Hartley's Rolled Platc.—Hartley's rolled plate is made in the same way as plate-glass, only on a diminished scale. The molten glass, instead of being poured from the crucible, is ladled out of the crucible by means of large iron ladles. Although the plates produced are necessarily very much smaller and thinner than east plates, nevertheless great economy is attained through the continuity of the process. The glass produced has an undulating surface, and is suitable for glazing roofs or windows, where translucency, rather than transparency, is required. The glass is made in a consi derable variety of tints, as well as with impressed flutes, ribs, and ornamental patterns. The impressions are given by projections on the easting-plate, which acts as a mould, while the advancing roller performs the office of a "plunger." The rolled plate is exceedingly cheap. Tank furnaces are well adapted for use in the manufacture of rolled plate. (See Bottle-glass.) Mechanical Processes applied to Plate-Glass.—So soon as the kiln is sufficiently cool, a workman enters to examine the plates, and, if any are cracked, to lead the creeks by means of a heated iron, in whatever direction may be least injurious to the value of the plates. The plates of glass are now carefully drawn forward to the mouth of the kiln, and carried thence to another part of the manu factory, to undergo the processes of grinding and polishing. A plate is first cut with a diamond, to any required dimensions, or so as to get rid of imperfections in the glass, and is then cemented upon an iron horizontal table, in such a manner as to expose one surface to be ground. The processes of grinding, smoothing, and polishing are almost identical in action, and differ only in the media employed. In the first, the plate is ground by en iron rubber, and a medium of sand and water; in the second, it is smoothed by a glass rubber, and a medium of emery-powder and water ; and in the third, it is polished by a wooden rubber, covered with felt, leather, or some soft material, and with a medium of rouge of increasing degrees of fineness.
Fig. 756 represents an apparatus for grind ing : A is a fixed frame ; B, an iron horizontal table, revolving, and carrying the plate with it; D, rubbers shod with iron, to which a partial rotary motion is conveyed by friction with the glass carried round upon the revolving table B. Sand and water are fed through a hopper. In another arrangement, the lower table has a backward and forward motion ; and the superimposed rubber, a similar motion in a direction at right angles to that of the lower table.
Figs. 757 and 758 show respectively a side elevation partly in section, and a plan of another arrangement, invented by Pilkington, for grinding glass :—a is the grinding-runner, with a conti nuous iron-shod surface b, sufficiently large to cover the whole of the plates c to be ground; d, the bed or table. A cireular motion is imparted to the runner a, through the arm e, which receives motion from the crank-pin f, actuated from the shaft g, receiving motion from any convenient source of power ; f are guide-cranks. Rotary, oscillating, or other motion, may be given to the runner.
The overhead tramway h has wheels i running on it, and carrying screw-jacks], coupled to the ends of the runner by means of pins k. When it is desired to remove the runner from the grinding-bed or table, it is raised by means of screw jacks j, until it is freely suspended from the overhead tram way. The runner is then run along the tramway, until it is clear of the bed or table, when it may
be turned on the pins k, into any required position, for examination and repairs. The operation is reversed, when it is desired to replace the runner on the bed or table.
Rods 1 are attached to the back of the runner, and to adjustable hooks m, which are raised and lowered by means of the cbain-wheel n, through which motion is given to the screw-wheel o. By adjusting the height of the hooks m, the strain on the rods I and cross-bars p is made greater or less, as desired, and the pressure of the runner on the plates to be ground is increased or diminished at will. If the rods and cross-bars have not sufficient elasticity to produce the desired effect, springs may be interposed. In every instance, after one surface has been ground or smoothed, the plate has to be reversed, so as to expose the second surface.
A fourth plan is to bed one plate of glass in plaster of Paris, upon a flat receptacle, and to attach a second plate by the same means, exactly parallel with the first, to an overhanging movable arm. To this arm, an elbow motion is communicated, and it is caused to press the upper plate upon the lower, and to grind them together, sand and water beiug thrown upon the lower plate. Thus the opposed surfaces grind each other, and, so soon as one surface of each plate is ground, both plates are reversed, so as to expose the other surfaces to the same action. The same apparatus serves for the smoothing process, fine emery-powder being supplied as a grinding medium, in place of sand. The polishing process generally employed is as follows. The glass plate is cemented to a table, possessing a backward and forward motion, and lying immediately beneath a beam, moving in a direction at right angles to the motion of the table, through which pass rods, having at their ends wooden rubbers coated with felt or leather. These rubbers are pressed upon the surface of the glass by strong springs. The grinding or polishing medium is rouge (peroxide of iron) and water. The loss in weight of each plate, by the three processes of grinding, smoothing, and polishing, amounts to almost 40 per cent. It has been suggested to use, for the purpose of grinding, only the best sand, or calcined flints, with a view to remelting the waste from the glass and the grinding medium with new raw materials. It has also been suggested to perform the levelling process by means of the abrading chemical action of hydrofluoric acid.
In the processes of smoothing and polishing, great care is required in the preparation of the grinding media, i. e. the emery-powder and rouge ; neither of these materials is soluble in water, and it is by water that they are respectively sifted into lots of varying degrees of fineness. The sifting is based upon the principle that the largest particles will sink first, and the finest powder will remain longest in suspension. If emery-powder or rouge be agitated with water, and the water, in an uniform stream, be suffered to pass through a succession of troughs or cylinders of increasing size, the powder will be deposited in each succeeding trough in a condition of increased fineness, owing to the fact that the larger the trough the longer will the water be in travers ing it.