This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Glass is an amorphous substance (that is, of no regular shape or form), hard and liable to break at ordinary temperatures, liquid or plastic at a high temperature, transparent or translucent, white or colored, having a peculiar brilliant and smooth fracture, called "vitreous". It is composed of silica with some of the following bases: Potash, soda, lime, magnesia, lead, iron and alumina. Several kinds of glass are known, such as window and plate glass, flint, white and bottle glass, made up in different proportions of sand, soda, potash, lime, red lead, etc. Bohemian glass, used in the making of ordinary and fine hollow ware, is a silicate, with potash and lime base. It contains, like all other kinds of glass, a small quantity of alumina from the pots and oxide of iron from the impurities contained in the materials used. Potash is often replaced by soda, owing to the lower cost of the latter.
Bottle glass contains - besides silica - soda or potash, lime, magnesia, alumina, and oxide of iron. Flint glass, or crystal, is known as a glass with a base of lead potash. This denomination, however, is not accepted by all nations, as, in Bohemia, lime-glass used for fine table ware is known as crystal. Glass used for optical purposes, with a great density, owing to the lead it contains, is called flint. Strass is another variety of lead glass, used for making imitations of diamonds and precious stones.
Enamels contain, besides lead, oxide of tin or arsenious acid. Colored glasses are produced by using various metallic oxides, charcoal or sulphur. Oxide of manganese is introduced to correct the green coloration of glass by giving it a purple tint. In larger proportions it produces various colored glasses.
Glass at a white heat becomes almost as liquid as water, but when cold is quite rigid; however, at a cherry-red heat it is plastic and malleable. This property of glass enables the blower to work with facility. At the cherry-red heat it is plastic enough to be blown by means of a pipe and shaped with tools. When it becomes rigid by cooling it may be reheated and worked until the proper shape is obtained. Glass rolled on a metallic table is made into plates; by blowing it into a mold all kinds of bottles are made. By pressing the plastic mass by means of a press, plunger and metallic mold, glass can be shaped into all kinds of wares. By means of the glass-blower's lamp this material can be drawn into very fine threads and reeled up like ordinary thread. Glass can also be reduced to almost impalpable threads, as fine as filaments of cotton, by means of a steam or air blast acting upon a very fine stream of molten glass. Glass is a bad conductor of heat, and when heated and suddenly cooled flies to pieces. While being worked it cools very rapidly by the action of the ambient air; it becomes necessary to correct this defect by annealing. This operation consists in carrying the glass objects when still hot to a special furnace, where they are reheated to a low cherry-red, and gradually and slowly cooled.