Sec. XXXIX. Of Clay.
Clay is an argillo-siliceous substance, of a colour more or less yellow, and containing a variable quantity of silica and alumina, with oxide of iron. There are a variety of clays; the common potter's clay, pipe clay, porcelain clay, &c. Some contain, and others are free from iron. Those that contain this metal burn red; while those which remain, or become white in the process of burning, are free from it.
The use of clay in fire-works is confined nearly altogether to rockets. In the driving of sky-rockets, &c. the charge must always be driven one diameter above the piercer, and on it there is sometimes rammed one-third of a diameter of clay, through the middle of which a hole is bored to the composition, so that, when the charge is burnt to the top, it may communicate its fire through the hole, to the stars in the head. This, however, is not always the case. See [Rockets.]
The clay for fire-works, is usually prepared of the common kind, which contains neither stones nor sand. It must be first baked in an oven, until perfectly dry, and then pulverized, and sifted through a common hair sieve. In China, the Chinese mostly employ, for this purpose, their white porcelain clay.
Sec. XL. Of Quicklime.
Lime, as it is found in nature, is combined with carbonic and sulphuric acids, and less frequently with some of the other acids, as the nitric, fluoric and phosphoric. Calcareous carbonates are the most abundant; in which we include marble, limestone, and chalk; and the sulphate, or gypsum, may be considered the next. Lime constitutes the basis of marine shells; for, when burnt, they furnish quicklime. Its union with nitric acid is well known, forming the calcareous nitre of the saltpetre caves of Kentucky, &c. We have mentioned this combination under the head of nitre.
Without enumerating all the chemical properties of lime, it will be sufficient to remark, that it is composed of calcium and oxygen, and, when slaked with water, will evolve caloric in a free state, while the water solidifies or combines with the lime; that it forms with water, a solid hydrate, an example of which combination is afforded by the preparation of mortar; that it dissolves in water, and forms lime-water, and is slaked by exposure to the air, absorbing, at the same time, carbonic acid; that it unites with acids, like other salifiable bases, and forms salts, some of which are soluble in water, and others not; that it deprives the alkalies of carbonic acid, and renders them caustic, being itself changed into a carbonate; and, that it unites with sulphur and phosphorus, forming a sulphuret and phosphuret, and, also, with hydroguretted sulphur, and sulphuretted hydrogen, forming a hydroguretted sulphuret, and a hydro-sulphuret.
When limestone, marble, &c. are burnt in a kiln, the carbonic acid is expelled, and quicklime formed. Quicklime and lime, chemically speaking, are synonimous terms.
The fluor, or Derbyshire spar, is a fluate of lime. When this substance is distilled in a leaden retort, with sulphuric acid, we have sulphate of lime, and fluoric acid gas, called by some hydro-fluoric acid. This acid, when received in water, is used to etch on glass, in the same manner as nitric acid on copper; and while applied in a liquid state, or in that of gas, it acts on the glass, by combining with the silicon, and is changed from the hydrofluoric, into the silicated fluoric acid. If, instead of employing a leaden vessel, we make use of a glass retort, or introduce powdered glass or silica, into the leaden vessel, in either case, we obtain another acid, which we have just mentioned, the silicated fluoric acid; in consequence of the union of silicon with the supposed radical of the fluoric acid, known by the name of fluorine.
Quicklime is occasionally, though but rarely, employed in fire-works. That it increases the strength of powder, is asserted by Dr. Bayne. See [Gunpowder.] Its use in making slow match, along with other substances, is given in the article on that subject.