Crucible
Overview
 
A crucible is a container used for metal, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, and pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents. Historically, they have usually been made of clay, but they can be made of any material with a higher temperature resistance than the substances they are designed to hold.
The form of the crucibles has varied through time, with designs reflecting the process for which they are used, as well as regional variation.
Encyclopedia
A crucible is a container used for metal, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, and pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

 production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents. Historically, they have usually been made of clay, but they can be made of any material with a higher temperature resistance than the substances they are designed to hold.

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 earliest crucible forms derive from the sixth/fifth millennium B.C. in Eastern Europe and Iran.

Chalcolithic

Crucibles used for copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 smelting
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...

 were generally wide shallow vessels made from clay that lacks refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...

 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.

Iron Age

The use of crucibles in the Iron Age
Iron Age
The Iron Age is the archaeological period generally occurring after the Bronze Age, marked by the prevalent use of iron. The early period of the age is characterized by the widespread use of iron or steel. The adoption of such material coincided with other changes in society, including differing...

 remains very similar to that of the Bronze Age
Bronze Age
The Bronze Age is a period characterized by the use of copper and its alloy bronze as the chief hard materials in the manufacture of some implements and weapons. Chronologically, it stands between the Stone Age and Iron Age...

 with copper and tin smelting was being used to produce bronze. The Iron Age crucible designs remain the same as the Bronze Age.

Roman period

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
Cementation process
The cementation process is an obsolete technique for making steel by carburization of iron. Unlike modern steelmaking, it increased the amount of carbon in the iron. It was apparently developed before the 17th century. Derwentcote Steel Furnace, built in 1720, is the earliest surviving example...

, used in the production of brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

. 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 in with zinc oxide or carbonate which comes in the form of calamine
Calamine
Calamine is a mixture of zinc oxide with about 0.5% ferric oxide . It is the main ingredient in calamine lotion and is used as an antipruritic to treat mild pruritic conditions such as sunburn, eczema, rashes, poison ivy, chickenpox, and insect bites and stings...

 or smithsonite
Smithsonite
Smithsonite, or zinc spar, is zinc carbonate , a mineral ore of zinc. Historically, smithsonite was identified with hemimorphite before it was realised that they were two distinct minerals. The two minerals are very similar in appearance and the term calamine has been used for both, leading to some...

 (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.

Medieval period

Smelting and melting of copper and it 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 (Tylecote 1976: p73). 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
Wootz steel
Wootz steel is a steel characterized by a pattern of bands or sheets of micro carbides within a tempered martensite or pearlite matrix. It was developed in India around 300 BCE...

 from India (Craddock 1995: p276), 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 little amount of steel and would have to be broken once the process has finished.

By the late Medieval period steel production had move 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
Cupellation is a metallurgical process in which ores or alloyed metals are treated under high temperatures and carefully controlled operations in order to separate noble metals, like gold and silver, from base metals like lead, copper, zinc, arsenic, antimony or bismuth, that might be present in...

. 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: p208). 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 assaying 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.

Laboratory crucibles

A crucible is a cup-shaped piece of laboratory
Laboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...

 equipment used to contain chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s when heated to extremely high temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

s. Crucibles are available in several sizes and typically come with a correspondingly-sized crucible cover (or lid
Lid (container)
A lid, also known as a cap, is part of a container, and serves as the cover or seal, usually one that completely closes the object.-History:...

).

Crucible materials and description

Crucibles and their covers are made of high temperature-resistant materials, usually porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

 or an inert
Inert
-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...

 metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

. One of the earliest uses of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 was to make crucibles. Ceramics such as alumina, zirconia, and especially magnesia
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

 will tolerate the highest temperatures. More recently, metals such as nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

 and zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

 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.

Use in chemical analysis

In the area of chemical analysis, crucibles are used in quantitative gravimetric chemical analysis (analysis by measuring mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 of an analyte
Analyte
An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...

 or its derivative). 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
Filter paper
Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.-Properties:Filter paper comes in various porosities and grades depending on the applications it is meant for...

. 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 (heated at very high temperature) until all the volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...

s and moisture
Moisture
Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

 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
Desiccator
thumb|right|A desiccator and a vacuum desiccator - note the stopcock which allows a vacuum to be applied. The blue [[silica gel]] in the space below the platform is used as the [[desiccant]]....

. The crucible and lid with the sample inside is weighed very accurately again only after it has completely cooled to room temperature (higher temperature would cause air currents around the balance giving inaccurate results). 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
Gooch crucible
A Gooch crucible, named after Frank Austen Gooch, is a filtration device for laboratory use . It is convenient for collecting a precipitate directly within the vessel in which it is to be dried, possibly ashed, and finally to be weighed in gravimetric analysis.The device was originally a standard...

 after its inventor, Frank Austen Gooch
Frank Austen Gooch
Frank Austen Gooch was a chemist and engineer. He was born to Joshua G. & Sarah Gates Gooch in Watertown, Massachusetts...

.

For completely accurate results, the crucible is handled with clean tongs
Tongs
Tongs are used for gripping and lifting tools, of which there are many forms adapted to their specific use. Some are merely large pincers or nippers, but the greatest number fall into three classes:...

 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 (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 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
Desiccant
A desiccant is a hygroscopic substance that induces or sustains a state of dryness in its local vicinity in a moderately well-sealed container....

 to absorb moisture from the air inside, so the air inside will be completely dry.

Use in ash content determination

Ash
Ash (analytical chemistry)
In analytical chemistry, ashing is the process of mineralization for preconcentration of trace substances prior to chemical analysis. Ash is the name given to all non-aqueous residue that remains after a sample is burned, and consist mostly of metal oxides....

 is the completely unburnable inorganic salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

s 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
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

, wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, or oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

. 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.
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