| Refractory materials retain their strength at high temperatures. They are used to make crucibles and to make refractory linings, which line furnaces, kilns and incinerators. The oxides
of magnesium (magnesite) and calcium (lime) are the most important refractory materials, though fireclay is widely used as well. Zirconia is used when the material
must withstand extremely high temperatures. Silicon carbide is another refractory material. It is very strong at high temperatures, but will burn in the presence of oxygen, if the protective silica coating comes off. Refractories must be chosen according to the conditions they will face. For
example, carbon cannot be used when it will be in contact with oxygen, as it will burn. Acidic refractories cannot be used in a basic environment and basic refractories cannot be used in acidic
environment because they will be eroded. Zircon, fireclay and silica are acidic, dolomite and magnesite are basic and alumina,
chromite, silicon carbide, carbon and mullite are neutral. Refractory materials must be strong at high temperatures, resistant to thermal shock, chemically inert, and have low thermal conductivities and coefficients of expansion.
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