To harden leather, it is heat treated. Here is the short version:
I very strongly recommend you read a very comprehensive document about this subject that have been around even when the internet was text-only:
"HARDENED LEATHER" by I. Marc Carlson from the University of Tulsa, Oklahoma.
I also attach a snapshot of this document as it was on 6/1/09.
This is from Leatherworker.net (by Daggrim - see his leatherwork at leatherhelms.com):
I've had lots of experience with Cuir Bouilli, since every helmet I make is treated to harden the leather. I just use the hot water, and the leather is 10 to 13 oz saddle skirting.
The procedure is very time and temperature critical. I aim for a water temp of 175 to 180 degrees, and the immersion time varies from 2 to 3 minutes. A difference of just 5 degrees can cut your time in half, or double it, so you need to watch closely. I pull it out for inspection every 30 seconds or so. As you treat more pieces, the water turns dark red, and you can't see the leather.
I presoak the peices in room temp water, because if they go in dry, the thermal shock will just shrivel them. Do not allow the peices to touch the pot, it'll make hot spots, and shrivel it there. I use a folding round vegetable steamer in the pot to isolate the leather.
Water at 170 will have some hardening effect, but you'll need to leave it in there for maybe 4 minutes, and it'll never get really, really hard. It'll dry more like heavy cardboard than armour. If you start with stiff leather, it hardens very well, and if you start with floppy leather, it'll also never get really hard. When the leather starts to turn dark...get it out asap, because that's the point when it starts to shrivel. Leather brought to that point gets super hard. The flesh side dries like sandpaper, and the edges have just begun to curl.
I've hardened 8 oz leather, but it's tricky. You can't fudge on the temp...hit it with the full monty...175 degrees, but watch it like a hawk. Itll start to shrink earlier than heavier leather, and so you might need to do some tugging to reshape it. You have about 30 seconds to get the leather into a mold after you pull it. It's gonna be rubbery, so be careful or you can distort it with careless handling. It'll start to set up in about 15 minutes, and then you can moderately shape it for about an hour.
I've taken pieces that never got hard enough, and resoaked them, them dried them in the oven at 200 degrees. They kinda flatten out and sag, but you can reshape them. I leave them in for 30 or 40 minutes. They scorch where they touch the pan, or the screen.
So, there it is...way more than you asked for. I read lotsa stuff on the internet when learning this technique, and I took the parts that made the most sense to me. I'm also a little lazy, so I always try to K.I.S.S. a job as much as possible. I hate the Fuss Factor, and try to cut it out of all my processes. And lastly, the temps are what I see on my candy thermometer, so there might be variations with your thermometer.