These techniques will work for pretty much any pourable mold-making material (although with urethanes you will
need mold release), I just happen to be using high-temp silicone because it's what I have, and I have it because I can cast pewter (and other low temp alloys) with it. You would also be able to use this method to make waxes which you then cast using investment and more conventional metals (gold, silver, etc.).
The pictures of pouring are from when I molded an aluminum flashlight reflector, and the mold pictured in the last step (and this step) is one of my ring molds