At Zelikovitz Leathers we have been asked over and over how to harden leather for Mediaeval Armor. I haven't found any really good books on the subject so I asked some of our experienced clients to help us out with some tips, and came up with this article. If you enjoy it or find it usefull, please rate it at the end.
The period term for hardened leather is "cuir boulli," which translates as "boiled (or cooked) leather." Since we are a distributor for Mediaeval Miscellanea (you can find all the patterns in our ebay store)
There are two schools of thought here. Some say to use the Wax method and some say the Water method is better. We are providing you with both. You should experiment and see which works best for you.
Hardened Leather - The Wax Method
Hardened leather was used for armor from classical antiquity through the end of the Elizabethan era. Its popularity can be trace to its strength, weight, and relative ease of manufacture.
For this method of hardening you will need a good sized double boiler, a large supply of wax (paraffin works very well), candle wax hardener, candle wax dye (both are available at a good hobby shop), and, of course, a supply of leather.
The minimum thickness of leather you should use is about 8 oz. (1/8") to 11 oz. (3/16"). Sole leather, which is about 10-14 oz. (1/4") thick, is very efficient and has great stopping power. Very heavy leather (such as bulls throat) is superlative.
For small plates, the simple immersion method is best; I will deal with hardening larger plates later on in the article.
Always use a double boiler to melt wax. The boiler I use is a large kettle that I no longer love, which rests on three legs inside a larger kettle which is filled halfway with water.
Put in your wax: after it melts, add about half again as much hardener as the manufacturer recommends. Since you are going to be inside this leather, you want it very rigid.
If you want to dye the leather, add the dye at this point. You can use either Zelikovitz Professional waterbased dye or Fiebing's dye. Make certain that you have enough wax to complete the job at one sitting. If you run out halfway through you will have a wretched time mixing a new batch that matches the original. Always use a test piece of leather, and let it dry completely before you decide you like it. If you don't use dye, the leather will usually turn a very rich, dark brown, which I personally prefer to dyed leather.
Now, the actual mechanics of the hardening. Take about five or ten plates and drop them into the wax.
About fifteen seconds after you immerse the plates, you will see a stream of air bubbles rising from the leather. Wait until these bubbles have completely stopped rising, and remove the leather. This will take anywhere from two to ten minutes, depending on your leather. Place the plates smooth (grain) side up, on waxed paper or foil. Don't use newspaper, the plates willl stick to it. Once you have the process down, you can produce an unbelievable number of plates in an evening.
For any plate too large to fit into your boiler you will have to take a few risks and do the hardening in your oven. I can hear your castle's cook screaming already.
Melt a good quantity of wax in your double boiler, as you would for small plates. Use a brush to coat the plates completely with wax. The wax should be about 1/4 of an inch thick. Put the plates onto an old cookie sheet, and then pour on more liquid wax.
Set your oven to bake at about 150 degrees. Don't use broil; you'll ruin an expensive piece of leather and create a stench that must be smelled to be believed.
Place your leather into the oven and watch it carefully. "Baste" the leather with liquid wax at least once a minute. If necessary be sloppy, just never let the leather dry.
When your leather is quite saturated with wax, remove it and start shaping. The sane way is to put on work gloves, hold the leather in your hands. and shape it. This method only produces and approximate fit. A second method (practiced by our local shire's Armorsmiths and Masochists Guild) calls for putting on a heavy garment, finding someone who doesn't like you very much. and having him strap the plate onto you, molding it to an exact fit. This method is only moderately painful. (Make sure that you really don't like the clothing very much. No matter what you do, you will always have a ghost image of your harness.)
I. Do all of your cutting, and punch any rivet holes before hardening the leather. This is armor: it was intended to be hard to cut.
2. If the harness becomes deformed by being crammed into a car trunk for several hours, you can usually repair it by laying it on a dark cloth in the sun for about half an hour. It will become just barely pliable enough and from there can be fitted back onto your person.
3. Don't attach straps to your harness until after you have hardened it. Wax will get into the thin leather and make it just stiff enough to be maddeningly uncomfortable.
4. Couir Boulli is an ideal medium for other craftsmen. The Metropolitan Museum has many cups, cases, and chests made from hardened leather.
S. Couir Boulli is ideal for articulated armor. This style uses square or rectangular plates riveted or sewn to a leather or canvas tunic, in a brickwork pattern. This armor is most effective if you use very thick plates and cut the edges to a good right angle. The plates will lock, and become very rigid when struck.
Hardened Leather - The Water Method
Take a piece of vegetable tanned leather. Immerse it in water long enough to get it soaked--ten minutes will do. Heat a pot of water to 180°. Immerse the leather in the hot water. Watch it.
In about a minute, the leather will begin to darken, go limp, and curl up. If you pull it out at that point, it will have shrunk a little, thickened a little, and be stretchy, like a thick sheet of rubber; at this point it can be stretched and formed. In a minute or two the stretchiness will go away, but the leather will still be flexible. Over the course of the next few hours it will become increasingly stiff. You will end up with a piece a little thicker and a little harder than what you started with.
The longer you leave the leather in the hot water after the process has started, the more it shrinks, the more it darkens, the thicker it gets--and the harder the final piece will be. A sufficiently long immersion gives you something that feels like wood. Unfortunately, when the piece gets harder and stiffer it also gets more brittle. If I were making lamellar armor to defend myself against real weapons, I would use a long immersion--and plan on replacing a few cracked lamellae after each fight. For SCA purposes, I normally leave the leather in the hot water for about thirty seconds after the process starts. This gives me, very roughly, shrinkage to about 7/8 of the original dimensions, an increase in thickness of about 25%, and a piece that is hard but not totally inflexible.
The process is very sensitive to the temperature of the water, so you will want an accurate thermometer. The timing and the result also depend to some degree on the particular piece of leather. Instead of trying to work entirely by the clock, experiment with pieces of scrap until you have a reasonably good idea of how the leather looks at various stages in the process and how it comes out when finished, then judge the progress of your piece in part by time and in part by appearance.
You can also harden leather in in boiling water--considerably faster. In my experience, about a twenty second boil gives shrinkage to 7/8ths, about a forty second gives you a shrinkage to 2/3 and roughly doubles the thickness. That has the advantage of not requiring a thermometer.
It has two disadvantages. First, the faster process is harder to control precisely. Second, the hotter water produces a less uniform hardening--you tend to get pieces where the surface is harder and more brittle than the interior, eventually producing surface cracks. I therefore prefer the lower temperature process. I have not done any extensive experimentation on what happens at intermediate temperatures.
Cuirboulli can also serve a variety of more peaceful purposes. Think of it as a medieval plastic-literally true, if you take "plastic" in its general sense and consider the condition of the leather immediately after it comes out of the water.