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In general, a crucible is a vessel, or dish, that is used to hold a sample within a high temperature furnace. A ceramic crucible is made of a ceramic material, such as kiln-fired clay. Ceramic crucibles are chemically and physically stable at high temperatures, and are thus used for working with materials that must be manipulated at very high temperatures, such as metals and glasses.
Ceramic crucibles are commonly used in manufacturing, particularly metallurgy, analysis and quality control. They are also common tools in research environments in fields such as material science, Earth science, and engineering. Crucibles can also be found in use in the arts, either for work with metals or pigments.
The characteristic that makes a crucible useful is the ability to withstand higher temperature than whatever material is placed within. This allows the material within the crucible to be melted and manipulated without destroying the container. Being physically and chemically stable at high temperatures is a characteristic also referred to as refractory.
Ceramics themselves are non-metal, inorganic, crystalline, or amorphous solids that have been fired, cooled, and glazed during their creation. Commonly known in the context of pottery, ceramics are characterized by their smooth, relatively inert surface. An inert surface, or non-reactive surface, is not only useful, but is essential for a crucible. If the surface is reactive, the materials within the crucible itself might begin to mix with the contents of the crucible at high temperatures.
A ceramic crucible can be made from a variety of starting materials including silica, zircon, spinel, alumina, and magnesite. The desired chemical composition of the ceramic crucible depends on the application, since each composition reacts differently to temperature and pressure. Alumina crucibles are particularly common as they are relatively inexpensive and withstand a wide variety of temperature environments.
Regardless of application, a variety of characteristics should be considered when selecting a ceramic crucible. Chemical composition is usually the most significant characteristic, since it most directly impacts the temperatures and pressures at which the crucible will be useful. Other relevant characteristics include total capacity, overall shape, and wall and bottom thickness.
Ceramic crucibles have been used for working with metals since around 5000 BC. Used in the smelting of copper, tin, and iron through history, the design of crucibles has changed and evolved as metallurgy techniques have developed. The fact that the design and use of crucibles has changed through time makes them historically interesting and quite useful to archeologists. Early crucibles were irregular in shape, often inconsistent in thickness, and usually not extremely refractory. More modern crucibles are manufactured to a nearly limitless set of specifications, and are used at higher temperatures and pressures than would have once been dreamed possible.