For those who might remember the 18th century recipe I've described at presentations, posted elsewhere, and probably somewhere here (ie oak gall, rainwater, iron sulfate, gum arabic), one small addition I neglected to mention in online discussions:
Some old recipes add cloves to the ink mixture. The idea is that cloves prevents or at least inhibits growth of mould in the organic concoction. I've wondered. I've tried it, with some success, but unsure of the correct amount to work.
A new experiment using a 1/4 tsp in a 1/4 C. of the liquid drained off oak galls soaked in snow water has now been mould free for almost a month (though the tannic acid is beginning to etch the plastic container). Previously, this same liquid had moulded within 3 days of being drained.
Too small a sample, perhaps, to make a general assertion, but . . .