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 per cent. 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.

Lime-water is the name given to the solution in water of slaked lime, called in chemical language hydrate of lime, calcium hydrate, and calcium hydroxide. To prepare it, quicklime, which has been made by burning (as it is commonly called) a pure marble, or, preferably, Iceland spar, is slaked with distilled water. The calcium hydrate formed is placed in a wide-mouth stoppered bottle, and covered with several times its bulk of distilled water. The object of this treatment is to dissolve soda and some other soluble impurities, the major part of which will be removed when the watery liquid in the bottle is decanted from the undissolved excess of calcium hydrate which should then be again covered with distilled water which has been recently boiled. The stopper should be well ground and smeared with vaseline. The bottle should be shaken at intervals in order that the water may take up as much calcium hydrate as it can dissolve. After all, this amount is very small, not exceeding, at 15° C, 0.172 part by weight per hundred measures of lime-water. Thus a gallon of lime-water, saturated at about 60° F., could not contain more than 120 grains of calcium hydrate, corresponding to 90 grains of pure lime or calcium oxide, CaO. In ordinary practice such a perfectly saturated solution is not attainable, while the most carefully prepared and strongest solution is sure to become weakened each time the stopper of the containing vessel is withdrawn by the removal of some of the lime in solution in the form of carbonate of lime.

The clearest lime-water, from this cause and from its action on glass, always appears turbid after a time.

Although so dilute a solution, lime-water gives the most marked reactions of an alkali: it turns red litmus paper blue, embrowns yellow turmeric paper, and imparts a crimson hue to colourless phenolphthalein paper. It acts energetically upon many organic and some inorganic pigments, owing to its alkaline or basic properties. The ease with which the lime in lime-water unites with carbonic acid, forming carbonate of lime (= calcium carbonate), and the bearing of this action, and of other properties of caustic lime upon the materials and processes of painting are discussed in Chapters II. and XXIII.

Baryta-water has its uses, but cannot replace lime-water in fresco-painting. It is a solution of hydrate of baryta, barium hydrate, barium hydroxide, for these names all belong to the compound, in distilled water. The distilled water used should have been recently boiled and then cooled out of contact with the carbonic acid of the air. The barium hydrate used may be purchased in the form of colourless crystals having the formula BaO2H2 + 8 aq. These, if not sufficiently pure, may be washed with cold distilled water, or recrystallized from boiling water, in which they dissolve very abundantly. A saturated cold solution is made by placing rather more than 1 ounce of these crystals in a bottle containing a pint of distilled water: the bottle should be almost full, the stopper should be smeared with a little vaseline. If the crystals dissolve completely, after repeated agitation, a few more should be added so as to leave a small excess at the bottom of the bottle. If the solution be clear it may be used directly from the bottle, as required; if filtration be needed, a glass plate should be placed on the funnel during the operation to prevent free access of air, and the clear filtrate should be received at once in the bottle in which it is to be preserved.

A solution of barium hydrate saturated at 15° C, contains nearly 2.9 grams of BaO in 100 cubic centimetres, or 2,023 grains per gallon. It is thus about seventeen times stronger than a solution of calcium hydrate saturated at the same temperature. Baryta-water, as it is called, is a powerfully alkaline liquid, becoming covered with a film of white barium carbonate on exposure to the air. By blowing air from the lungs through a glass tube into baryta-water, a dense white precipitate is formed.

Unfortunately, the binding power of barium carbonate is almost nil, so that baryta-water in mural painting is of service, not directly as a medium, but for destroying traces of calcium sulphate (gypsum) in the plaster-ground, and thus liberating a corresponding amount of lime-water. It may also be used for testing the effect of an alkaline earth on the powdered pigments which it is proposed to use in the work, in order to see if they can withstand its action; those unaffected by baryta will prove to be unchanged by lime.