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, the zinc oxide vaporizes into a gas, and the zinc gas bonds with the molten copper (Rehren 1999: p1085). This reaction has to take place in a part-closed or closed container otherwise the zinc vapor 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 smallmouth 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.