water glass or soluble glass, colorless, transparent, glasslike substance available commercially as a powder or as a transparent, viscous solution in water. Chemically it is sodium silicate, potassium silicate, or a mixture of these. It is prepared by fusing sodium or potassium carbonate with sand or by heating sodium or potassium hydroxide with sand under pressure. Water glass is very soluble in water, but the glassy solid dissolves slowly, even in boiling water. Water glass has adhesive properties and is fire resistant. It is used as a detergent; as a cement for glass, pottery, and stoneware; for fireproofing paper, wood, cement, and other substances; for fixing pigments in paintings and cloth printing; and for preserving eggs (it fills the pores in the eggshell, preventing entrance of air).