Knowing nothing about hardening leather apart from what I've just read here, if reaching a certain temp without going too high or too low is the way to harden l

Knowing nothing about hardening leather apart from what I've just read here, if reaching a certain temp without going too high or too low is the way to harden leather armor sous vide might be a good experiment. No chance of overheating like in an oven. I guess it would depend on how much of the hardening process is the drying action and how much is the heat itself. You can vacuum seal around a mold of the shape you want, and then sous vide the sealed leather. Perfect temp control, and no risk of scorching.