Obs. These crucibles of Stourbridge clay are made large enough to hold forty pounds or more of melted brass. They are only dried, and not baked. For use they are warmed, placed on the furnace, bottom upwards, the burning coke gradually heaped round them, and the firing continued until they acquire a fully red heat. They are then quickly taken out of the furnace, and put in again with the mouth upwards. If placed in the furnace with the mouth upwards at first, they are sure to crack. After they have been once used and allowed to become cold they are worthless.
Crucibles, Plat′inum. These are indispensable instruments in the laboratory of the analytical chemist. They are chiefly employed in the ignition of precipitates, and in the fusion of silicates with carbonated alkalies to render them soluble, a preliminary step to their analysis. The most ordinary form of the platinum crucible is that of a cup with a flat bottom. They are always provided with lids, which are sometimes so constructed that they may be used, when separated from the crucibles, as capsules for ignitions and evaporations. Platinum crucibles are not acted on by carbonated alkalies at a high temperature, but they are liable to be seriously damaged by the caustic alkalies. Precipitates of the more reducible metals must never be ignited in these crucibles, as the reduction of the metals would infallibly destroy the vessels.
Crucibles, Gold, are exceedingly useful for many operations, on account of the way which they stand caustic and carbonated alkalies, and nitric acid, which destroy platinum or silver crucibles respectively. Their drawbacks are their great expense and ready fusibility.
Crucibles, Silver. These are much used for fusions of alkalies, being much less acted on than platinum crucibles, and also for water analyses, from their cheapness and light weight. They are easily destroyed, however, by acids.
Crucibles, Plumba′go. Syn. Graphite c., Blacklead c., Blue pots. From graphite, ground and sifted, mixed with sufficient refractory clay to render it plastic. They are shaped by hand on an ordinary potter’s wheel, or by moulds of metal like that figured above under the head of Crucibles, Earthen.
Prop., &c. Good blacklead crucibles, even when of the largest size, support the greatest and most sudden alternations of temperature without cracking, and may be used after repeated heating and cooling. Their surface, within as well as without, may be made very smooth, so that particles of melted metal will not hang about the sides. They are now almost universally used for melting the precious metals.
Crucibles, Por′celain. These beautiful vessels are now made in Germany and France of all shapes and sizes. They are formed of the most exquisitely white, thin, and hard porcelain, which does not crack when heated, and which is but little acted on by the most energetic chemical reagents. For some operations they supersede platinum crucibles, particularly in the ignition of the precipitates
of the more reducible metals. They do not retain colouring matter, and are not porous. Their covers are excellently adapted for delicate cases of testing, the whiteness of the porcelain showing the changes of colour in a single drop of liquid most distinctly.
Crucibles, Iron. Used chiefly for preparing common reagents, as sulphide of iron, calcic chloride, &c., and also for preparing pure caustic potassa from the nitrate.
CRUMP′ET. A sort of muffin or tea-cake, very light and spongy. Prep. From flour, 2 lbs., made into a dough with warm milk-and-water, adding a little salt, 3 eggs (well beaten), and 3 teaspoonfuls of yeast, mixed to the consistence of thick batter; after standing before the fire for a short time, to rise, it is poured into buttered tins, and baked slowly to a fine yellow. For the table, crumpets are toasted lightly on both sides, buttered, piled on a hot dish, and cut into halves.