Working with leather to make the material stiffer requires time and effort. You can achieve an adequately stiffened leather piece by soaking it in cold water for several minutes or hours. You can further stiffen wet leather by hammering it, as well as by baking it until it dries. It can be hard to determine the exact requirements for these activities because of the unique nature of individual pieces of leather. These methods are best used for vegetable tanned leather, although other leather types can be stiffened through these techniques as well.
Submerge your leather in cold water, making sure every part is covered. Let the leather soak for at least 15 minutes. Thicker leather can soak for up to 12 hours. Make sure to check your submerged leather every half hour or so to make sure there are no problems with the material. This step will make your leather noticeably stiffer.
Work your leather with a leather-working hammer once you remove it from the water if you want to increase the material's stiffness. Gently strike around the surface of the leather, making sure not to overlook any areas. This will toughen the leather up as it dries and stiffens.
Bake your dampened leather in the oven at about 48.9 degrees Celsius. This step is optional, but it may make your leather stiffer than soaking and hammering alone. The baking process runs the risk of scalding or shrinking. You may want to experiment with a scrap of your leather first to survey the results. Tie, stitch or nail your leather into place if necessary when you bake it to make sure it keeps its intended shape. Bake it until the leather is dry.