Na2O.xSiO2. An aqueous solution of sodium silicate prepared from silicon dioxide (SiO2) and sodium oxide (Na2O) in a ratio that varies from 2 to 3.5. Water glass forms a hard glasslike mass when it dries. A water glass solution is viscous and has little tack, so when it is used as an adhesive pressure must be applied to hold materials together while bonding. The dried product is brittle and water sensitive. Aluminium salts can be added to the formulation to improve water resistance. Water glass has been used to make artificial stone. It was tried unsuccessfully as a binder in the 19th century for fresco paintings. Calcium water glass was used in the Ransome process of stone preservation. This procedure used alternating solutions of an alkaline silicate and calcium chloride to form insoluble calcium silicate (calcium water glass) in the pores of the stone. Potash water glass is composed of potassium silicate. Double water glass is a mixture of equal parts potassium silicate and sodium silicate. Synonyms: soluble glass; soluble silicate; liquid glass; aqueous sodium silicate; flint liquor; silicate of soda; waterglass; water-glass; calcium water glass; double water glass; potash water glass Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]