The plates are next removed to the first pcilishing shed, where each is imbedded it a matrix of stucco, leaving one surface exposed; the whole is inclosed in a frame, which holds both glass and stucco securely. Two of these frames are placed one over the other, with the two exposed surfaces of glass in contact. The lower frame is fixed, and the upper is made to move by machinery with great rapidity backward and for ward with a swinging motion, so as to describe an opposite curve with each backward and forward motion. Sand and water are continually thrown on the surface of the fixed plate, and thus the first stage' of polishing is performed. The plates are then readjusted in the frames, and the other surfaces are brought upwards, and receive a similar rubbing clown with sand and water. The plates are next removed to the second polishing-room, where women are usually employed; here they are again fixed on low tables, and each woman rubs the surface for a long time with a piece' of plate-glass, cov ering from time to time the whole face of the plate with emery powder and water. After both sides have received this hand-polishing, the plates are removed to a third room, where they arc again imbedded .on tables which are movable by machinery, so that the whole surface of the plate may be brought under the action of the polishers. These arc large movable blocks, covered with woolen cloth and leather, and loaded so as to press on the glass: the polishing material used is colcothar, the red oxide of iron; this completes tire polish which gives so much beauty to plate-glass. It is a long and laborious process, and is the chief cause of the high price of plate as compared with other sheet-glass. British plate is only the cylinder glass polished by the processes just described; its comparative cheapness is due to the rapidity with which the cylinder can be blown. Of tliis rapidity, the best estimate may be formed from a single well-authen ticated statement concerning the first crystal palace, which had 18 acres of roof; when the sides are added to this, and a fair addition for the increase caused by the ridge-and furrow system of the roof, the whole extent may be stated at 25 acres; and yet this vast surface of glass was .supplied by Messrs. Chance Co. of Birmingham, with only an interruption of three weekS td theinordinary business.
Flint-glass and Optical Glass.—The general principle of themanufactnre of these two varieties of glass is identical with those already described, the chief difference consisting in the great care taken to insure perfect purity in the materials. The pots used are so made, that the metal is protected from the chance of being contaminated by any accidental impurities falling in or from the gaSes of the furnace; they are made with a dome-shaped roof and a lateral arch-shaped opening, which is placed opposite the fur nace-mouth, so that the workman has easy access to the contents of the pot, which is necessarily smaller, otherwise the workman couldnot dip to the bottom.
The materials used for the best flint-glass are varied in other proportions, according to the judgment of the manufacturer; they consist of the whitest sand which can be procured, fine American pearl-ashes (impure carbonate of potash, which is purified by dissolving out the carbonate from its impurities, and evaporating it to dryness in leaden evaporating pans), red lead, or else litharge (the semi-vitrified protoxide of lead), and a small quantity of niter (nitrate of potash). To these, according to their greater or less
purity, the manipulator adds more or less of oxide of mangandse and arsenic, as cor rectives; the former removes the green discoloration which the presence of even a small quantity of iron in the sand will produce; and the latter corrects the tendency the manganese has to give a purple tint to the glass. Both substances require the utmost care and judgment in their use, otherwise they are more injurious than beneficial. The following are the usual proportions: sand, 51; pearl-ashes, prepared, 16; litharge. 28 (or red lead, 29); niter, 4t; white arsenic, peroxide of manganese, of flint glass in any proportion the manufacturer thinks proQer.
Formerly, the silica was obtained by calcining flints, hence the name applied to this kind of glass, but now sand is used instead; and although beautifully white sands are obtained from Lynn, in Norfolk, from the isle of Wight and other parts of Hampshire, from Aylesbury, from France, and even frbm North America and Australia, it never theless requires most careful preparation by washing, calcining, and sifting.
But however carefully flint-glass is made, and however pure and transparent the crystal may be which is so made, it nevertheless possesses some defects, which interfere with its fitness for telescopes, microscopes, lighthouses, and other optical purposes. These defects consist in almost imperceptible strite in the material, which produce cer tain optical aberrations. These stria; are known to be caused by the imperfect mixture of the materials, and the want, consequently, of a uniform density. This has been obviated by M. Guinaud and his associate, M. Frauenhofer, by stirring the metal in the pot with an iron rod; but greater improvements have been effected by our own chemist Faraday, who not only improved upon the manipulation of Messrs. Gninaucl and Frauenhofer, but suggested also an improvement in the materials, by the addition of carbonate of baryta, and a little carbonate of lime, which produces a glass of greater density and clearness than has ever been known before. Instead of the iron rod for stirring, which of itself is apt to discolor the glass, an iron rod coated with platina is used. In the manufacture of this particular kind of glasS, the Messrs. Chance of Bir mingham are unrivaled, and they have produced very perfect disks for lenses, weighing as much as two-hundred weights each.
Flint-glass is employed in the manufacture of all the articles of utility and ornament for table and other domestic uses; and as the manufacture of each article requires dif ferent management, it would he impossible here to give any satisfactory explanation of the manipulative processes. Suffice it to say, that at present Great Britain is unrivaled in the production of the so-called crystal or flint-glass, which we manufacture of the greatest purity and brilliancy; but in the colored kinds the Venetians take the lead, and excel both in the design and in the art of coloring.