I don’t think it really shrinks more. I don’t run the heat higher than 150° I will say the leather gets firmer using heat.
Horsewright
shows this technique in one of his tutorials. That’s where I picked it up from. I’m sure Dave will be along to fill in the blanks.
This is after binding cement cures overnight. I’ve done it sooner but you have to go light on water and really pay attention !!!
I damp the sheath with room temp water. Then dunk for oh say 40 secs in very hot too the touch water. Then wet mold or fit to knife, then oven for hours like 3 + I like lower heat longer times. I try to not make jerky out of the sheath lol.