You can melt down almost any kind of aluminum—cans, even—but a piece that has already been cast (such as a water pump, wheel, or intake manifold) will have an alloy that's better suited for casting.

Making silicone molds seems easy, but there are a lot of missteps to be made along the way that can mean the difference between a great, reusable mold, and one that’s a sad waste of silicone. If ...