I initially saw this on Hackaday, but thought the armorers, cosplay, and re-enactors at DMS might find it of interest. In this article, several (16) techniques for hardening leather (wax, boiling, epoxy, steric acid, and others) are tried and tested, then improvements on the initial tests tried.
Good, practical advice on making hardened leather items (including food-safe leather bottles) as well as a good “design of experiments” article.
Worth a read, even if you’re NOT planning to make something of hardened leather.