Gravity casting

Gravity casting is among the oldest known processes for fabricating metals and metal alloys.^[1] It involves the pouring of molten metal from a crucible into a mold under only the force of gravity, without the use of pressurized gases, vacuums, or centrifugal force. Small-scale molds used for this form of casting have most commonly been made of sand, tufa stone, and cuttlebone^[2] as well as charcoal and plaster^[3] as these materials are generally easy to shape (unlike iron or steel), do not break down when suddenly exposed to high temperatures (unlike glass, wood, or plastic), do not deform easily (unlike silicone) and are widely available.

References

1. Pratip Kumar Gupta; J.E. Mannar (1969). Recent Developments in Non-ferrous Metals Technology: Nickel, lead, zinc, rare earth, and nuclear metals. National Metallurgical Laboratory (India). p. 102.
2. Marcia Chamberlain (1976). Metal jewelry techniques. Watson-Guptill Publications. p. 79.
3. Philip Morton (1970). Contemporary jewelry: a studio handbook. p. 205.