Crucible

While crucibles were historically usually made from clay, they can be made from any material that withstands temperatures high enough to melt or otherwise alter its contents. The form of the crucible has varied through time, with designs reflecting the process for which they are used, as well as regional variation. The earliest crucible forms derive from the sixth/fifth millennium B.C. in Eastern Europe and Iran. Crucibles used for copper smelting were generally wide shallow vessels made from clay that lacks refractory properties which is similar to the types of clay used in other ceramics of the time. Crucibles have added handles to allow for better manipulation, however, due to the poor preservation of the crucibles there is no evidence of a pouring spout. The main purpose of the crucible during this period was to keep the ore in the area where the heat was concentrated to separate it from impurities before shaping. A crucible furnace dating to 2300–1900 BC for bronze casting has been found at a religious precinct of Kerma. The use of crucibles in the Iron Age remains very similar to that of the Bronze Age with copper and tin smelting being used to produce bronze. The Iron Age crucible designs remain the same as the Bronze Age. The Roman period shows technical innovations, with crucibles for new methods used to produce new alloys. The smelting and melting process also changed with both the heating technique and the crucible design. Crucibles in some cases have thinner walls and have more refractory properties. During the Roman period a new process of metalworking started, cementation, used in the production of brass. This process involves the combination of a metal and a gas to produce an alloy. This is heated to about 900 °C, the zinc oxide vaporizes into a gas, and the zinc gas bonds with the molten copper. Cementation crucibles, therefore, have a lid or cap which limits the amount of gas loss from the crucible.