Casts are used with the Smeltery to make metal tool parts. To start, take a stone tool part, (ex., pickaxe head) and put it in a Casting Table hooked up to your Smeltery. Then pour some Aluminum Brass or Gold into the table. It will form a Cast around the tool part. You then remove the stone tool part, and are now free to pour any metal you like into the newly created Cast.