Page:The American Cyclopædia (1879) Volume VIII.djvu/25

 GLASS 17 centre. To this centre a punty with a lump of molten glass at its end is next attached, and the blowing pipe is separated by applying a piece of cold iron around the nose. As it breaks away it takes a portion of glass with it, leaving a circular opening. Taken up by the punty, the glass is held with the nose (or por- tion to which the blowing pipe had been at- tached) presented to the nose hole of the fur- nace. Here it is softened almost to melting, while it is all the time twirled around ; it is then presented to the flame issuing from the great circular opening of the flashing furnace, the man holding it being protected from the fire by a covering over his head and face. Rapidly revolving in this flame, the opening in the end grows larger ; the heated air within prevents the two opposite faces of the flattened spheroid from coming together, and the cen- trifugal force is constantly enlarging its diame- ter. The opening rapidly increases, until the glass becomes a flat circular disk, which being removed from the fire is kept rapidly revolving until it is cool enough to retain its form. The punty is then cracked off, and the disk or table is removed upon a fork to the annealing oven and set upon edge with the rest, arranged in rows and supported by iron rods so as not to press against each other, and the thicker part in the centre, called the bullion point or bull's-eye, also keeping the tables apart and open for the circulation of air. The anneal- ing is completed in from 24 to 48 hours. Ta- bles are thus commonly made of 54 inches diameter, and some have been produced of 70 inches ; but the difficulty of manipulation and the uncertainty of the result render the making of very large sizes unprofitable. A pot containing half a ton commonly produces 100 tables ; and in the crown glass houses it is customary to empty eight such pots in three days every week. From the annealing kiln the tables are taken to the warehouse and sort- ed according to their different qualities and defects. Each one is then laid in turn upon a " nest " or cushion, and is divided by the dia- mond into two pieces, the larger one contain- ing the bull's-eye. These are next cut up into rectangular panes. The shape and the bull's- eye involve considerable waste in cutting; and numerous other defects are found in many of the sheets. These, however, are compensated for by the remarkable brilliancy of surface pe- culiar to glass made in this way, which is at- tributed by some to the influence of the mar- ver, and by others to the effect produced by flashing the surface. Crown glass is also free from the undulations, or cockles, which often disfigure the surface of glass made by the cy- lindrical process. In the manufacture of sheet glass two furnaces are generally used, one for melting or making the glass, and the other for reheating it during the process of blowing. The latter is usually of an oblong form, with four, five, or six holes on each side for as many workmen. On each side of this furnace is a pit about 7 ft. deep, 16ft. wide, and as long as the furnace; over this at intervals of about 2 ft. are erected in front of each hole of the furnace wooden stagings or platforms, upon which the workman stands when swinging the cylinder to and fro and over his head. The manufacture of this kind of glass may be divided into three processes : 1, blowing the cylinder ; 2, flattening it out into a sheet ; 3, polishing the sheet. The first step is to gather from the pot a lump of melted glass of the required weight, which experience enables the workman to do with great accuracy. Dipping the end of a blowpipe into the melted metal and twirling it round, he gathers a pear- shaped lump of 2 or 3 Ibs. After thia has cooled to a dull red, it is again dipped into the glass in the pot, and a larger amount withdrawn. Thus by degrees a sufficient quantity is collect- ed, usually about 20 Ibs., to produce a sheet of glass of the required size. When this mass has become somewhat cooled, the workman places it in a block of wood so hollowed as to allow the lump of glass when placed upon it to be FIG. 13. Blowing Cylinder Glass. blown to the required diameter of the cylinder. Here, while a stream of cold water is turned upon the block to prevent the wood from being burnt and the glass from being scratched, the workman revolves the pipe, and blows through it, occasionally raising it to an angle of about 75, until he has formed a hollow pear-shaped mass, with its largest diameter, which is the same as that of the finished cylinder, next to the pipe. It is now taken to the blowing fur- nace, where after being heated it is swung to and fro in the pit and round in a vertical plane over the head of the workman, who stands upon the platform above mentioned and keeps the lengthening cylinder full of air by occa- sionally blowing through the tube. Uniformity of thickness and of diameter, which was de- termined by the wooden block, is secured by the skill of the workman, who when the metal runs out too freely holds the cylinder vertically