PREPARATION OF THE MATERIAL. Sand being the commercial representative of silica, it is evi dent that the quality of glass depends, apart from the effect of different processes of manufacture, upon the quality of the sand, and the more thor oughly the sand can be cleansed of impurities be fore it is melted, the better will be the quality of the glass. Hence, for the finer qualities of glass, the sand is often subjected to a preliminary process of purification by washing, burning, and sifting. The sand is washed by stirring it thor oughly in large volumes of water and then allowing it to settle; the lighter particles of dirt, chalk, and other extraneous matter will remain in suspension after the heavier sand has settled, and can be drawn off. The sand is burned to remove the moisture and organic mat ter; it is placed in the bed of an oven and played upon directly by the flame. Last of all, it is sifted through copper gauze. The chief impuri ties found in sand are iron, lime, alumina, chalk, and magnesia, besides organic matter and dirt. Of these the iron is the most troublesome, so that it is customary to estimate the value of sand according to the amount of iron which it con tains.
The process of mixing the prepared sand with the other ingredients is called `mixing the batch,' the batch being the mechanical mixture of the materials whose chemical combination, brought :about by heat, produces glass. It is evident that the materials of which glass is to be made must be mixed with the greatest nicety, knowledge, and skill, if a perfect product is to be obtained. Here tofore the proportioning of materials has been done empirically, but in the best modern fac tories there is more dependence put upon chem ical analyses to determine the proper mixture.
The accompanying table, compiled from figures in The Mineral Industry, previously referred to, shows the mixes for American glass as prescribed in three different glass - makers' recipe - books. While it is true that a practical glass-maker might not follow any of these recipes exactly, but would have to modify them to suit the peculiar requirements of his own material and furnaces, yet they will serve to give a general idea of the average composition of the batches from which some of the different kinds of glass are made in America.
yields only a few weeks of service before it breaks. For window, plate, and bottle glass manufacture the furnace is rectangular, with doors at each end for the moving of the pots, which are simply great cups with flat bottoms and sides broadening upward to an open top. But the pots are always unreliable and trouble some affairs. When one suddenly cracks, per haps a few days after its inauguration, every thing must be stopped for a day till another is built in and its contents fused; when the glass is all worked, half a day is taken for another batch to be melted; while the molten glass is worked the furnace must be cooled; moreover, the sulphur and soot from coal fuel are con stant annoyances. To escape these difficulties the `tank furnaces,' heated by gas, are now used, and are revolutionizing the whole glass industry. In 1861 the first regenerative glass-furnace was introduced into Germany by its inventor, Sie