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. The stone part is consumed and your cast is ready to be filled with metal of your choice.
You can make Casts for any tool part except Bowstring and Fletching. You can also make Ingot and Gem Casts. The ingot cast requires any ingot. The gem cast requires an emerald (the emerald is NOT consumed in the process).
There are also clay versions of cast which act the same way as the normal cast however they are one use items.
Making a cast takes the same materials as making the part.