Silica, formerly styled the earth of flints, which constitutes the basis of all commercial glass, is infusible by itself in the strongest fire of our furnaces ; but its vitreous fusion is easily effected by a competent addition of potash or soda, either alone or mixed with lime or li Marge. The silica, which may be re garded as belonging to the class of acids, combines at the heat of fusion with those bases into saline compounds ; and hence glass may be viewed as a silicate of cer tain oxides, in which the acid and the bases exist in equivalent proportions. Were these proportions, or the quantities of the bases which silica requires for its saturation at the melting point, exactly ascertained, we might readily determine beforehand the best proportions of ma terials for the glass manufacture.
Glass-houses are commonly large coni cal buildings, from 60 to 100 feet high, and from 50 to 80 feet in diameter.
The furnace is in the middle, over a large vault, which is connected with it by means of an opening. This opening is covered with an iron grate, upon which the fire is made, and it is kept up by the draught of air from the vault.
The most important part, however, of the apparatus of the glass-house is the crucible, made from clay, found at Stour bridge. This is first pounded fine, then sifted, moistened, and worked into a thick dough. Sometimes old crucibles are used, which are broken into powder, and then mixed with a red clay. Some pots, for bottle and flint glass, are made 40 inches deep and wide. They are from 2 to 4 inches in thickness. They remain several days at a white beat, before they are placedin the furnace.
The basis of glass is silica. When flints or quartz are used, they are first reduced to powder by being heated red hot, and then plunged into cold water. This causes them to whiten and fall to pieces, after which they are ground and sifted. The second ingredient is potash or soda. The alkali used is more or lees pure, according to the fineness of the glass to be made. Lime is often em ployed in small quantities ; also borax.
Of the metallic oxides added in dif ferent cases, the deutoxide of lead is the most common. It renders flint glass more fusible, heavy, and tough, more easy to he ground and cut, and increases its bril liancy and refractive power.
A small quantity of black oxide of man ganese renders the glass more transpa rent; too much gives a purple tinge, which, however, may be destroyed by a little charcoal or wood.
Arsenious acid (white arsenic), in small quantities, promotes the clearness of glass ; too much of it gives the glass a milky whiteness. Its use in drinking
vessels is not free from danger, if the glass contains so much alkali that any part is soluble in acids.
The various materials are carefully washed, and, after the extraction of all the impurities, are conveyed to the fur nace in pots made of tobacco-pipe clay. The produce of this process is called the frit, which is again melted in large pots or crucibles, till the whole mass becomes beautifully clear, and the dross rises to the top.
Blowing is the next process, which, in round glass, as phials, drinking-glasses, cte., is thus performed ;—Thec workmen dip the end of long iron pipes, red hot, into the liquid glass, then roll it on a polished iron plate to give it an external even surface • they next blow down the iron pipe, till it enlarges the metal like a bladder, and, if necessary, roll it again on the iron plate, and proceed to form it into a globular or any other one required. The glass is then transferred from the by dipping the end of another iron rod into the liquid glass, which adheres to the heated rod, and with which the workman sticks it to the bottom of the vessel; then, with a pair of pincers, wetted with water, he touches the neck, which immediately cracks, and, on being slightly struck, separates at the end of the blowing-pipe, and becomes attached to the iron rod. The vessel is next carried up to the mouth of the furnace, to be heated and softened, that the operator may finish it. If the vessel require a handle, the opera tor forms it separately, and unites it while melting hot, forming it with pincers to the requisite shape and pattern.
Annealing is the removing of the glass, after it has been blown or cast, into 'a furnace, whose heat is not sufficiently intense to melt it ; and gradually with drawing the article from the hottest to a cooler part of the annealing chamber, till it is cold enough to be taken out for use. If cooled too suddenly, it is extremely brittle.
Coloring .—The different colored glasses owe their tints to the different metallic oxides mixed with the materials while in a state of fusion. In this manner are made those elegant pastes, which so faith fully imitate, and not unfrequently excel,. in brilliancy their originals, the gems of antiquity. The glass, however, for this purpose is preserved in a peculiar man ner, and requires great nicety. It com bines purity and durability.