There are several schools of thought on how this was done in the past, and no one knows for sure who is right. There are many opinions, but few facts.
The schools are:
- Chemically altering the leather through the use of heat and some form of heated liquid to polymerize the tannins in the leather and these will harden into some form of plastic. A variation of this involves the use of pressure in the form of molding and/or tooling. The most common liquid used in this school is heated water.
- Impregnating the leather with some other substance that then hardens and forces the leather to retain a shape. The most common substance used in the SCA for this is wax (although there is some evidence that resin was used to harden Cuir Bouilli late and post period).
- The term may have meant a specific technique originally, but eventually came to mean any sort of hardened leather.