Soluble Glass. A simple silicate of potassa or soda, which unites perfect solubility in boiling water to some of the general properties of common glass. In the liquid state, it may be applied to cloth or wood, for the purpose of rendering them incombustible. In fact, by the evaporation of the water in which it is dissolved, a layer of a substance capable of fusing when heated, is deposited on these bodies, that protects them from the contact of air necessary for their combustion. The following account of its manufacture and uses is derived from a translation by Professor Renwick, of the Traite de Chimie applique aux Arts, par M. Dumas.


Soluble glass may be obtained by dissolving pure silica, obtained by precipitation, in a boiling solution of caustic potassa; but this process, being both inconvenient and costly, cannot be practised upon a large scale. When sand and carbonate of potassa are heated together, the carbonic acid is never wholly driven off, except when the sand is in excess; but the whole of the carbonic acid may be expelled by adding powdered charcoal to the mixture, in such proportion that the carbonic acid of that part of the carbonate which is not decomposed may meet with a sufficient quantity of carbon to convert it into carbonic oxide. In this way the silica first forms a silicate in the proportions contained in common glass, and drives off the appropriate equivalent of carbonic acid; then, at a high heat, the rest of the carbonate of potassa is decomposed by the carbon, the carbonic oxide escapes, and the potassa thus freed, either sublimes, or combines with the glass already formed.

The sand (freed from lime and alumina) and carbonate of potassa (pearl ash) are taken in the proportion of 2 of the latter to 3 of the former, and to 10 parts of pearlash and 15 of sand, 4 parts of charcoal are added. A less portion of charcoal must not be taken; on the contrary, if the form of potash employed be not sufficiently pure, a larger proportion of charcoal may be advantageously employed. This substance accelerates the fusion of the glass, and separates from it all the carbonic acid, of which there would otherwise remain a small quantity that would have an injurious effect. In other respects, the same precautions that are employed in the manufacture of common glass are to be observed. The materials must be first well mixed, then fritted, and finally melted in a glass pot, until the mass becomes liquid and homogeneous. The melted matter is taken out of the pot with an iron ladle, and the pot is then filled with fresh frit. Thirty pounds of pearlash, 45 of sand, and 121bs. of powdered charcoal may be taken for a charge; with this quantity the heat must be continued for five or six hours.

The crude glass thus obtained, is usually full of air bubbles; it is as hard as common glass, of a blackish gray colour, and transparent at the edges; sometimes it has a colour approaching to whiteness, and at others is yellowish or reddish these are indications that the quantity of charcoal has not been sufficient. If it be exposed for some weeks to the air, it undergoes slight changes, which rather tend to improve than injure its qualities. It attracts a little moisture from the air, which slowly penetrates its mass, without changing its aggregation or its appearance; it merely cracks; and a slight efflorescence appears at its surface. If it be exposed to heat, after it have undergone this change, it swells up, owing to the escape of the aqueous matter it has absorbed. In order to prepare it for solution in boiling water, it must be reduced to powder by stampers; if this were not done, it would dissolve too slowly. One part of glass requires from 4 to 5 of water for its solution. The water is first heated to ebullition in an open boiler, the powdered glass is then added by degrees, and must be continually stirred, to prevent it from adhering to the bottom.

The ebullition must be continued three or four hours, until no more glass is dissolved: the liquor will then have acquired the proper degree of concentration. If the ebullition be checked before this state is attained, carbonic acid will be absorbed by the potassa from the air, which will produce an injurious effect; for the same reason, too great a quantity of water must not be employed, for during the long evaporation which will then become necessary, the carbonic acid of the water will readily combine with the potassa, and cause a precipitation of the silica. When the liquor becomes too thick, before the whole of the glass is dissolved, boiling water must be added. When the solution has acquired the consistence of syrup, and a density of 1.24 to 1.25, it is sufficiently concentrated, and fit for use. It is then permitted to rest, in order that the insoluble parts may be deposited; while it is cooling, a pellicle forms upon the surface, which after a time disappears of itself, or may be redissolved by depressing it in the liquor. This pellicle begins to appear during the ebullition, and indicates its concentration.

When the crude glass is of a proper composition it contains but a few saline impurities, and no sulphuret of potassium, it may be treated in the way we have described; but it' it contain any notable proportion of these substances, they must be separated before it is dissolved; this separation may be effected in the following manner: - The powdered glass is exposed to the action of the air for three or four weeks, during which time it must be frequently stirred; and if it run into lumps, which will happen in moist weather, they must be broken up. The glass, as we have stated, attracts moisture from the air, and the foreign substances either separate or effloresce. It then becomes easy to remove them from the glass. It is sprinkled with water, and frequently stirred. At the end of three hours the liquor is removed, it will then contain a part of all the saline impurities, and a little of the silicate of potassa; the powder is again to be washed with fresh water. Soluble glass thus treated readily dissolves in boiling water, and the solution leaves nothing to be desired. To preserve it in the liquid form no particular care is necessary, as even after a long space of time it undergoes no perceptible change, if the solution have been properly prepared.

The only precaution is not to allow air too free an access to it. A similar product may be obtained by using a carbonate of soda instead of one of potassa. In this case, two parts of the soda of the shops is required for one of silica. This glass has the same properties as the other, but is more valuable in its uses. The solutions of these two kinds of glass may be mixed in any proportion whatever, and this mixture is more serviceable in some cases, than either of them separately.