ECONOMIES IN every glass-manufactory, there must be considerable waste : prosperity much depends upon its reduction to the smallest limits. The production of highest-quality goods necessitates an accumulation of disqualified glass, for which profitable uses must be found. Before discussing the subject, it is necessary to briefly review the course of the manufacture. The raw materials are mixed, and introduced into crucibles standing around the grate of a furnace ; the heat is raised, and the fusion of the raw materials, and the purification of the molten glass, take place. Purification consists in the escape of gases generated by the decomposition of the materials, and in the rising of infusible impurities to the surface of the glass. The latter are removed by a process of skimming, and may be regarded as the first instalment of waste or inferior glass. The glass at the bottom of a crucible is generally impure, and, though fit for remelting with the raw materials, is unfit to be worked into goods, and is therefore ladled out. Whenever a crucible breaks, the glass which runs into the furnace is utterly lost, and the remainder can only be saved by ladling.
Glass is liable to various blemishes, oven though it be prepared with the greatest care. Discoloration arises, not only from impurity in the raw materials, but is often caused by a variation of temperature in the furnace : if too low, the manganese dioxide will remain unreduced, and the glass will he pink ; if too high, the reduction will be carried too far, and the glass will be green or brown. A cold furnace will account for the presence of bubbles in the substance of glass ; white solid specks are caused by decay of the crucible ; cords or stria; are due to variations of temperature, or to imperfect combination. The waste caused by these blemishes may be exemplified by the fact, that a shade of either pink or brown, or a single bubble, speck, or cord, is sufficient to condemn a first-class wine-glass, Considering the extent to which the temperature of the furnaco influences the purity of glass, it is astonishing that manufacturers are still in want of a reliable and handy pyrompter. Those at present in use depend either upon the contraction of unburnt clay, or upon the expansion of burnt clay, and are cnmbrous and untrustworthy.
When glass is gathered, scales from the gathering-rod adhere to the portion immediately in con tact with it. If glass, soiled with scales of iron, be remelted without the addition of a considerable quantity of the manganese dioxide, the resultant glass will be discoloured. To avoid the scaling of the iron, it has been proposed either to plate the ends of the gathering-rod with platinum or some other difficultly fusible metal, or to subject them to the process of oxidation by superheated steam, invented by Prof. Barff. For the manufacture of a wine-glass, at least two distinct gatherings
of molten glass are necessary. At each gathering, the weight taken from the crucible is at least double that actually used. Considering also the large number of wine-glasses rejected in course of manufacture, at least half the contents of the crucible is wasted. In annealing, in moving from the annealing-kiln, in smoothing and grinding, in cutting, and in carriage, there is necessarily a very large amount of breakage and of waste. All broken glass is carefully collected and sorted. The best, together with that ladled from the crucibles, is mixed with the raw material, and remelted. To the second best, i. e. that to which iron scales adhere, a proportion of manganese .dioxide is added, together with the raw material. The third quality must be used for the production of coloured glass, or be worked up into some inferior ware.
At the St. Gobain Works, inferior glass is worked into tiles and transparent paving-blocks, as represented in Fig. 803. In manufactories where mosaic windows are made, the discoloured glass may profitably be mixed with metallic oxides, and worked into coloured sheets or circles. It may also be slightly tinted, and pressed into glazing-quarries.
In addition to glass of inferior quality, thero is always a large quantity which has become so mixed with clay and dirt as to be unfit for use as transparent glass. If this final waste be carefully collected and sifted, and if, after it has been ground into an almost impalpable pow der, it be spread upon clay tiles, and partially melted in a kiln, it forms an opaque sub stance, which is exceedingly hard, and suitable for pavements or wall decorations. By the inter mixture of metallic oxides, almost any colour can be produced. The substance possesses a granular surface, and its colour is usually beautifully irregular ; in these respects, it compares favourably with ()lay tiles. Another use proposed for powdered glass waste is as a partial substitute for emery in emery-wheels.
Bibliography.—C. Winston, Hints on Glass-painting' (London : 1847); A. Pellatt, Curiosities of Glass-making ' (London : 1849); W. Stein, Glas-falirikation ' (Brunswick: 1862); P. Flamme, Le Vernier an xix. Siecle ' (Paris : 1863); C. Winston, Art of Glass-painting' (London : 1865); P. Flamme, Un Chapitre sur la Verrerie' (Paris : 1867); G. Bontempa, ' Guide du Verrier ' (Paris : 1868); F. S. Barff, ' Glass and Glass-painting' (Cantor Lecture, Soc. Arts, London : 1872) ; H. E. Bernath, Glas-fabrikation ' (Brunswiek: 1875); J. Fowler, Decay in Glass' (ArohrAnlnoi. vol. xlvi., London: 1880); R. Garner, Glas-fabrikation ' (Vienna and Leipzig).