A comparative study of leather hardening techniques (2018)

Several methods mentioned here were tested [PDF warning]:

The Waterer discussion, source of the “stearic acid” technique(s)

The Turner paper, source of the gelatin technique(s)

You may want to read those in full before continuing to familiarize yourself with common leather hardening techniques. I ended up with consistently sized and shaped leather samples well suited to the coming test methods. Instead of a completely untreated piece of leather, I needed a sample without significant hardening that could still maintain the domed shape required for certain tests.

Source: onicrafts.com