I am trying to make a sword hilt out of sterling,
I'd recommend casting this massive of an article in a sand mold,
such as Delft Clay. Many jewelry suppliers sell a "cope and drag"
design casting "flask", really a cast iron 2 part frame, a miniature
version of the old style used in industry before ceramic shell and
catalytic resin sand technologies. The jeweler's version is large
enough for something like a sword hilt, but Delft's largest flask is
only 100 millimeters, maybe large enough, maybe not. As for sprue
thickness, I would, if casting a 1" diameter sphere, sprue this with
a 1/4 sprue, but consider, with sand casting, you need some pretty
significant vent sprues too, at least 1/8" in this case. That all
said, you're biggest problem is going to be shrinkage, and the
attendant porosity problems, as well as gas porosity, which is less a
problem in sand molds. Think about casting these parts a little on
the thick side and doing a bit of forging towards the final shape,
which should compact the metal and reduce the porosity problems.
Another option would be to fabricate in heavy gauge sheet and weight
the parts by filling with lead.
David L. Huffman