A few words about Soluble Glass.
It appears from ancient testimony, that there have been many successful attempts at making glass mouldable in its cold state, or of so far altering its state as to render it malleable. There is a tradition, that a clever artist, in the time of Tiberius Cæsar, made glass both elastic and malleable, for which he was rewarded by the loss of his head. A similar discovery is said to have been made in France, in the reign of Louis XIII. The inventor presented a bust, formed of malleable glass, to the Cardinal Richelieu, and was rewarded for his ingenuity by perpetual imprisonment, lest the French glass-makers should be injured. In our day, a description of glass, perhaps more remarkable, has been discovered by Mr. Fuchs, the curious properties and important applications of which it will be proper to advert to in this place.
The preparation of soluble glass does not greatly differ in its early stages from that of common glass. It is a union of silica and an alkali, which has, in addition to some of the properties of common glass, the property of dissolving in boiling water.
To form this compound, carbonate of potash and stone sand, are taken in the proportion of two, to three, or four parts of charcoal, and added to every ten parts of potash, and fifteen of sand. The charcoal accellerates the fushion of the glass, and separates from it all the carbonic acid, a small quantity of which would otherwise remain and be injurious. The materials must first be well mixed, then fritted and finally melted at a high heat, until a liquid and homogenous mass be obtained. This is removed by means of an iron ladle, and the glass-pot filled with fresh grit.
The crude glass thus obtained is usually full of bubbles; it is as hard as common glass—it is of a blackish gray, and more or less transparent at the edges. In order to prepare it for solution in water, it must be reduced to powder by pounding. One part of this glass requires from four to five of water for its solution. The water is first boiled in an open vessel; the powdered glass is added gradually, and is continually stirred to prevent adhesion. The boiling is continued for three or four hours, until no more glass is dissolved. If the boiling be checked before the liquor has thus attained the proper degree of concentration, carbonic acid will be absorbed by the potash from the air, which will produce an injurious effect. When the solution has acquired the consistency of syrup, and a density of 1·24, it is fit for use, and will keep for any length of time.
The solution of soluble glass is viscid, and when concentrated, becomes turbid or opalescent. The solution unites with water in all proportions. At a density of 1·28 it contains nearly twenty-eight per cent. of glass, and if the concentration be carried beyond that point, it becomes so viscid that it may be drawn out in threads like molten glass. When the solution is applied to other bodies it dries rapidly in the air, and forms a coat like varnish, and possesses the quality of not being affected by cold water, and it is applied as a durable coating to a vast variety of articles.
None of the methods hitherto proposed for making cloth fire-proof appear so advantageous as the application of soluble glass, for it does not act upon the vegetable matter, and completely closes the spaces between the threads. It can also be applied to clothes; but one of its most useful properties is its application as a cement; and, for this purpose, it is superior to all those that have hitherto been employed for cementing broken glass, porcelain, &c., and may be used instead of glue or isinglass in applying colors.
