It also possesses the power of combining with most metallic oxides at high temperatures, forming borates, which in many cases show characteristic colours.
The theory most widely accepted at present is that glass is a quickly solidified solution, in which silica, silicates, borates, phosphates and aluminates may be either solvents or solutes, and metallic oxides and metals may be held either in solution or in suspension.
The metallic borates are generally obtained in the hydrated condition, and with the exception of those of the alkali metals, are insoluble in water.
The most important of the borates is sodium pyroborate or borax (q.v.).
Fused borax dissolves many metallic oxides, forming complex borates which in many cases show characteristic colours.
More recently, the extensive deposits of borates (chiefly, however, of calcium; see Colemanite) in the Mohave desert on the borders of California and Nevada, and in the Atacama desert in South America, have been the chief commercial sources of boron compounds.
The deposits formed by evaporation from these lakes and marshes or salines, are mixtures of borates, various alkaline salts (sodium carbonate, sulphate, chloride), gypsum, &c. In the mud of the lakes and in the surrounding marshy soil fine isolated crystals of borax are frequently found.