Crucibles

A crucible is a container that can withstand very high temperatures and is used for metal, glass, and pigment production as well as a number of modern laboratory processes. While crucibles historically were 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 crucibles 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. During the Chalcolithic period, crucibles were heated from the top by using blow pipes. Ceramic crucibles from this time had slight modifications to their designs such as handles, knobs or pouring spouts (Bayley & Rehren 2007: p47) allowing them to be more easily handled and poured. Early examples of this practice can be seen in Feinan, Jordan. These 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.

The use of crucibles in the Iron Age remains very similar to that of the Bronze Age with copper and tin smelting was 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. The crucible changed into rounded or pointed bottom vessels with a more conical shape; these were heated from below, unlike prehistoric types which were irregular in shape and were heated from above. These designs gave greater stability within the charcoal (Bayley & Rehren 2007: p49). These crucibles in some cases have thinner walls and have more refractory properties (Tylecote 1976: p20).

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 (Zwicker et al. 1985: p107). Brass is made by mixing solid copper metal with zinc oxide or carbonate which comes in the form of calamine or smithsonite (Rehren 2003: p209). This is heated to about 900 °C and the zinc oxide vaporises into a gas and the zinc gas bonds with the solid copper (Rehren 1999: p1085). This reaction has to take place in a part-closed or closed container otherwise the zinc vapour would escape before it can react with the copper. Cementation crucibles therefore have a lid or cap which limits the amount of gas loss from the crucible. The crucible design is similar to the smelting and melting crucibles of the period utilizing the same material as the smelting and melting crucibles. The conical shape and small mouth allowed the lid to be added. These small crucibles are seen in Colonia Ulpia Trajana (modern-day Xanten), Germany, where the crucibles are around 4 cm in size, however these are small examples. There are examples of larger vessels such as cooking pots and amphorae being used for cementation to process larger amounts of brass; since the reaction takes place at low temperatures lower fired ceramics could be used. The ceramic vessels which are used are important as the vessel must be able to lose gas through the walls otherwise the pressure would break the vessel. Cementation vessels are mass-produced due to crucibles having to be broken open to remove the brass once the reaction has finished as in most cases the lid would have baked hard to the vessel or the brass might have adhered to the vessel walls.

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