175. Polishing Glass

Polishing Glass. When the plates are withdrawn from the annealing oven, they are carefully examined for any defects, such as air bubbles, opaque spots, etc., and those plates which are free from imperfections are selected and set aside for polishing. This is accomplished in three operations: namely, grinding, smoothing, and polishing. The grinding and smoothing table consists of a revolving slab, with a bar fixed horizontally, about 10 inches above its surface, to which two iron runners are attached. The glass to be ground is cemented to the table with plaster of Paris and the runners set to rest evenly on its surface. This apparatus is shown in Fig. 71, where the slab or table is seen at a; the runners at b are pivoted at e to the fixed rod or bar c d, when the table is revolved by machinery. The runners are independently revolved on their bearings e, and being fed with emery or sharp sand and water, every part of the plate is evenly abrased and the surface is ground down until perfectly level and true; The smoothing process is conducted on the same table and in the same manner, but the runners fed with a finer grade of emery, which renders the surface of the plate perfectly smooth, but not transparent. When both sides of the plate are ground and smoothed, it is removed from the table and polished with felt rubbers, supplied with jewelers' rouge and water. This operation removes every trace of abrasion by the sand or emery and leaves the plate perfectly clear and transparent, but about 40 per cent. less in thickness than when it left the annealing oven. Ordinary finished plates vary in thickness from 1/4 inch to 3/8 inch, and may be obtained in regular sizes up to 17 ft. X 9 ft. 6 in.

Fig. 71.

176. Rolled plate is a form of unpolished cast plate glass used largely for floor and vault lights, the only difference between this and the regular unpolished plate is that the roller used to spread the semifluid glass, on the casting table, is cut in grooves or squares which mark the glass correspondingly, causing it to cool with a ribbed surface or with a diaper design on one side.

177. Colored glass is produced by adding various chemicals to the melting pots, while the glass is in a state of fusion, and may be made under any of the processes heretofore described. Colored crown or sheet glass is of two kinds, known as pot metal and flashed colors. The first is made by dipping the blowpipe in the pot of melted glass and blowing the globe or cylinder as described in Art. 171. The glass is therefore uniformly colored all the way through its thickness. Flashed color is produced by dipping the blowpipe in clear glass first, and then, before the globe is blown, covering the lump of glass accumulated on the end of the pipe, with a thin layer of colored glass, by dipping it in a pot of colored metal. When the globe or cylinder is then blown, the colored glass spreads evenly over the exterior, and the finished sheets or panes are of plain glass, with a thin veneer of color. Flashed color may be applied to both sides of the glass, by dipping in the colored metal first, then dipping in the clear glass, and again coating the plastic lump with a layer of colored glass. When the globe or cylinder is then blown, we have a tinted film on the inside and another on the outside, with a colorless layer between. When this is opened out to form tables or sheets, both sides of the panes have a veneer of colored glass. Colored plate glass can be made only of pot metal, and, owing to the expense of polishing, it is seldom made except in the form of unpolished plate. This is usually called cathedral glass, on account of its extensive use in church windows, etc.