Crucible, a small vessel made of refractory materials for withstanding high temperatures, and used in metallurgic and chemical operations for containing substances to be melted. The name is said to have been given to it by the alchemists, from the Latin crux, cruris, in consequence of their custom of marking it with the sign of the cross. Others derive it from the Latin crucio, to torment, because the contents, in the language of the same alchemists, were thus treated in the operations to which they were subjected. Crucibles are made in various forms and of different materials, according to the purposes required of them. The qualities they should possess are infusibility, capacity of bearing sudden changes of temperature without breaking, resistance to the chemical action of the substances fused in them, and a texture impermeable to liquids and gases. - The best earthenware or porous crucibles are made of the purest clays, such as consist only of alumina and silica. The texture depends upon the degree to which the materials are pulverized. The close Wedgwood crucibles are made of the best materials finely ground; but they do not withstand sudden changes of temperature so well as the coarser Hessian and English crucibles.
The former, which have been long known as the cheapest and among the best clay crucibles, are made in the vicinity of Almerode, Germany, of an aluminous clay, mixed with quartz sand. They are three-sided at top and round below. Their composition, according to Berthier, is silica 70.9, alumina 24.8, oxide of iron 3.3, with traces of magnesia. They are remarkable for withstanding sudden changes and high degrees of temperature. Small ones may even be heated to redness and thrown into cold water without breaking. They will soften, however, at the high heat of the furnaces in which they are used, and the coarseness of their material renders them very porous. Saltpetre and common salt, and other substances used as fluxes, are liable when fused to find their way through them. Porcelain or Wedgwood crucibles are more impervious to vapors and fluxes. The French crucibles of Beaufay are perhaps more refractory than the Hessian. They are made near Namur, of clay without additional mixture of sand; when moulded they are washed over with a thin coating of pure clay, prepared by pulverizing clay that has been baked. They are more dense than other clay crucibles, and hence better resist the passage of fluxes.
Their composition is, by the analysis of Berthier, silica 64.6, alumina 34.4, oxide of iron 1. The English or London crucibles are triangular or circular, and have covers of the same material. They are composed of two parts of Stourbridge clay and one of pulverized coke. The Cornish crucibles, made for the assayers of copper ores in Cornwall, have long been celebrated. They are made of Poole and Stourbridge clay, mixed with sand from St. Agnes, and ground pots. An analysis by Dr. Percy gives, in 100 parts, silica 72.29, alumina 25.32, protoxide of iron 1.07, lime 0.38, magnesia a trace, potash 1.14. - Blue pots, or black-lead crucibles, as they are often incorrectly called, are made of the mineral graphite or plumbago, which is composed of carbon with 4 to 10 per cent, of iron. The substance is finely pulverized, mixed with a third or half its weight of clay, moulded into the pots, some of which are large enough to serve for assaying furnaces, and then baked. These are excellent crucibles for resisting changes of temperature, as well as the chemical action of their contents; but their higher cost limits their use. They are used in melting cast steel in the large works where it is manufactured.
They may be protected on the inside from the action of the oxides, which tend to remove the carbonaceous material, by a lining of clay or other substance. These are made of excellent quality in Boston and Jersey City. - For different chemical operations crucibles made of various metals are employed. Those of platinum are in continual use in the operations connected with chemical analyses. But these, though they bear the highest temperature, are attacked by many substances which do not affect other metals, as silver particularly, and crucibles of this material are therefore required as occasional substitutes. Cast-iron crucibles are cheaply made, and are very serviceable in many assays of sulphurets especially. The iron itself serves to desulphurize the natural compound of this substance, as the carbon of the brasqued crucibles deoxidizes the oxides. Assays of galena may be rapidly made one after another in cast-iron crucibles, by introducing a portion mixed with twice and a half its weight of carbonate of soda and fusing; the galena is decomposed, and sulphuret of iron is produced at the expense of the crucible; the lead set free may be poured out, and a new portion instantly introduced, and thus the operation may be continued as long as the crucible lasts.