Although the manufacture of soluble glass does not strictly belong to the glass maker’s art, yet it is an allied process to that of manufacturing glass.  Of late soluble glass has been used with good effect as a preservative coating for stones, a fire-proofing solution for wood and textile fabrics.  Very thin gauze dipped in a solution of silicate of potash diluted with water, and dried, burns without flame, blackens, and carbonizes as if it were heated in a retort without contact of air.  As a fire-proofing material it would be excellent were it not that the alkaline reaction of this glass very often changes the coloring matters of paintings and textile fabrics.  Since soluble glass always remains somewhat deliquescent, even though the fabrics may have been thoroughly dried, the moisture of the atmosphere is attracted, and the goods remain damp.  This is the reason why its use has been abandoned for preserving theater decorations and wearing apparel.  Another application of soluble glass has been made by surgeons for forming a protecting coat of silicate around broken limbs as a substitute for plaster, starch, or dextrine.

The only use where soluble glass has met with success is in the preservation of porous stones, building materials, paintings in distemper, and painting on glass.  Before we describe these applications, we will give the processes used in making soluble glass.

The following ingredients are heated in a reverberatory furnace until fusion becomes quieted:  1,260 pounds white sand, 660 pounds potash of 78 deg..  This will produce 1,690 pounds of transparent, homogeneous glass, with a slight tinge of amber.  This glass is but little soluble in hot water.  To dissolve it, the broken fragments are introduced into a iron digester charged with a sufficient quantity of water, at a high pressure, to make a solution marking 33 deg. to 35 deg.  Baume.  Distilled or rain water should be used, as the calcareous salts contained in ordinary water would produce insoluble salts of lime, which would render the solution turbid and opalescent; this solution contains silica and potash combined together in the proportion of 70 to 30.

Silicate of soda is made with 180 parts of sand, 100 parts carbonate of soda (0.91), and is to be melted in the same manner as indicated previously.

Soluble glass may also be prepared by the following method:  A mixture of sand with a solution of caustic potash or soda is introduced into an iron boiler, under 5 or 6 atmospheres of pressure, and heated for a few hours.  The iron boiler contains an agitator, which is occasionally operated during the melting.  The liquid is allowed to cool until it reaches 212 deg., and is drawn out after it has been allowed to clear by settling; it is then concentrated until it reaches a density of 1.25, or it may be evaporated to dryness in an iron kettle.  The metal is not affected by alkaline liquors.

The glass is soluble in boiling water; cold water dissolves but little of it.  The solution is decomposed by all acids, even by carbonic acid.  Soluble glass is apparently coagulated by the addition of an alkaline salt; mixed with powdered matters upon which alkalies have no effect, it becomes sticky and agglutinative, a sort of mineral glue.