The centrifugal force presses the liquid pewter throughout the mold in the seconds before cooling, and returning to it's natural, solid state in the mold cavity. Molds are made of vulcanized or cured rubber, similar to that of an automobile tire. Molds can be designed to cast multiple items at the same time, or as production molds so that many like pieces are cast on the same pour. This process uses a mold made of bronze. The bronze mold allows the pewter to flow into the mold and is poured out again before cooling and completely hardening.
Pewter takes on the shape of the mold and remains hollow at the same time.
The liquid pewter cools faster closest to the bronze, allowing the center to be poured back out, leaving the inside hollow. Tankard handles that are large need to be hollow so that they do not make the piece top heavy or tip to one side. A bud vase that is slush cast is another example of when the hollow casting is deliberate and comes in very handy. Temporary molds, or molds that do not need to be used again after having been cast once, are made of various silicones that start out as a liquid and harden over night around a model or design. Once hardened, the silicone can then act as a mold for the pewtersmith to pour melted pewter into.