Crucible
A melting-pot for melting brass, cast iron, steel, etc., as well as for a numerous variety of chemical purposes. The common crucible is made of clay, and designed to withstand a strong heat. The best of this class are the "Hessian" crucibles, which contain a proportionate quantity of sand, mixed with clay of a highly refactory nature.
The ordinary crucible is made from two parts Stourbridge clay and one part very hard coke--the coke to be well ground and the two ingredients well mixed and tempered together.
Crucibles for melting steel must be made from the best ingredients, and should show a special toughness during the high temperature required for steel-melting. If soft, they are apt to crush under the pressure exerted by the tongs with their load of 75 pounds of molten steel. The materials generally employed are fire-clay, burnt and raw, graphite, and ground coke. The burnt fire-clay is simply old crucibles, etc., perfectly freed from slag and scoria and ground into a fine powder.
The Mushet crucibles for melting steel are made from Stourbridge clay 5, kaolin 5, ground crucible 1, ground coke 1 1/2. These crucibles are from 16 to 19 inches high and 6 to 8 inches diameter. Many steel melters make their own crucibles, claiming that they can suit the crucible to the kinds of steel they are making.
Krupp uses his crucibles but once for steel. They are then ground, and mixed with plumbago to make other crucibles with, the latter substance being the only ingredient used.
Black-lead crucibles were first made by Joseph Dixon, founder of the present company in Jersey City, in the year 1827. They became at once the standard at home and abroad, and continue so to be to the present day. They are made of this material from 1/4 pound to 1000 pounds capacity, and are for melting steel, brass, gold, copper, silver, German-silver, pure nickel, white metal, etc., and also for file-tempering.
Black-lead crucibles are all annealed before shipping, but immediately on arrival they should be unpacked and stored in a warm, dry place to expel any moisture that may have been absorbed in the transit. To provide against possible accident, it is always wise to re-anneal. Before placing in the hot furnace, the temperature of a crucible should be slowly raised to at least 212 deg. F., or a little above the temperature of boiling water. All clay crucibles, new or old, should be gradually heated, mouth down, up to a red heat before charging, and, whether in the preparatory heating, or subsequent firing for melting, the crucible should always be well covered with fuel; otherwise the unequal temperatures induced by carelessness in this particular will result in a broken pot. If clay crucibles could be kept constantly in use without allowing them to become cold alternately, there is no reason why they should not last from 15 to 20 meltings, especially if they are kept clear from scoria, and repaired occasionally with a paste made from silica, fire-clay, and coke-dust. Lifting-tongs should always be made to fit the crucible snugly, at as many places as convenient, to prevent splitting. By exerting the pressure unequally on the sides of a crucible very many accidents occur; especially is this to be observed in regard to large ones. Crucibles should always be returned to the furnace mouth downwards, after the heat is through, so that the cooling may occur gradually. The best graphite crucibles should last from 20 to 30 meltings, and, whilst they may be less liable to fracture from sudden changes in the temperature, it is well to observe, in some measure at least, the directions given for clay ones. The above instructions refer to brass-melting only. See STOURBRIDGE CLAY; KAOLIN; PLUMBAGO; GRAPHITE; FIRE-CLAY; HESSIAN-CRUCIBLE; CRUCIBLETONGS.