I've spent a lot of time searching on the Internet in regards to how to make cuirbolli (hardened leather) and found many methods involving heating, boiling, and waxing leather. What follows is what I've determined works for me.

Materials

Vegetable tanned leather is critical to making hardened leather. The other tanning methods create material that does not stiffen well due to fiber breakdown in the hide. Preferably, use undyed leather. Dye is shed in the water heating process, mucks up the water, and is again, more chemicals that affect the nature of the leather. In my experience, dyed leather shrinks more.

You can use any weight of leather, but remember that thinner leather will stretch/shape more easily when wet, and shrink more when drying. All weights of leather will thicken to some degree. You may get less shrinkage if you nail your leather to a wooden form, or sandwich it in a 2 piece mold with lots of weight on top. It's probably not so feasible to have a 2 piece mold for everything your working on. Instead, get to know your materials and test for shrinkage on scraps.

Make sure there are no major flaws in the piece you are hardening. large divots, cuts, and thin spots result in uneven shrinkage, and can warp the shape of whatever you are making.

Tools

Pans of various sizes, an accurate candy thermometer for low temperatures, cookie sheets, oven, gas range, and "forgiving" utensils are required.

References: leather http://oror.essortment.com/leathertanning_rdcu.htm other hardening links http://www.daviddfriedman.com/Medieval/Articles/Perfect_Armor_Improved.htm http://www.pbm.com/~lindahl/cariadoc/perfect_armor.html http://www.personal.utulsa.edu/~marc-carlson/leather/hl.html http://www.arador.com/construction/leather.html wax http://www.candlecauldron.com/waxandwicks.html http://www.candlewic.com/candle-wax/wax-overview.asp http://candles.genwax.com/candle_instructions/___0___wax_cndl_basics.htm

Soaking

Depending on how thick your leather is, you can get away with a quick soak, or have to soak the leather overnight. This soaking gets water fully into the material, and results in more even heating than just dipping dry leather into a pot of hot water. Soaking also gets a lot of dyes and tanning chemicals out of the leather.

Heating

I have found that it is important to use a fresh pot of water when heating the leather. If all the chemicals that came out of the leather are in the water, the leather may not harden as much. A fresh pot of clear water actually pulls more of the chemicals out of the hide as it heats, as well as permitting you to see what your leather is doing.

Use a pan that is just large enough to fully immerse your project. A larger pan will work, but that means the water takes longer to heat.

The thermometer needs to work in the 170 to 190 degree range. You don't want to boil your leather, just get it hot enough to relax the leather and get the water to soak fully to the core. Too high a temperature will cook the leather and make it brittle. Too low a temp will result in the leather never getting to the pliable stage.

You'll know the leather is ready when it easily flops and folds as you prod it with a stick. At this point, you need to get the leather out of the pot. You can heat it further, but this may result in your leather becoming too brittle.

Use a broad faced utensil to get the leather out of the pot. Wire tongs may leave a permanent impression in the leather where they squeeze into the hide. A forgiving utensil is anything that does not mar the leather in the process of getting it out of the water.

Shaping

At this point, you do your shaping. either drape the material on a form, stretch it over a curve, or squeeze it in a mold until it's cooled down and become stable. Thinner leathers may dry completely at this stage. Thicker leathers will not dry without further heating.

Oven Drying

As with the heating stage, you want the oven to never go above 170 to 190 degrees. Too much heat in too little time cooks and burns the leather, which destroys the fiber and makes the material brittle.

Most ovens do not have dials with numbers below 200 degrees. Use your thermometer to verify that your oven is working in the right range.

You can use cookie sheets for small items that slip between the bars on the rack. If your making flat pieces of leather (lamellar plates) you can sandwich them between two sheets. Larger pieces can sit directly on the rack.

Be aware that leaving the face of a piece of leather on a rack will result in grill lines like the ones you find on your favorite steak! Cookie sheets will result in a larger area of browning on leather faces. To avoid this stand the leather on an edge, or keep the flesh side down. Larger pieces are bound to be three dimensional enough to stand on an edge, or not touch the rack/cookie sheet at all with the use of various supports such as empty soda cans, paper towel tubes etc. Just make sure whatever you use does not combust at 200 degrees Fahrenheit.

You will want to pay attention to the leather as it is drying. Every two or three minutes, you'll should take the leather out of the oven, adjust the shape, turn the piece around, and put it back into the oven. This ensures that the leather shapes properly and does not get cooked. As you do this, the leather will become increasingly stiff, and lighter.

The longer you dry the leather, the harder it will be. Eventually, there will be no moisture in the leather at all, and the hide will crack when hit. This is a bad thing. There are two ways to avoid this result.

The first is to know when to stop drying the leather (trial and error time). The second is to wax the leather. You still have to stop drying the leather at a reasonable time. Once you have the leather out of the oven, coat it in Paraffin wax, and put it back into the oven. A cookie sheet is critical here. You won't drip wax into the oven, and you won't accidentally set fire to your leather. Coat both the inside (flesh) and the outside (skin) of the leather. Wax absorbs faster through the porous flesh than the skin, but it can't hurt to wax the skin side.

The wax replaces the water, makes the leather a little more flexible, and creates a waterproof barrier. More importantly, paraffin creates a crystalline matrix as it cools, filling in the gaps in the leather fibers and adding to the strength.

Paraffin has a variable melt point between 122 and 155 degrees. Most pillar and taper candles have a melt point of 140 degrees. Highly refined Paraffin has a melt point of 155 degrees.

As a general rule, don't leave hardened leather in the car on a sunny day. You should not have any issues with using waxed leather for armor, so long as you don't fight in 122 degree heat. If the leather softens and becomes malformed due to heat, put it back in the oven to soften and reshape it, and wax it again.