CHEMICAL AND PHYSICAL PROPERTIES. Chemi cally, any vitreous compound is called glass, but commercially glass is a fused mixture of two or more metallic silicates, and is often named from the predominant base. as 'soda glass.' pot ash glass,' lime glass,' and 'lead glass.' The es sential ingredients are silica and alkali. Flint glass is a mixture of the silicates of lead and potassium; Bohemian glass, of tile silicates of potassium and calcium; plate or sheet glass, of the silicates of calcium and sodium; bottle glass is a mixture of the silicates of sodium, nois, Missouri, Ohio, Indiana, New Jersey, New York, and Maryland, but good sand is abundant all over the United States.
Ancient glass was commonly a soda glass. one part of sand to three of crude soda, and, there fore, was soft, decomposing easily. The inferior soda, made from the ashes of sea-plants, was succeeded by soda-ash, made from salt by Le blanc's prize discovery of 1792, which opened a new era in glass-making. Since 1875 'salt-cake,' sulphate of soda, has been largely used, pro ducing a harder glass of a bluish color, while the carbonate of glass is softer and yellowish. The use of potash as an ingredient dates from the Middle Ages. It is made from the mineral chloride by the Leblanc process, similar to the manufacture of soda-ash from salt. Lime, next to silica in importance in producing toughness, is a modern discovery. Of old, it was used sparingly as a cheap substitute for soda and potash; now, with improved facilities, it pro duces 'lime-flint,' unequaled for lightness and beauty in common wares. It is used in the form of carbonate of lime, which is a staple compound, while burned or slaked lime constantly changes its chemical composition. Lead oxide (litharge) began in the seventeenth century to produce the heaviness and brilliancy of the English flint glass now made in every industrial country. Another oxide of lend, called red lead, is also used in modern glass, and is said to be preferable on account of its excess of oxygen. Other ingredi ents, as barium, zinc, and lithium, are used in special glasses. Manganese is introduced in mod ern glass as a decolorizer, to correct the greenish tint produced by iron, which is an unwelcome ingredient of nearly all materials. Arsenic, and carbon in the form of charcoal or coal, are aids to the decomposition of the batch. The propor
tions of the ingredients differ widely among va rious establishments.
Potassic silicate is colorless, and contributes brilliancy and fusibility; sodic silicate is also bright and fusible, but imparts a sea-green tint; plumbic silicate increases the fusibility, ductility, and brilliancy of glass. If an excess of any material is used it affects the quality of the glass. Too much alkali tends to make the glass spotted, scaly, rough, or cordy; excess of lime Molten glass is an amorphous substance, but when allowed to stand undisturbed for hours it will crystallize. This process is called devitri fication. The accompanying table, showing the components of different varieties of glass, was taken from an article on "Glass," by Robert Lin ton, in the Mineral Industry, for the year 1899 (New York, 1900) : prevents cords, but tends to make the glass brittle and liable to devitrify; if there is too much sand it is impossible thoroughly to melt the glass, and it is full of stones, seeds, strings, and other imperfections. The imperfections so com mon in flint glass are due to the fact that it is composed of the silicates of lead and potassium, materials so different in their specific gravities that when fused together veins and cords are likely to be produced. It is this tendency which makes a perfect optical flint glass so hard to produce. The specific gravities of variobs kinds of glass are as follows: Window-glass, 2.50 to 2.70; plate glass, 2.40 to 2.50; bottle-glass, 2.60 to 2.70; tead-flint glass, 2.80 to 3.25. Hy drofluoric acid is the only acid known which will dissolve glass. Glass is slightly acted on by alkaline solutions, by light, air, and boiling water. It is usually transparent, or at least translucent.
The most important property of glass, however, is the condition of viscosity, intermediate between solidity and liquidity, which it assumes when the ingredients of which it is composed have been thoroughly fused and are maintained at the proper temperature. In this semi-liquid state it may be blown into any hollow form like a bubble, spun into the finest thread, or pressed or cast into any desired shape. This property of glass will be considered farther on as the different methods of manufacture are described.