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How to stiffen leather
Soak the leather in cool water.
Fill a large bucket or sink with cool to room temperature water. Immerse the leather in the water for about 10 minutes, or until it becomes thoroughly soaked.
Note that this process works best when used with vegetable tanned leather. You can technically stiffen the leather by soaking it in nothing but room temperature water, but it will only become mildly harder and you will not be able to shape it.