At the present time probably the greater bulk of plate-glass is used for the purpose of glazing windows of various kinds, principally the show windows of shops, etc. As used for this purpose the glass is finished when polished and cut to size. The only further manipulation that is sometimes required is that of bending the glass to some desired curvature, examples of bent plate-glass window-panes being very frequently seen. This bending is carried out on the finished glass, _i.e._, after it has been polished; the glass is carefully heated in a special furnace until softened, and is then gently made to lie against a stone or metal mould which has been provided with the desired curvature. It is obvious that during this operation there are great risks of spoiling the glass; roughening of the surface by contact with irregular surfaces on either the mould, the floor of the kiln, or the implements used in handling the glass, can only be avoided by the exercise of much skill and care, while all dust must also be excluded since any particles settling on the surface of the hot glass would be “burnt in,” and could not afterwards be detached. Small defects can, of course, be subsequently removed by local hand-polishing, and this operation is nearly always resorted to where polished glass has to undergo fire-treatment for the purpose of bending.
In addition to its use for glazing in the ordinary sense, plate-glass is employed for a number of purposes; the most important and frequent of these is in the construction of the better varieties of mirrors. For this purpose the glass is frequently bevelled at the edges, and sometimes a certain amount of cutting is also introduced on the face of the mirror. Bevelling is carried out on special grinding and polishing machines, and a great variety of these are in use at the present time. The process consists in grinding off the corners of the sheet of glass and replacing the rough perpendicular edge left by the cutting diamond by a smooth polished slope running down from the front surface to the lower edge at an angle of from 45 to 60 degrees. Since only relatively small quantities of glass have to be removed, small grinding rubbers only are used, and in some of the latest machines these take the form of rapidly-revolving emery or carborundum wheels. These grinding wheels have proved so successful in grinding even the hardest metals that it is surprising to find their use in the glass industry almost entirely restricted to the “cutting” of the better kinds of flint and “crystal” glass for table ware or other ornamental purposes. The reason probably lies in the fact that the use of such grinding wheels results in the generation of a very considerable amount of local heat, this effect being intensified on account of the low heat-conducting power of glass. If a piece of glass be held even lightly against a rapidly-revolving emery wheel it will be seen that the part in contact with the wheel is visibly red-hot. This local heating is liable to lead to chipping and cracking of the glass, and these troubles are those actually experienced when emery or carborundum grinding is attempted on larger pieces of glass. In the case of at least one modern bevel-grinding machine, however, it is claimed that the injurious effects of local heating are avoided by carrying out the entire operation under water.