Silica
One of the most abundant substances found in nature. Silica is the chief component of a number of precious stones, of rock-crystals, agates, porphyry, granite, flints, sandstone, and sand. When perfectly pure it is a fine powder, very hard, and will wear away glass. When mixed with water it does not adhere, but falls to the bottom, leaving the water clear. It fuses in the oxyhydrogen blowpipe, and may be drawn into threads after the manner of glass. When silica is mixed with alkalies it melts at a lower temperature, and combines with them to form glass. The minerals, feldspar, mica, hornblende, serpentine, etc., which form the granitic and many other rocks are silicates of the alkalies and alkaline earths. Glass and pottery are compounds of silica with various metallic oxides. See GRANITE.
Silica Bricks are made by incorporating about 50 pounds of lime-paste with a ton of crushed Dinas rock from the Swansea Valley. This rock contains about 97 per cent of silica, and the bricks produced from it are employed chiefly for the roofs and all exposed parts of the open-hearth steel-melting furnaces, and other similar purposes where the operations demand the most intense heat. See OPEN-HEARTH CAST STEEL.