GLASS

is a complex solution of many Substances, chiefly silicates and borates. Solidification occurs without any change in structure, and g. is often spoken of as a ‘congealed liquid.’ G. is a typical vitreous body with an amorphous structure. Although a large number of bodies | can be obtained in the vitreous state, very few are used in g., because with many the cooling of the molten solution cannot be carried out quickly enough to avoid a component crystallizing out at its own freezing-point (devitrification.) | Cost, as well as the risk of devitrification and the temperature of the furnaces, also restricts the choice of the substances used in g. manufacture. | Industrially, therefore, g. is composed of the mixed silicates of bases, those most in use being the alkalies (Sodium and Pota ium), the alkaline earths Calcium, Magnesium, Strontium, and arium), and oxides of Iron, Aluminum and Lead. The properties of the g. depend on the base’chosen, as does also its behavior during manufacture. Lead and Barium give weight and brilliancy, and increase the fusibility and softne during manufacture. Magnesium and Aluminum give a g. which stands red heat without softening. The alkalies increase the fusibility of g. and ensure its easy working, but reduce the chemical resistance of the finished article, which is hygroscopic and apt to decompose rapidly in a damp atmosphere, EHven good g.—that is, g. containing le than 15 per cent. alkali—is not stable. The prolonged action of water on the best g. extracts a small quantity of alkali, and steam actively corrodes it. Water and alkali remove silica, Water and acid tend to protect g., since it prevents the formation of silicon hydrate, which permits the water to di olve out the alkali. Phosphoric and Hydrofluoric Acids both corrode g.; the latter is used to etch g. Carbonic Acid in the presence of water di olves the surface of g.,’ rendering it liable to decomposition by bacteria. G. is photo-sensitive. Light produces color changes in it, bringing out a brown or purple tint in g. made with manganese, The Tensile Strength of g. is said to be about 21% to 5 tons per sq. inch, and the Crushing Strength 3. to 8 tons per sq. inch. These two properties are both affected by rate of cooling. The hardne depends on the, rate of cooling as well as on the constituents. G. is not a good conductor of heat, and the thicker it is, the more easily it is fractured, since the inner wall expands, while the outer wall, remaining stationary, suffers from this tension and fractures. This has to be overcome in the manufacture of boiler gauge gla es, gla and metal junctions, etc. The harder g. is, the better it acts as an electric insulator. Transparency is the most e ential property of g., and it is estimated that over 90 per cent of it is used for this, Even the best g. absorbs a certain amount of light, The manufacture of g. is controlled by the above facts. and the constituents, melting, and rate of cooling are so chosen as to adapt the g. as nearly as po ible to the required object. The quality of the g. decreases with the impurities in the raw material; therefore, the raw material should be pure and uniform. The silica is got from sand, which must be dry and of uniform grain. For the best g. it must contain le than.05 per cent. of Jron and not more than.05 per cent. of other impurities. Sandstone is difficult to grind to uniform grain, and therefore cannot be used. Felspar is too expensive. The alkali used in the best g. is obtained from the Le Blanc or Ammonia Soda proce , while sal i-cake is used in the manufacture of inferior g. The calcium used in sheet-g., pre ed and. blown g. is obtained as the carbonate or as slaked lime. The Lead for flini g. is introduced as red lead. For the special scientific, optical and technical gla es specially prepared salts of Barium, Magnesium, and Zinc are employed. Aluminum is added in the form of felspar or as hydrate. The tanks or pots used in the manufacture are made of fire-clay if they have to resist heat and the solvent action of g., or of silica brick if they have only the latter to withstand. | The furnaces are fed by gas generated in gas producers. In the furnace the gas meets with hot air and the mixture burns rapidly, and if properly proportioned gives very high temperatures. The materials are mixed very carefully, and when the mixture is uniform in color and material the heating commences. The silica is supposed to act as an acid; all the carbon dioxide is expelled, and silicates of sodium and calcium remain in combination and solution with one another, so that the contents of the furnace are a ma of transparent gla ,with many bubbles. The bubbles are disengaged by boiling, the g. then being | ‘fine.’ The surface is skimmed and the temperature reduced. The g. is now ready to be worked. If the g. is to be blown it is removed by gathering. A hot rod is dipped repeatedly into the mixture, and when some pounds have adhered the worker blows through the rod and a hollow sphere of g. is obtained. from this sphere articles are made by pre ure, moulds or blocks with or}. without further blowing. Stained Gla is an art of N. Europe Which arose in the XII cent., and reached its turning point in Tudor times. The development of mosaics and frescoes on the wall of Ital. churches rendered it an unnece ary art in that country, consequently the finest examples are to be found in tke older cathedrals and churches of France and England. The best work is now done, as it was in the Middle Ages, by the method known as ‘mosaic gla .’ First a design is prepared representing the figures in the proposed picture; then the glazier works out the main portions in various kinds of colored gla ; after which the artist develops in pigment all the delicate shadings of draperies, the lines of the figures, etc., and when this is completed the colors are fixed by firing. ‘Mosaic gla ’ may be distinguished by the characteristic way in which the leads, ' or calms, do not follow the outlines of the figures, but boldly intersect them, ' enclosing rich and variegated plots of color. An inferior method, known as ‘enameled gla ,’ proceeds by painting entirely on white gla and fusing the pigments to fix them.