Crucibles, Earth′en

Syn. Clay crucibles . From fire-clay, mixed with silica, coke, burnt clay, or other infusible matter. Manuf. The materials, having been ground and kneaded, are generally moulded by hand upon a wooden block of the shape of the cavity of the crucible. Another method of shaping a crucible consists in ramming the ingredients into a suitable mould, formed of steel or gun-metal. (See engr. ) IMG:596405606043057898_i533.png: a a. External steel mould. b b, Clay or composition for forming the crucible. c, Internal steel mould. d d, Wooden stand. e, Cord or chain to withdraw the internal mould or plug. 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 thickne 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 le 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 Stour bridge clay ‘casting-pots,’ which are extensively employed in bra foundries, are never previously burnt. The following kinds of earthen crucibles are much used in the arts:—