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How to Water Harden Leather

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.