OIL AND THE GLASS INDUSTRY.
In order to make clear the application of fuel oil to the glass industry, a general description of the pro cess of making glass will be given. Broadly speaking, glass is made by properly melting. together, cooling, and shaping, certain raw materials, of which silicates play the most important part. At the outset, the finely ground raw materials are thoroughly mixed, sometimes by regrinding the entire "batch." The "batch" 'is then shovelled into a pot together with a certain amount of broken glass called "cullet," which melts at a comparatively low temperature, and thus assists in liquefying the rest of the charge. More of the batch is added until the pot is filled to the desired height with the fused mass, then volatile substances. such as arsenious acid, used to decolorize the glass. are added. During the melting much gas (CO2 SO2 and 0) escapes, and the bubbles rise through the melt, stirring it and causing frothing. A considerable amount of alkali and other constituents volatilize.
When the melt has come to a state of quiet fusion the temperature is generally raised somewhat, and the liquid glass allowed to stand for a time. This is called "refining," and its object is to form a homo geneous mass, 'free from bubbles and bits of uncom bined silicia or other matter. The scum which collects is skimmed off ; it is called "glass gall," and consists of undecomposed sulphates and chlorides of lime and alkali, alumina compounds from the furnace, and vari ous other impurities. If too little carbon is used in the batch, the melt is covered with a layer of fused so dium sulphate ; this is known to the workman as "salt water." Samples of the glass are examined dur ing the refining, to determine the exact time of heating. After refining, the glass is too liquid to blow, or to work to. advantage, and is cooled until it becomes pasty.
Glass is known under various names in commerce, depending to the method of its manufacture or the uses to which it is put.
Plate glass is cast on a large iron plate or "cast ing table," made of thick narrow segments of cast iron, bolted together and planed on top. These tables were
formerly cast in one piece, and, being large and thick were very expensive. But when used, they soon be came warped and dished, owing to unequal expansion of the top and bottom. This caused much loss of time and glass in the subsequent grinding of the plate. The built-up casting table is much cheaper and retains its even surface much longer. The melted glass is poured on the table and spreads out in an even layer. But to give the plate a uniform thickness, and to smooth down any inequalities of the surface, a heavy iron roller, traveling on adjustable guides at the edge of the table, is passed over it. The height of these guides determines the thickness of the plate. Both the cast ing table and the roller are heated before use, so that the glass may not be cooled too rapidly. As soon as the plate is rolled it is transferred to the floor of the annealing furnace, which has been heated to the temperature of the glass, and which is directly in front of the casting table. The annealing oven is then closed tightly and the fire drawn, leaving the plate to cool slowly for a number of days.
All glass must be annealed. This process prob ably allows the molecules to arrange themselves so that there is no great internal stress when the mass is cooled. Unannealed glass, which has been suddenly cooled, is always under high internal strain which makes it exceedingly brittle, and may even cause it to fly to pieces spontaneously, or when slightly scratched.
When removed from the annealing furnace, the plate is uneven and rough and may be somewhat devitrified on the surface. It is fastened on a hori zontal table, and heavy cast iron rollers are made to slide over its surface with a rotary motion, while coarse sand and water are sprinkled on it. When the glass is smoothed off and of a uniform thickness, it is pol ished by rubbing with buffers, covered with leather or felt, and used with fine emery dust or putty powder. About one-half of the thickness of the plate is cut away during the grinding and polishing.