|Certain of the metallic silicates, especially alkali silicates, show a remarkable tendency to super-cool, giving a transparent structureless solid, known as a glass, which may be regarded as a liquid in a highly viscous condition. The alkaline earth silicates, aluminium silicate, and others, do not possess this property, or only to a very limited degree, extremely rapid cooling being necessary to obtain them in a vitreous condition. The silicates of iron, manganese, and lead do not readily crystallise unless cooled slowly. By mixing together silicates of the alkaline earths with the more readily crystallisable silicates, glasses are easily obtained which are at the same time capable of resisting the action of water and chemical reagents as the alkali silicates alone cannot do. In this possibility lies the foundation of the glass industry. |
Glass of a very imperfect type was produced and employed as early as 1400 b.c., and probably before, in Syria and in Egypt, but it was not until the time of the Roman Empire that glass became an article of general domestic and industrial use. Roman glass was almost invariably of the soda-lime type, rich in silica. After the collapse of the Roman Empire, the Emperor Constantine encouraged the construction of glass furnaces at Byzantium, and these became celebrated throughout the civilised world until, after the taking of Constantinople in 1453, their products were eclipsed in beauty and delicacy by the work of the Venetian glass-blowers. The latter held first place for several centuries, until, in fact, in spite of stringent laws to prevent it, their secrets became known in England, France, and Germany. The Venetians, no doubt, owed part of their superiority in glass manufacture to the natural sodium carbonate which they imported and used. In other countries during mediaeval times the manufacture degenerated, owing to the use of crude wood ashes and the production of a potash-lime glass, very low in silica and with an excess of lime and magnesia. So far as is known, the first English glass factory, if we except those put up during the Roman occupation and one probably conducted by French workmen brought by St. Wilfred to York about 700 a.d., was erected at Chiddingfold in Surrey about the year 1230, and this district remained the only centre of glass manufacture until restrictions on the use of timber for fuel in the time of Elizabeth caused its decline. The fashion for ornamental glass vessels, which prevailed in the sixteenth century, resulted in the establishment, in London and other parts of England, of glass-blowers from France and the Low Countries, as well as a few from Italy. The introduction of coal as a fuel, and the necessity for covering the pots to protect from contamination by the coal, facilitated the use of lead oxide in glass and led ultimately to the manufacture of lead or flint glass in the form now known.
The composition of glass is varied according to the use to which it is to be put. Comparatively small changes in the composition may bring about rather serious changes in properties, and much experimental research has been necessary, and is still required, to discover suitable formulae for glasses for special purposes. Before the War, the manufacture of chemical glass-ware and optical glass was developed on the Continent, but not in Britain. Since then, however, British manufacturers have made remarkable progress.
Ordinary soda-lime glass approximates substantially to the composition Na2O.CaO.6SiO2, but there are very large possibilities of variation by replacing more soda by lime, exchanging soda for potash, or lime for baryta, magnesia, lead oxide, zinc oxide, or alumina, or, finally, by exchanging silica for phosphoric or boric acids. Small quantities of other substances may also be introduced, with the object of aiding in the refining of the glass or of producing coloured or opaque glass.
The earliest pioneers in the systematic study of glass, largely for optical and chemical purposes, were Schott and Abbe, who began their work in 1881, and set up a factory at Jena in 1884.