Water-Glass

The name water-glass appears to have been first applied to those silicates of potash and of soda which are soluble in water by Professor J. N. von Fuchs, in 1825; but Glauber, so early as 1648, made a soluble potash silicate, which he termed fluid silica. Van Helmont had prepared a similar compound in 1640. The actual manufacture on a commercial scale of these salts dates, however, from 1825 only, and the credit of originating their production belongs to Von Fuchs. They differ from the compounds constituting ordinary and insoluble glass by containing no lime, baryta, alumina, or other earthy base. They are made in several ways. The purest sand obtainable is fused with carbonate of potash, or carbonate of soda, or a mixture in the desired proportions of these two carbonates, in the presence of a little powdered charcoal. The fused mass dissolves by long continued boiling in water, and yields a heavy syrupy liquid of strongly alkaline reaction. By evaporating this liquid to dryness, and fusing the residue, the water-glass may be obtained in a solid form, and then closely resembles ordinary glass in appearance.

Water-glass may also be made by heating flints red-hot, quenching them in water, and then digesting the powdered silica thus obtained with soda-lye or potash-lye under pressure.

Three kinds of water-glass have been used in water-glass painting or stereochromy. One of these is a potash silicate, another is a soda silicate, the third is a mixture of these two, or a potash-soda silicate, called double water-glass. The solutions of the two former silicates as met with in commerce vary a good deal in their relative proportions of silica and alkali; it is not desirable that they should contain so much silica as was recommended in the original papers of Von Fuchs, the inventor of stereo-chromy, and of Kuhlmann, who subsequently modified the process. Indeed, it has often been found useful to add a little pure caustic potash or caustic soda-solution or ammonia to the commercial solutions of water-glass before diluting them with distilled water for use in this process of painting.

A solution of water-glass, if allowed to dry upon a piece of ordinary glass, leaves an opaque white irremovable stain. Water-glass alters or destroys, in virtue of its strong alkalinity, the great majority of organic pigments. On the same account it cannot be used with flake-white, aureolin, the chromates, vermilion, and several other mineral pigments. It hardens zinc-white, some of the ochres, earths, and terre verte, forming with them, or with some of their constituents, double silicates, which are quite insoluble in water. The fixative power of water-glass in stereochromy depends indeed mainly upon actions of this order which occur between it and ingredients of the plaster or painting-ground, and of the pigments. It was formerly supposed that when an alkaline silicate acted upon carbonate of lime a double decomposition occurred, of which the only products were an alkaline carbonate, and lime silicate. But subsequent investigation has proved that the change in question is more complex, a considerable quantity of a double and insoluble silicate of lime and alkali being produced. Similar double silicates of potash or soda and zinc, of potash or soda and baryta, and of potash or soda and alumina, have been proved to exist in stereochromic work; doubtless many others are also present.

They are not only insoluble in water, but are harder than the materials out of which they have been formed.

Commercial solutions of water-glass contain from 28 to 60 percent of the alkaline silicate or silicates. They should be carefully preserved from access of air, the carbonic acid of which produces much alkaline carbonate (often separating in crystals in the case of soda), and finally causes the separation of gelatinous silica hydrates. The entrance of calcareous matters, gypsum, zinc-white, etc, should also be guarded against.

The subject of water-glass is here treated very briefly, partly because the various processes of stereochromy, even with their latest improvements, are very little used in this country, and partly because the preparations of water-glass specially made for the use of painters may be trusted. To this latter observation I might add the remark that the problem of thoroughly examining a commercial water-glass solution for strength, purity, and due proportion of silica to alkali, is too complex to be undertaken except by a trained chemist.