I don't think it matters very much whether it's soft or hard-wood saw-dust. Someone earlier mentioned, they could get a kiln to a temp. of above a thousand... You kight, but if you want to use your pottery in a fire, you don't want to fire it to that temperature. At that temperature, it basicly turns into stone. If it is fired at that temperature, the earthenware will not be flexible enough anymore to allow uneven heating and cooling. It will then break.