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and again dried. It is then finely sifted, and most accurately mingled with quartz, ground very fine; to which, then, is added some burnt and finely pulverized gypsum. This mass is worked with water to a paste, and duly kneaded; it is usually suffered to lie in this state for
The vessels and other goods formed of this mass, are first moderately burnt in earthen pots, to receive a certain degree of compactness, and to be ready for glazing. The glazing consists of an easily melted mixture of some species of earths, as the petrosilex or chert, fragments of porcelain and gypsum, which, when fused together, produce a crystalline, or vitreous mass, that, after cooling, is very finely ground, and suspended in a sufficient quantity of water. Into this fluid the rough ware is dipped, by which the glazing matter is deposited uniformly on every part of its surface. After drying, each article is thoroughly baked or burned in the violent heat of the porcelain furnace. It is usual to decorate porcelain by paintings, for which purpose, enamels or pastes, coloured by metallic oxydes, are used, so easy of fusion as to run in a heat less intense than that in which the glazing of the ware melts.
Delft-ware, so called because first made at Delft in Holland, is a kind of pottery made of sand and clay, and but slightly baked, so that it resists sud. den application of heat. Articles made of this are glazed with an enamel, composed of common salt, sand ground fine, oxyde of lead, and oxyde of tin. The use of the latter is to give opacity to the glaze.
Tobacco-pipes require a very fine tenacious, and refractory clay, which is either naturally of a perfectly white colour, or, if it have somewhat of a grey
cast will necessarily burn white. A clay of this kind must contain no calcareous or ferruginous earth, and must also be carefully deprived of any sand it may contain, by washing. It ought to possess, besides, the capital property of shrinking but little in the fire. If it should not prove sufficiently ductile, it may be meliorated by the admixture of another sort. Last of all, it is beaten, kneaded, ground, washed, and sifted, tillit acquires the requisite degree of fineness and ductility.
When, after this preparation, the clay has obtained a due degree of ductility, it is rolled out in small portions to the usual length of a pipe, perforated with a wire; and put together with the wire, into a brass mould rubbed over with oil, to give it its external form; after which it is fixed into a vice, and the hollow part of the head formed with a stopper. The pipes, thus brought into form, are cleared of the redundant clay that adheres to the seams, å rim or border is made round the head, they are then marked with an iron stamp upon the heel, and the surfaces smoothed and polished. When they are well dried, they are put into boxes and baked in a furnace.
In the Dutch manufactories, these boxes consist of conical pots made of clay, with conical lids, with a tube passing through the middle of them, by which the pipes are supported; or else, they are long clay boxes, in which the pipes are laid horizontally, and stratified with fragments of pipes pounded small. Lastly, the pipes, when baked, are covered with a glazing or varnish, and afterwards rubbed with a cloth. This glazing consists of a quarter of a pound of soap, two ounces of white wax, and one ounce of gum arabic, or tragacanth, which are all
boiled together in five pints of water, for the space of a few minutes.
MANUFACTURE OF GLASS.
This beautiful material is not of modern invention; it was known to the ancient Romans, but it was by no mealis common among them; and they do not appear to have had the method of forming it into vessels of various shapes as is practised at present.
Glass is made by fusing together silex ånd potash, or soda, in proper proportions. Sea sand; which consists almost entirely of quartz and Aliits reduced to powder, is generally used for this purpose. The alcali is generallý procured from the bürning of sea weeds; these are cut; dried, and burned in pits dug in the ground; after a sufficient quantity of them have burned in the same pit, v melted or liquid mass is found in the bottom, which, after being well stirred, is suffered to cool, it is then called kelp, and consists of a mixture of sóda; potash, and parts of hálf burnt weeds, together with shells, sand, and other impurities.
When the ingredients of which glass is composed are perfectly fused, and have acquired a certain degree of heat; which is known by the Auidity of the mass, part of the melted matter is taken out at the end of a long · hollow tube, which is dipped into it, and turned about, till a sufficient quantity is taken up, the workmen then rolls it gently upon a piece of iron, to unite it more intimately. He then blows through the tube; till the melted mass at the extremity swells into a bubble, after
which he rolls it again on a smooth surface to polish it, and repeats the blowing, until the glass is brought as near the size and form of the vessel required as he thinks necessary.
If it be a common bottle, the melted glass at the end of the tube is put into a mould of the exact size and shape of its body, and the neck is formed on the outside, by drawing out the ductile glass.
If it be a vessel with a wide orifice, the glass in its melted state is opened and widened with an iron tool; after which being again heated, it is whirled about with a circular motion, and by means of the centrifugal force thus produced, is extended to the size required. Should a handle, foot, or any thing else of the kind, be required, these are made separately, and stuck on in its melted state.
Window-glass is made in a similar manner, except that the liquid mass at the end of the tube is formed into a cylindrical shape, which being cut longitudinally by scissars or sheers, is gradually bent back until it becomes a fat plate.
Large plate glass, for looking-glasses, &c. is made by suffering the mass in a state of complete fusion to flow upon a table, with iron ledges to confine the melted matter, and as it cools, a metal. lic roller is passed over it, to reduce it to an uniform thickness. There are various kinds of glass manufactured for different purposes; the principal of these are flint-glass, crown-glass, and bottle-glass.
Flint-glass is the densest, most transparent, colourless, and beautiful. It is sometimes called crystal. The best kind is said to be manufactured in London, from 120 parts of white siliceous sand, 40 parts of pearl-ash, 35 of red oxyde of lead, 13 of nitrate of potash, and 25 of black oxyde of manganese. It is the most fusible glass. It is used
for bottles, and other utensils, intended to be cut and polished, and for various ornamental purposes.
Crown-glass differs from the last, in containing no lead. It is made of soda and fine sand. It is used for panes of windows, &c.
Bottle-glass is the coarsest sort of all. It is made from kelp and common sand. colour is owing to iron. It is the least fusible.
Glass is sometimes coloured by mixing with it while in a fluid state, various metallic oxyds. It is coloured blue, by the oxyd of cobalt ; red, by the oxyd of gold; green, by the oxyd of copper or iron; yellow, by the oxyd of silver or antimony, and violet, by the oxyd of manganese.
The hardness of glass is very considerable; its specific gravity varies from 2.3 to 4, according to the quantity of metallic oxyd which enters into its composition. Though glass, when cold, is brittle, it is one of the most ductile bodies known. When liquid, if a thread of melted glass be drawn out, and fastened to a reel, the whole of the glass can be spun off'; and by cutting the threads of a certain length, there is obtained a sort of feather of glass. A thread of glass may be thus drawn or spun so fine, as to be scarcely visible to the naked eye. Glass is almost perfectly elastic, and is one of the most sonorous bodies. Fluoric acid dissolves it at common temperatures, and alcalis in a great degree of heat. These are the only substances known which act upon it.
Glass utensils require to be gradually cooled in an oven: this operation is called annealing, and is necessary to prevent their breaking by change of temperature, wiping, or slight accidental scratches.