CHAMBERS'S INFORMATION FOR THE PEOPLE. 



heats the metal twice, blowing it out between the 

 heatings, when it is brought to a globe shape. 

 The glass is then allowed to cool a little, ana a 

 rod of iron, called the punty-rod, is attached to 

 the side immediately opposite to the tube. This 

 is done by dipping the end of the rod in the liquid 

 metal, which adheres readily to the half-cooled 

 glass, and the tube is detached by touching it 

 with a piece of iron clipped in cold water, leaving 

 an aperture in the glass about two inches in 

 diameter. The glass is again put into the furnace 

 until it has become sufficiently ductile to yield 

 readily to any impression. The workman then 

 twirls the globe round, slowly at first, but after- 

 wards with great velocity, during which the aper- 

 ture formerly mentioned gradually widens, until it 

 reaches a certain point, when the globe suddenly 

 flies open with a loud ruffling noise, and becomes 

 a plane or circular sheet of glass, about fifty inches 

 in diameter. This is an exceedingly beautiful 

 operation, and requires considerable skill on the 

 part of the workman. The circular motion is still 

 continued, until the sheet is sufficiently cool to 

 retain its form, when it is carried to the annealing- 

 arch to be tempered. The punty-rod is detached 

 by a slight blow with an iron tool, and the sheet 

 of glass is lifted on a wide-pronged fork, and set 

 up edgewise in the kiln. A kiln will hold from 

 400 to 600 sheets. When full, the mouth is built 

 up, the fire withdrawn, and the kiln allowed to 

 cool as gradually as possible. This process of 

 gradual cooling is known by the name of anneal- 

 ing a. process without undergoing which, glass 

 would be so brittle as to break on the application 

 of the slightest force. The glass is then taken 

 out, the circular sheet cut into halves, and assorted 

 into different qualities, known by the names of 

 firsts, seconds, and thirds, 



The manufacture of crown-glass has rapidly 

 declined of late years, owing to the introduction 

 and perfection of sheet-glass, and especially of the 

 means of polishing it, so that it may be made to 

 have all the brilliancy and perfectly smooth surface 

 of polished plate-glass. 



Sheet-glass. 



The process we have described is only appli- 

 cable to crown or window glass ; the manufac- 

 ture of sheet-glass is very different. In making 

 sheet-glass, the same materials are used as in 

 crown-glass, the difference being in the manner of 

 forming the sheet. When the metal is melted, 

 the workman dips his tube into the pot, and when 

 he has gathered a sufficient quantity of the liquid 

 glass upon it, he places it in a horizontal position 

 upon a hollowed block of wood. He turns the 

 rod round in his hand, with the metal resting 

 upon the hollowed block, which forms it into a 

 solid cylindrical mass. Water is poured upon the 

 block during this operation, to prevent the burning 

 of the wood and the scratching of the glass. If 

 the glass was only red hot, on coming in contact 

 with the water, it would crack ; but at the great 

 heat at which it must be kept so as to be ductile, 

 no injury takes place. When the metal is suffi- 

 ciently formed and cooled, the workman blows 

 into the tube until he perceives the diameter to be 

 of the dimensibns required, which depends upon 

 the size of the sheet to be made. The bulb-shape 

 body is again put into the furnace, and when 



362 



softened, the workman swings it round his h<>nl, 

 re-heats, and continues to swing it, until the 

 cylindrical mass has attained what he thinks a 

 sufficient length. He then fills it with air, at a 

 slight pressure, by blowing down the blow-pipe, 

 and closes up the hole with his thumb, so that 

 none may escape ; after which the end of the 

 cylinder is again put into the furnace, and as it 

 becomes soft, the air bursts from the end opposite 

 to the tube, leaving an aperture. The cylinder 

 is now turned round very quickly, which renders 

 the ruptured end perfectly regular ; and then, by 

 applying cold iron to the end of the glass next the 

 tube, a sudden contraction takes place, which 

 separates the cylinder of glass from the iron tube. 

 The different forms through which the workman 

 makes the first gathering of metal pass until the 

 perfect cylinder is formed and split, are seen in 

 fig. 13. The cylinder thus formed is allowed to 



A A 



Fig- 13- 



cool for about five seconds, and is then split up 

 lengthwise by drawing a diamond along the inner 

 side. The glass has next to be flattened, which 

 is done by softening it in a furnace upon a smooth 

 plate, where, as it begins to melt, it gradually 

 opens, and is smoothed with a piece of charred 

 wood. It is then carried to the annealing-furnace 

 to be tempered, in the same manner as crown- 

 glass. The method above described of making 

 sheet or cylinder glass was introduced, in 1842, 

 from Belgium by the Messrs Chance. These 

 gentlemen have effected many improvements in 

 the manufacture. 



Sheet-glass may be made of any thickness, and 

 possesses considerable advantages over crown- 

 glass, from the greater size of sheets obtainable. 

 For the Great Exhibition of 1851, cylinders were 

 blown yielding sheets 49 by 30 inches, which were 

 cut into panes 49 by 10. Of these, 300,000 in number, 

 or 896,000 superficial feet, and 400 tons in weight, 

 were made in a few weeks, and without disturb- 

 ing the ordinary economy of the manufactory in 

 which this large order was executed ; in fact, the 

 ordinary business under this immense strain was 

 only retarded three weeks. The introduction of 

 machinery for grinding and polishing by Mr 

 James Chance, of Messrs Chance & Co. of Bir- 

 mingham, effected quite a revolution in the trade 

 in this country, and is characterised as one of 

 the 'greatest improvements which has ever been 

 introduced in the manufacture of glass.' By this 

 means, large sheets are obtained, in appearance 

 equal to plate-glass, at a much lower price. In 

 Mr Chance's plan, each sheet of glass is laid 

 upon a flat surface, covered with damp leather 

 'the sheet adhering completely to the leather, 

 after having been pressed against it, producing 

 in truth a vacuum, which maintains the whole 

 sheet in a flat position. Two sheets having 

 been placed in this manner, each on a retaining 





