CHAPTER III.

THE RAW MATERIALS OF GLASS MANUFACTURE.

General considerations--Chemical purity, moisture, and physical condition, constancy of quality--Sources of silica, sand and sandstone--Felspar--Sources of alkali: Soda ash (carbonate of soda), salt-cake (sulphate of soda), pearl ash (carbonate of potash)--Alkali nitrates--Natural minerals containing alkalies--Sources of other bases: Lime, chalk, limestone, slaked lime-- Gypsum (sulphate of lime)--Barium compounds--Magnesia and zinc--Lead oxide, red lead--Aluminium, manganese, arsenic--Carbon--Coke, charcoal, anthracite coal _p._ 35