Soluble Glass. This is a combination of silica with an alkali, soluble in boiling water, yielding a fine, transparent, semi-elastic varnish. When made according to Liemen's or Kuhlman's method, under increased pressure and heat, it is unaffected by cold water, and the object painted or covered by the same can only be deprived of its coating by undergoing the same heat and pressure as was required to prepare the original solution. Soluble glass prepared from potash is usually called silicate of potash; that from soda being silicate of soda. The most extensive use which is made, at present, of soluble glass produced after the other methods, is for the adulteration of soap; in fact, such a preparation is a kind of soap, in which the expensive fatty acids are replaced by the cheap silicic acid or sand; but it is a bad soap, very caustic, as the silicic acid but very imperfectly neutralizes the alkali. Another use of water glass is that of hardening cements, mortar, etc., so as to render them impermeable by water.

2817. Fuchs' Soluble Potash Glass

2817. Fuchs' Soluble Potash Glass. A mixture of 15 parts pulverized quartz, or pure quartz sand, 10 parts of well purified potash, and 1 part powdered charcoal, may be conveniently employed. These ingredients are to be well mixed and exposed to a strong heat in a fire-proof melting-pot for 5 or 6 hours, until the whole fuses uniformly and steadily; as much heat is required as is necessary to melt common glass. The melted mass is then taken out by means of an iron spoon, and the melting-pot immediately refilled with a fresh quantity. (At this stage of the process it is said by another authority, that, by pulverizing and exposing it to the air, it will absorb acidity, and by degrees the foreign salts will, after frequent agitation and stirring, be completely separated, particularly after pouring over the mass some cold water, which dissolves them, but not the soluble glass.) It is then broken up, pulverized, and dissolved in about 5 parts of boiling water, by introducing it in small portions into an iron vessel and constantly stirring the liquid, replacing the water as it evaporates, by adding hot water from time to time, and continuing to boil for 3 or 4 hours, until the whole is dissolved - a slimy deposit excepted - and until a pellicle begins to form on the surface of the liquid, which indicates that the solution is in a state of great concentration; it disappears, however, when the liquid is stirred; and the boiling may then be continued for a short time, in order to obtain the solution in the proper state of concentration, when it has a specific gravity of from 1.24 to 1.25 (about 28° Baum'e). In this state it is sufficiently liquid to be used in many operations; in some instances it will be necessary to dilute it with more or less water. "When evaporated to a syrupy consistence, it can be employed with advantage in but few cases. Very frequently it is found contaminated with a little sulphide of potassium, and it becomes necessary to add a little oxide of copper or copper scales towards the end of the boiling, which liberates a small quantity of potash, but which renders it rather more suitable for many practical purposes than otherwise. If it is desirable, however, to have a water - glass which is entirely neutral, it requires to be boiled - with freshly-precipitated silica as long as any silica is dissolved.