Small crucibles are sometimes formed by pouring ‘slip,’ that is, clay mixed with sufficient water to give it the consistence of cream, into porous moulds, made of a species of stucco. A series of these moulds are placed upon a table and filled with the semifluid composition. By the time the whole (say 50 or 60) are filled, the ‘slip’ may be poured out of the one first filled, leaving only a very small quantity behind to give the requisite thickness to the bottom. The second and third may then be treated in the same way, until the whole number have been attended to. In each mould a perfect crucible is formed, by the abstraction of the water of that portion of the ‘slip’ in immediate contact with the stucco, and the crucible is either thicker or thinner in proportion to the time this absorbent action has been allowed to go on. 70 or
80 crucibles may thus be easily made in less than 15 minutes. The moulds and their contents are next placed in a stove or slow oven. In a short time, from the contraction of the clay in drying, the crucibles may be removed, and the moulds, as soon as they have become dry, may be again filled; by care they will last for years.
Earthen crucibles are used both in the burnt and unburnt state. Small crucibles are generally kiln-burnt before they are used, but the large Stourbridge clay ‘casting-pots,’ which are extensively employed in brass foundries, are never previously burnt.
The following kinds of earthen crucibles are much used in the arts:—
Crucibles, Cornish. From Teignmouth clay, 1 part; Poole clay, 1 part; sand from St. Agnes’s Beacon, Cornwall, 2 parts. When smaller and less refractory crucibles are needed, the same mixture is employed, with the addition of an eighth part of China clay, or Kaolinite from St. Austell. These crucibles are generally made round, and of two sizes, of which one fits into the other; the larger being 3 inches in diameter at the top, and 31⁄2 inches high outside measure. They are coarse in grain, and of a greyish-white colour, spotted with dark specks. They are always kiln-burnt. Of all crucibles, none are more generally useful for metallurgical experiments.
Crucibles, Hessian. From a mixture of equal weights of Almerode clay and sand. They are generally triangular in shape, so that the melted metal may be conveniently poured out from each corner. They are usually sold in ‘nests’ of six crucibles, fitting one in another. In the character of their body, and in composition and qualities, they closely resemble the Cornish.
Crucibles, London. From a very refractory clay. They have a reddish-brown colour, and are close in grain. They are exceedingly useful in assaying, as they resist the action of fused oxide of lead much better than most clay crucibles. Being very liable to crack, they require to be used with care.
White Fluxing-pots. From a peculiar kind of foreign clay. They are manufactured by the Patent Plumbago Crucible Company, and are much esteemed by metallurgists, being well moulded and very refractory. They have a smooth surface, and withstand the action of fluxes satisfactorily.
Crucibles, Stourbridge-clay. From Stourbridge clay, 4 parts; burnt clay, obtained by pounding and grinding old glass pots, 2 parts; pipe-clay and coke-powder, of each 1 part.
Anstey’s Patent. From Stourbridge clay, 2 parts; hard gas-coke (previously ground and sifted through a sieve of 1⁄8th-inch mesh), 1 part.