Spin casting is a common method for mass-producing
small metal art pieces and figurines. If you've ever bought small
pewter RPG figurines or sculptures, they were likely spin-cast.
First, a sculptor makes the figurine, usually out of wax. A
mold maker takes the figurine and makes an impression (an inverse) of
it in a rubber mold which is then vulcanized. These molds are round
disks and can be of various sizes, though a reasonable size is about
15 inches in diameter and two or three inches thick. (The mold is
actually two slices, each about an inch and a half thick.) In addition
to the impression, the mold maker also cuts veins in the mold which will
allow the molten metal to flow into the impression(s). The center of the
disk is open (like an LP record), which serves both as a way to mount
the mold within a centrifuge, and as the entry point for the molten metal.
The rubber mold is loaded into a centrifuge, and spun at several hundred revolutions per minute.
Molten metal (most often pewter) is then poured down into the centrifuge, which
flows into the mold, through the veins and into the impressions. Once
the mold is full, it is removed from the centrifuge, and allowed to cool for
several hours. The mold is split apart, the cast of the figurines
and veins are removed, and the mold set aside for reuse. The figurines are
then cut or broken off of the veins; the figurines are sent to be finished,
and the veins and bad casts thrown back into the smelter.
In the case of pewter figurines, the pieces are buffed with a wire brush to
remove burrs and seams, then dipped in pewter black (an oxidizing acid). The pewter black
artificially adds shadowing and
contrast to the piece, a process sometimes called "antiquing". The piece is then
re-buffed to add shine and highlights.
The result -- a little pewter figurine used by legions of RPGers the world over.
I spent several summers in high school working in the finishing and packing department of Perth Pewter, a
now defunct casting operation in Claymont, Delaware.
Perth was responsible for some rather famous lines of pewter figurines and models, including the Superior Models and "Authenticast" naval ships, and the MAATAC/Star Fleet Wars RPG space ships.
Perth shut down for good around 1999, its spin casting molds scattered to the four winds, and its owner, Ian L. "John" Carter, died in 2002. Ah well -- thanks for all the cool stuff, John.