Make Ink without Even Trying, Or, What Are Those Purple Spots?

In both the History of Atlantian Monarchs and Nastasiia's girdle book, I used a combination of oak boards and alum-tawed goatskin. In both those books, there were one or two small places on the leather that got slightly discolored: a light purplish stain. Luckily, I could either cut away those patches and replace them with inlays or cover them with furnishings. But more troubling was the fact that I had no idea what was causing the discoloration: was I stretching the skin too much? Using too much paste?

When I covered my next book, a small girdle book with a separate overcover, the problem got much worse: not only was the skin more discolored than on previous books, but there were also two large purplish-black spots on the inside of one of the boards. What was going on?

After asking other binders and posting my problem to various mailing lists, the answer appears to be that a chemical reaction occurs to create a type of iron-gall ink. The tannic acid from the oak boards reacts with metals ions, either from the alum leather, the brass nails I used to secure the thongs, or iron particles from the knife I use to cut the corner miters. The two large spots on the inside board are centered around the brass nails, so the brass is clearly contributing something. The discoloration of the leather is worst around the board edges, where I spent the most time handling the leather and the boards while doing the turnins. Two useful websites about the chemistry of ink are http://www.knaw.nl/ecpa/ink/index.html and http://www.scienceinschool.org/2007/issue6/galls/, but as I am not a chemist, some of that information is over my head.

Other binders have suggested covering books in stages (spine one day, boards another day, and turnins yet another day) to keep the process as clean and dry as possible. The less moisture, from sweat or paste, the less the chemicals will interact with each other. Of course, another possibility is to avoid using oak with alum-tawed leather. Only future books will tell whether these steps are successful.