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.
History Typology and chronology
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 crucible helps to prevent the
heat from affecting the solution. The earliest crucible forms derive from the sixth/fifth
millennium B.C. in Eastern Europe and Iran. Chalcolithic
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 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. Iron Age
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. These 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.
Brass is made by mixing solid copper metal in with zinc oxide or carbonate which comes
in the form of calamine or smithsonite. This is heated to about 900°C and the zinc oxide
vaporises into a gas and the zinc gas bonds with the solid copper. 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, 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. Medieval period
Smelting and melting of copper and its alloys such as leaded bronze were smelted in crucibles
similar to those of the Roman period which have thinner walls and flat bases to sit within
the furnaces. The technology for this type of smelting started to change at the end of
the Medieval period with the introduction of new tempering material for the ceramic
crucibles. Some of these copper alloy crucibles were used in the making of bells. Bell foundry
crucibles had to be larger at about 60 cm. These later medieval crucibles were a more
mass-produced product. 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. However, during this period a vast and highly
important technological innovation happened using the cementation process, the production
of steel. Steel production using iron and carbon works in the same way as brass with
the iron metal being mixed with carbon to produce steel. The first examples of cementation
steel is wootz steel from India, where the crucibles were filled with the good quality
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. These were sandy clay crucibles which had been formed
around a fabric tube. These crucibles are used in the same way as other cementation
vessels but with a hole in the top of the vessel to allow pressure to escape.
Post Medieval At the end of the Medieval and into the Post
Medieval new types of crucible designs and processes started. Smelting and melting crucibles
types started to become more limited in designs which are produced by a few specialists. The
main types used during the Post Medieval period are the Hessian Crucibles which were made
in the Hesse region in Germany. These are triangular vessels made on a wheel or within
a mould using high alumina clay and tempered with pure quartz sand. Furthermore another
specialised crucible which was made at the same time was that of a graphite crucible
from southern Germany. These had a very similar design to that of the triangular crucibles
from Hesse but they also occur in conical forms. These crucibles were traded all across
Europe and the New World. 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.
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. Modern day uses
Laboratory crucibles
A crucible is a cup-shaped piece of laboratory equipment used to contain chemical compounds
when heated to extremely high temperatures. Crucibles are available in several sizes and
typically come with a correspondingly-sized crucible cover.
When heated over a flame, the crucible is often held inside a pipeclay triangle which
itself is held on top of a tripod. Materials and description
Crucibles and their covers are made of high temperature-resistant materials, usually porcelain,
alumina or an inert metal. One of the earliest uses of platinum was to make crucibles. Ceramics
such as alumina, zirconia, and especially magnesia will tolerate the highest temperatures.
More recently, metals such as nickel and zirconium have been used. The lids are typically loose-fitting
to allow gases to escape during 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 10–15
ml size porcelain crucibles are commonly used for gravimetric chemical analysis. These small
size crucibles and their covers made of porcelain are quite cheap when sold in quantity 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 hobby shops.
Chemical analysis
In the area of chemical analysis, crucibles are used in quantitative gravimetric chemical
analysis. Common crucible use may be as follows. A residue or precipitate in a chemical analysis
method can be collected or filtered from some sample or solution on special "ashless" filter
paper. The crucible and lid to be used are pre-weighed very accurately on an analytical
balance. After some possible washing and/or pre-drying of this filtrate, the residue on
the filter paper can be placed in the crucible and fired until all the volatiles and moisture
are driven out of the sample residue in the crucible. The "ashless" filter paper is completely
burned up in this process. The crucible with the sample and lid is allowed to cool in a
desiccator. The crucible and lid with the sample inside is weighed very accurately again
only after it has completely cooled to room temperature. The mass of the empty, pre-weighed
crucible and lid is subtracted from this result to yield the mass of the completely dried
residue in the crucible.
A crucible with a bottom perforated with small holes which is designed specifically for use
in filtration, especially for gravimetric analysis as just described, is called a Gooch
crucible after its inventor, Frank Austen Gooch.
For completely accurate results, the crucible is handled with clean tongs because fingerprints
can add 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 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 constant 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.
Ash content determination Ash is the completely unburnable inorganic
salts in a sample. A crucible can be similarly used to determine the percentage of ash contained
in an otherwise burnable sample of material such as coal, wood, or oil. A crucible and
its lid are pre-weighed at constant mass as described above. The sample is added to the
completely dry crucible and lid and together they are weighed to determine the mass of
the sample by difference. See also
Hessian crucible Micro-pulling-down
References
Craddock P., 1995, Early Metal Mining and Production, Edinburgh University Press Ltd,
Edinburgh Hauptmann A., T. Rehren & Schmitt-Strecker
S., 2003, Early Bronze Age copper metallurgy at Shahr-i Sokhta, reconsidered, T. Stollner,
G. Korlin, G. Steffens & J. Cierny, Eds., Man and mining, studies in honour of Gerd
Weisgerber on occasion of his 65th birthday, Deutsches Bergbau Museum, Bochum
Martinon-Torres M. & Rehren Th., 2009, Post Medieval crucible Production and Distribution:
A Study of Materials and Materialities, Archaeometry Vol.51 No.1 pp49–74
O. Faolain S., 2004, Bronze Artefact Production in Late Bronze Age Ireland: A Survey, British
Archaeological Report, British Series 382, Archaeopress, Oxford
Rehren, Th. and Papakhristu, O., 2000, Cutting Edge Technology – The Ferghana Process of
Medieval crucible steel Smelting, Metalla, Bochum, 7(2) pp55–69
Rehren T. & Thornton C. P, 2009, A truly refractory crucible from fourth millennium Tepe Hissar,
Northeast Iran, Journal of Archaeological Science, Vol. 36, pp2700–2712
Rehren Th., 1999, Small Size, Large Scale Roman brass Production in Germania Inferior,
Journal of Archaeological Science, Vol. 26, pp 1083–1087
Rehren Th., 2003, Crucibles as Reaction Vessels in Ancient Metallurgy, Ed in P. Craddock & J.
Lang, Mining and Metal Production Through the Ages, British Museum Press, London pp207–215
Roberts B. W., Thornton C. P. & Pigott V. C., 2009, Development of Metallurgy in Eurasia,
Antiquity Vol. 83 pp 1012–1022 Scheel B., 1989, Egyptian Metalworking and
Tools, Shire Egyptology, Bucks Vavelidis M. & Andreou S., 2003, Gold and
Gold working in Later Bronze Age Northern Greece, Naturwissenschaften, Vol. 95, pp 361–366
Zwicker U., Greiner H., Hofmann K. & Reithinger M., 1985, Smelting, Refining and Alloying
of copper and copper Alloys in Crucible Furnaces During Prehistoric up to Roman Times, P. Craddock
& M. Hughes, Furnaces and smelting Technology in Antiquity, British Museum, London