Cuir bouilli? More like cuir boo-illi!

Part 2 of a 3 part series on material modifications

Boiled leather is one of those things that I learned about from poring over RPG sourcebooks as a kid, attempting to figure out what “leather armor” actually was and how in the hell it’d stop a sword—at some point discovering leather armor wasn’t just cowhide draped over your back, but rather a treated material called “cuir bouilli” or boiled leather. Some totally unsourced article informed impressionable me that leather was immersed in boiling water, and that it then became super hard. And it turns out that’s kind of right, but only kind of.

Leather is an interesting material, especially when it’s “veg tan”, one of the simpler and more traditional ways of prepping hide to make it handleable. Veg tan leather is particularly malleable—it changes color over time in the sun, it can be polished to a shine, and if you get it wet, you can reshape it into just about any form you want using a mold. And if you heat it, it’ll shrink and harden—but if the leather gets too hot, it’ll “burn” and shrivel up and become unusable.

The short version is that you get leather to the shape that you want it, and then you heat it up to the point where it contracts, thickens, and hardens. The details are argued, some people say you dunk it in hot water, or hot oil, or just heat it in an oven. But the overall consensus is that heat hardens leather into something a lot tougher than its original form.

A lot of what I’ve learned about cuir bouilli is from this amazing piece of web 2.0 scholarship by Marc Carlson at the University of Tulsa (it seems to have succumbed to linkrot, but the wayback machine has it) where he discusses much of the remaining historical instructions on boiling leather, and delves into some experimentation—notably into using hot oil compared to hot water. It’s a lot of SCA nerds talking about how they make their armor, and early internet pedantry about if it’s the heat or using the oil, or something else, that makes it “boiled leather”.

At one point, I was planning on attempting a few experiments myself, just to find out what it’s actually like to do this with some scraps of leather and a few types of oil. But it turns out someone has beaten me to the punch (and in much greater detail). Jason F. Timmermans over on his blog Oni Crafts did a truly comprehensive comparison test of 16 different hardening methods using materials both modern (elmer’s glue) and ancient (beeswax). It’s a really fun read, and I thoroughly recommend it for a much much deeper dive on the topic. He ends up on stearic acid, a material I’ve never heard of, but it’s used to make cosmetics and is a major component of cocoa butter. This technique makes a final product that is briefly malleable, and then very hard, water-resistant, and generally resilient.

He’s since modified the method to use carnauba wax, which is significantly more expensive than stearic acid, for only a moderate gain in toughness.

Reading through his techniques, one thing that’s tricky is temperature control. The leather and oil need to reach 150°F, then the oil is raised to 200°F with the leather submerged, before cooling back down to 150°F. Gives me flashbacks to trying to temper chocolate. But you might be able to improve the precision of the process using sous vide. When people use sous vide for confit cooking and similar, they don’t heat the oil directly with the immersion circulator. Instead, one option is to put a dish of oil inside the water bath, with the sous vide controlling the water’s temperature. Essentially a bain-marie. Over time the oil will come to reach the same temp as the water. Using the same technique you could accurately control the temperature on the stearic acid, and you could also double bag the leather in ziplocks and immerse it in the water to bring the leather to 150°F too.

I might have to give that a try, actually.

There’s not a lot of historical remnants of this sort of armor, since leather doesn’t last extremely well over hundreds of years, so much of what you see currently are modern reproductions and cosplay—most surviving cuir bouilli is either decorative, or small hard containers (which are much more practical than a suit of damn armor). But it’s clearly a very effective way to transform soft, flexible leather into something rigid.