Sometimes old plate glass, that has become scratched, is re-polished; when the plates are large, and sufficiently numerous, they are repolished by machinery, just the same as new glass, but more generally old plates are repolished by hand, as the process can be then restricted principally to the scratched portions of the surface.
The polishing is commenced with tripoly on cloth rubbers of the usual form, and finished with putty powder or crocus. The pressure is generally given as in hand calendering, by attaching the rubber to the lower end of an upright pole, suspended from a long horizontal spring fixed overhead, like that of a pole lathe. The elasticity of the spring supplies the pressure, and the workman has only to push the rubber backwards and forwards, but the process is both laborious and tedious with large plates, and from the irregular action of the hand, the surfaces of glass thus polished present a wavy appearance much inferior to those polished by machinery.
Sheet glass or flattened glass, is manufactured by blowing the glass first into the form of a spherical bulb, which is afterwards elongated, by alternate heating, blowing and swinging, into a cylinder about 3 feet long and 8 inches diameter, with rounded ends, which as the last process of blowing are opened out, and the ends are cut smooth with a diamond traversed in an upright frame around the cylinder, which is then cut through on the one side longitudinally, with a diamond inserted near the extremity of a light rod, and drawn through the inside of the cylinder under the guidance of a straight edge. The cylinder is then placed with the cut upwards in a reverberatory furnace, and the heat causes the cylinder gradually to open as a sheet, which is gently flattened down on the bed of the furnace, with tools like blunt garden rakes made of iron or wood.
To improve the flatness, several sheets are afterwards laid upon each other in a second reverberatory furnace with a leve bed, the heat of the furnace and the weight of the superincumbent mass, causes the lower sheets of glass to become sufficiently flat for ordinary use, notwithstanding that there are many little irregularities in its surface, arising from the imperfect action of the flattening process. For the best purposes these irregularities are removed by grinding and polishing, and a brief, notice of the method pursued in an extensive manufactory will be here subjoined.
The grinding room contains about 140 grinding machines, arranged in double rows of 10 each, and the annexed diagram fig. 1104, may be considered to represent roughly the moving parts of every machine, the framing being represented by the dotted lines.
The framework consists of continuous beams 1, 1, united by vertical posts 2, 2, bounding every machine, the whole firmly united. Above the framing extends an axis 3, 3, carrying for every machine one pair of bevil wheels which turn the upright shaft 4, and its crank 5, to the right or left at pleasure. The pin of the crank 5 communicates a circular motion to that point of the moving table to which it is attached, while the fixed radius bar 6, 7, 8 restrains the center of the table to describe an arc about the point 6, the two motions conjointly bring all parts of the running surface successively in opposition to nearly every part of the lower bed, which latter lies on railway bars 9, 9, and is very slowly reciprocated to and fro by the bar 10, which runs through the building, and is traversed about two feet by a crank, that is made slowly to revolve by a worm wheel and tangent screw, one screw serving for two cranks united to two lines of the machines. The whole arrangement is most massive and imposing.
The circle described by the crank 5, is about two-thirds of the length of the moving table, the lower face of which is covered with slate upon which the glass is cemented, another sheet of glass is cemented upon the lower table, and the upper table is loaded with 4 or 8 weights placed in the respective panels of the frame. When from swinging the upper table about by hand, it is judged that one of the corners bears too hard, the weights are removed from this corner. Coarse emery and water are used for the grinding, and when the machines are used with finer emeries for smoothing, the whole apparatus is carefully washed, for the convenience of which there are numerous racks and tanks between the rows of machines. In some manufactories the plates of glass are smoothed by rubbing them one upon the other by hand.
After the sheet glass has been ground flat and smoothed, it is polished in another room by the machinery rudely shown in figs. 1105 to 1107. 1, 1, is a long main shaft extending throughout the length of the building, and having for every row of the machines one double and two single cranks, which move the two long central beams, 2, 2, to the right, and the two exterior beams, 2', 2', to the left at the same instant, by the intervention of connecting rods, as usual.
The travelling beams or rods carry rubbers, 3, 3, 3, 3, about 12 by 5 inches on the face, and covered with leather; they are suspended by a joint to the loaded levers, 4, 4', which press them on the glass. To raise them up and retain them, the piece 5 is laid down in the position 5', which holds up the lever as at 4', as the joint which unites 4 and 5 is situated in the mortise through the long travelling beam 2, at the part represented by the dot in the figure to the left; so that when the rubber is at work the weight 5 cannot be misplaced, and when 5 is laid down as at 5', no shaking will allow the rubber to descend accidentally. The rubbers all assume an inclined position, from the several tables carrying the glass having a very slow transverse motion, simultaneously throughout the entire line of machines, which is effected somewhat after the manner of the annexed figure, 1108.