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METHOD OF MAKING POTTERY

converted into carbonic acid gas, which escapes; the sulphuricacid is thus partly converted into sulphur, which sublimes andburns. During these changes, the potash, which is present inthe alum of commerce, unites to part of the sulphur and formswith it a sulphuret of potash; and there remains the superfluouspart of the coal that had been blended with the sulphate of alu-mine. The constituent parts of the pyrophorus are consequentlyexceedingly dry potash, sulphur, charcoal, and alumine. Onexposure to air, the first rapidly attracts its moisture, and isheated with it. This heat is sufficient to inflame the sulphur,because sulphur is already, by its own nature, when combinedwith alcaline substances, By far more disposed to decompose ox-igen gas. Lastly, this ignition of the sulphur in the pyrophorusis communicated to the coaly particles, that, at the beginning ofits preparation, were mingled with the sulphate of alumine.

METHOD OF MAKING POTTERY.

Potter)' or stone wa're of all sorts, from the coarsest brownpitcher to the finest porcelain is made of alumine and silex. Thepse of the flint is to give strength to the ware, so that it maypreserve its solidity during baking. In making the English stoneware, care is taken to employ only the fine parts of the alumineand chert, a species of flint. With this view the alumine is muchbeaten in water, by which the fine parts, being mixed with thefluid, are suspended, while the coarser sink to the bottom of thevessel. The thick liquid is further purified by passing it throughhair and lawn sieves of different fineness; after this the liquidis mixed in various proportions for different wares, with another

liquor of about the same density, and consisting of ground flints.The mixture is then dried in a kiln, and after being beatento a proper consistence it becomes fit for being formed at thewheel, into dishes, plates, bowls, &rc. When the ware hasaeen exposed to heat tor a certain time it is glazed, or is made tomdergo a partial vitrification at the surface, without which it