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A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay,Percy, John. Natural Refractory Materials Employed in the Construction of Crucibles, Retorts, Forunaces &c. Metallurgy. London: W. Clowes and Sons, 1861. 208–09. Print. they can be made from any material that withstands temperatures high enough to melt or otherwise alter its contents.
A crucible furnace dating to 2300–1900 BC for bronze casting has been found at a religious precinct of Kerma.
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: p 49 These crucibles in some cases have thinner walls and have more refractory properties.Tylecote 1976: p 20
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: p 107 Brass is made by mixing solid copper metal with zinc oxide or carbonate which comes in the form of calamine or smithsonite.Rehren 2003: p 209 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: p 1085 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.Rehren Th., 1999, Small Size, Large Scale Roman brass Production in Germania Inferior, Journal of Archaeological Science, Vol. 26, pp 1083–1087 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.
The cementation process, which was lost from the end of the Roman to the early medieval period, continued in the same way with brass. Brass production increased during the medieval period due to a better understanding of the technology behind it. Furthermore, the process for carrying out cementation for brass did not change greatly until the 19th century.Craddock P., 1995, Early Metal Mining and Production, Edinburgh University Press Ltd, Edinburgh
However, during this period a vast and highly important technological innovation happened using the cementation process, the production of crucible steel. Steel production using iron and carbon works similarly to brass, with the iron metal being mixed with carbon to produce steel. The first examples of cementation steel are wootz steel from India,Craddock 1995: p 276 where the crucibles were filled with good quality low-carbon wrought iron and carbon in the form of organics such as leaves, wood, etc. However, no charcoal was used within the crucible. These early crucibles would only produce a small amount of steel as they would have to be broken once the process has finished.
By the late medieval period, steel production had moved from India to modern-day Uzbekistan, where new materials were being used in the production of steel crucibles; for example, Mullite crucibles were introduced.Rehren, Th. and Papakhristu, O., 2000, Cutting Edge Technology – The Ferghana Process of Medieval crucible steel Smelting, Metalla, Bochum, 7(2) pp55–69 These were sandy clay crucibles which had been formed around a fabric tube. These crucibles were used in the same way as other cementation vessels but with a hole in the top of the vessel to allow pressure to escape.
The refining of methods during the medieval and post-medieval periods led to the invention of the cupel which resembles a small egg cup, made of ceramic or bone ash which was used to separate base metals from noble metals. This process is known as cupellation. Cupellation started long before the post-medieval period; however, the first vessels made to carry out this process started in the 16th century.Rehren 2003: p 208 Another vessel used for the same process is a scorifier, which is similar to a cupel but slightly larger and removes the lead and leaves the noble metals behind. Cupels and scorifiers were mass-produced as after each reduction the vessels would have absorbed all of the lead and become fully saturated. These vessels were also used in the process of metallurgical assay where the noble metals are removed from a coin or a weight of metal to determine the amount of the noble metals within the object.
Crucibles and their covers are made of heat-resistant materials, usually porcelain, alumina or an Chemically inert metal. One of the earliest uses of platinum was to make crucibles. Ceramics such as alumina, zirconia, and especially magnesium oxide will tolerate the highest temperatures. However, chemical reactions with the material in the crucible must be kept in mind; the emergence of melting point-lowering Eutectic system is an especially important consideration. More recently, metals such as nickel and zirconium have been used. The lids are typically loose-fitting in order to allow gases to escape during the heating of a sample inside. Crucibles and their lids can come in high form and low form shapes and in various sizes, but rather small 10 to 15 milliliter size porcelain crucibles are commonly used for gravimetric chemical analysis. These smaller crucibles and their covers made of porcelain are quite cheap when sold in large quantities to laboratories, and the crucibles are sometimes disposed of after use in precise quantitative chemical analysis. There is usually a large mark-up when they are sold individually in .
A crucible with a bottom perforated with small holes which are designed specifically for use in filtration, especially for gravimetric analysis as just described, is called a Gooch crucible after its inventor, Frank Austin Gooch.
For completely accurate results, the crucible is handled with clean tongs because fingerprints can add a weighable mass to the crucible. Porcelain crucibles are hygroscopic, i. e. they absorb a bit of weighable moisture from the air. For this reason, the porcelain crucible and lid is also pre-fired (pre-heating to high temperature) to constant mass before the pre-weighing. This determines the mass of the completely dry crucible and lid. At least two firings, coolings, and weighings resulting in exactly the same mass are needed to confirm the constant (completely dry) mass of the crucible and lid and similarly again for the crucible, lid, and sample residue inside. Since the mass of every crucible and lid is different, the pre-firing/pre-weighing must be done for every new crucible/lid used. The desiccator contains desiccant to absorb moisture from the air inside, so the air inside will be completely dry.
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