The cylinder is now ready for tho flattener, who, having prepared it by a preliminary warming in the flue, by which it is introduced into his furnace, passes it, by means of a " croppie," or iron instrument, on to the flattening-stone, from the slight irregularities of whose surface it is protected by a "lagro," or sheet of glass, laid upon the stone. Upon this lagre, the cylinder, lying with the split uppermost, is soon opened by the flame passing over it, and falls back into a wavy sheet. The flattener now applies another instrument, a " polissoir," or rod of iron, furnished at the end with a block of wood, and rubs down the waviness into a flat surface, often, upon a refractory piece, using considerable force. Some cylinders are so distorted in the blowing that no rubbing can flatten them, but all, good, bad, and indifferent, pass through the same treatment. The flattening-stone is now moved on wheels to a cooler portion of the furnace, and, by the aid of the flattening-fork, delivers its sheet to another stone, called the cooling-stone. From this, when sufficiently stiff, it is again lifted, and then piled, generally on its edge, in order to be annealed.
When this manufacture was new in England, the size usually blown was 36 in. long, and 20 wide ; the usual size now is 47 in. long by 32 wide, and cylinders are occasionally blown 77 in. long ; but large sizes and heavy weights are accomplished only by first-class workmen. A sheet of the last size, containing 21 oz. to the foot, would require for its formation a lump of glass upon the gatherer's pipe, of no less than 38 lb. weight. The size which sheet-glass can Bans reach, is obviously a great advantage, and adapts it to many purposes from which the limited dimensions of crown are excluded. But sheet-glass has its faults—it is devoid of that brilliancy of surface for which crown is so remarkable ; and is subject to undulations on the surface, the precise origin of which it is difficult to explain, but it is probable that this undulation is produced in the operation of blowing, and is due to the double movement of the particles of glass which accompanies the formation of every cylinder, the one movement being parallel to the axis of the cylinder, and the other in planes at right angles to that axis.
Sheet-glass, by the mode of its production, has the polish of its surface spoiled in some degree ; but its greatest defect is that the inside and outside surfaces of the cylinder, not being of the same length after being developed by the flattening operation, cannot be in juxtaposition on two parallel planes, without one being forced to contract or the other to expend ; but, as the glass remains too hard during the flattening process to change its molecular arrangement, one surface contracts the other, and the result is a kind of undulating or wavy appearance, called "cockles." These cockles
reflect and refract light in various and contrary directions, and the objects seen through the glass are thereby distorted. Several attempts have been made to grind and polish this glass, in order to destroy these inequalities and imperfections. James Chance conceived the idea of laying every sheet to be ground and polished upon a flat surface, covered with a damp piece of soft leather. The sheet adheres completely to the leather, after having been pressed against it, producing, in truth, a vacuum, maintaining the whole sheet in a flat position. Two sheets having been placed in this manner, each on the retaining or sucking surface, they are turned one against the other in a hori zontal position, sand and water being constantly supplied between them, and by means of machi nery, the two surfaces are rapidly rubbed one against the other in all directions, and are ground at the same time by the sand. When the grinding has been performed on one surface, the sheets are turned, td have the other surface ground in the same manner. The sheets are smoothed and polished in the same way as plate-glass.
The sheet being ground while it is kept perfectly flat, it is not necessary to wear out more than a thin layer of the whole surface. Of course, after the operation, the sheet, by its own elasticity, resumes its former more or less curved shape ; but the whole surface of both sides has a polish as perfect as plate-glass, and this is the desired condition for glazing purposes.
Spreading- and Annealing-Kiln.—Figs. 764, 765, 766, and 767 show the various arrangements of the spreading- and annealing-kiln. The split cylinder is introduced through the tube x upon the movable flattening-carriage b', which, when the cylinder is flattened, can be moved from the tramway zp x to the tramway y p y , placed upon the " traversing-car riage" a. This moves upon the tramway x, and conveys the spreading-carriage b' to position C, at the same time bringing the second spread ing-carriage into such a position that it can be moved upon the tramway x p z, just vacated by V. The sheet can now be removed from b' to the carriage D, upon which it is gradually carried down the annealing-kiln, and away from the fire. The carriage b' is moved back to the position V.