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crucibles, temperature and clay

CRUCIBLE, a vessel used in chemistry and the arts for containing substances that are to be subjected to high temperatures. A good crucible should be capable of withstanding great and sudden changes of temperature without fracture or disintegration; it should not be at tacked by the substance it is to contain ; and it should be infusible at the temperatures to which it is exposed. Numerous materials are used in the manufacture of crucibles, each having its own peculiar advantages and disadvantages. Platinum is an ideal material for many purposes, but it is exceedingly expensive and it cannot be used for the fusion of metals. Nickel is sometimes used instead on account of its cheap ness. Clay, or a mixture of clay with sand, graphite, or old broken crucibles, is a favorite material, and Hessian crucibles, composed of equal parts of clay and sand, are in very general use. Hessian crucibles are commonly triangular in shape and coarse in texture. They are porous, they fracture easily from sudden changes in temperature, and they will not with stand the action of litharge; but they are cheap, and will not fuse at any temperature that is attained in ordinary operations. The clay that

is used in the manufacture of crucibles should be °weathered° for some months, by exposure to air in a moist condition, in order to effect the decomposition or elimination of certain impurities, such as pyrites, that would be prej udIcAal to the finished vessel. Graphite is used to a considerable extent in the manufacture of crucibles, especially for those that are to be used for melting metals. Lime crucibles, cat from blocks of well-burned lime, are practically infusible, but they will not stand exposure to air for any length of time, since they absorb mois ture, becoming converted into calcium hydrate, and then disintegrate. Magnesia crucibles are also practically infusible, and are not affected by exposure to the air. A mixture of equal parts of magnesia and bauxite (q.v.) makes excellent crucibles. Alumina (oxide of alumi num) is also highly recommended as a material for crucibles, as it will withstand sudden changes of temperature quite well, is practically infusible for all ordinary purposes, and is not attacked even by melted sodium. Carborundum is also used