CRUCIBLES are small conical vessels, narrower at the bottom than the mouth, for reducing ores in docimacy by the dry analysis, for fusing mixtures of earthy and other substances, for melting metals, and compounding metallic alloys. They ought to be refractory in the strongest heats, not readily acted upon by the sub stances ignited in them, not porous to li quids, and capable of bearing considera ble alternations of temperature without cracking ; on which account they should not be made too thick. The best cruci bles are formed from a pure fire-clay, mixed with finely ground cement of old crucibles, and a portion of black lead or graphite. Some pounded coke may be mixed with the plumbago. The clay should be prepared in a similar way as for making pottery-ware ; the vessels af ter being formed must be slowly dried and then properly baked in the kiln. Crucibles formed of a mixture of 8 parts in bulk of Stourbridge clay and cement, 5 of coke, and 4 of graphite, have been found to stand 23 meltings of 76 pounds of iron each, in the Royal Berlin foundry. Such crucibles resisted the greatest possi ble heat that could be produced, in which even wrought iron was incited, equal to 150° or 155° Wedgewood, and bore sud den cooling without cracking. Another composition for brass-founding crucibles is the following : Stourbridge clay, 1. burned clay cement, coke powder, pipe clay. The pasty mass must be com pressed in moulds. The Hessian cruci
bles of Germany are made from a fire clay which contains a little iron, but no lime ; it is incorporated with silicious sand. The dough is compressed in a mould, dried, and strongly kilned. They stand saline and leaden fluxes in docimas tic operations very well ; are rather po rous on account of the coarseness of the sand, but arc thereby less apt to crack from sudden heating or cooling. They melt under the fusing point of iron. Beaufay in Paris has lately succeeded in making a tolerable imitation 01 the Hes sian crucibles with a fire-clay found near Namur in the Ardennes.
Berthier has published the following elaborate analysis of several kinds of cru cibles : Wurzer states the composition of the sand and clay in the Hessian crucibles as follows : My—silica 10.1, alumina 65.4, oxides of iron and manganese P2, lime 0.3, water 23.
Send—silica 95.6, alumina 2.1, oxides of iron and manganese P5, lime 0.8. Black lead crucibles arc made of two parts of graphite and one of fire-clay, mixed with water into a paste, pressed in moulds, and well dried, but not baked hard in the kiln. They bear a higher heat than the Hessian crucibles, as well as sudden changes of temperature, have a smooth surface, and are, therefore, pre ferred by the melters of gold and silver. This compound forms excellent small or portable furnaces.