GLASS 



241 



'...ii. .in l.eiirj ].n-he.| M|. \\iili H |M.iitil, ami the ring 

 romnl tin- ni. mil aii'-i \, .u.U made by the addition 

 1 1 1|. ni' metal. 



Anni'iilintj. When the gloss-blower ha* lini-hed 



a I... ill.-, it i- inn lintrly tnken to (lit- annealing 



own, where it remain- fur some thirty nix hours, 

 <luiin^ \\liic-li time it c<x>ls very gradually from 

 almost ;i softening heal to the onliruiry tempera- 

 ture of the air. Tliis process is a very important 

 one in the manufacture of all kinds of -la~~, 

 lii-caiise when newly made into vessels or sheets 

 it i- -.1 fragile that it will scarcely endure touch- 

 ing. The molecule-, are then under a -t rain from 

 the .uit>i.le portion of the glass cooling much 

 quicker than the inner; hut this is in a great 

 measure rectified lv annealing it, and so a proper 

 strength is acquired. 



VVinilmr -if/iiKa. Crown and sheet glass are the 

 very same in respect to composition, and plate- 

 glass only differs from them in that the proportion 

 of lime is usually less, and the materials more care- 

 fully selected with regard to purity. In England 

 sheet-glass is made from mixtures of which the 

 following is an example. Sand, 100 ; sulphate of 

 soda, 40 to 45 ; chalk or limestone, 40 ; powdered 

 anthracite coal, 2 ; cullet, 100 ; small quantities 

 of those bleaching or oxidising agents already 

 noted being added as required. 



('riiirn-if/a-88. Before the repeal of the duty on 

 .J ass in 1845 this glass, then almost the only kind 

 used in Kngland for windows, sold at 12 per crate, 

 from which price it had fallen in 1865 to 2, 8s. 

 Its manufacture is now practically given up in 

 favour of sheet-glass, at first called German or 

 Bohemian sheet, the price of which has fallen in a 

 like degree. Crown-glass l>eing in large circular 

 discs, much waste is caused hy cutting these into 

 rectangular pieces, and by the thickened lump or 

 bull's-eye in the centre ot the disc. In past days 

 these objectionable bull's-eye pieces were stuck 

 into cellar-windows, and it is not a little curious 

 that these are now being made, though of smaller 

 size and in coloured glass, in large numbers for 

 ornamental windows. 



The metal being brought to a workable con- 

 dition and skimmed, a sheet of crown-glass is 

 made in this way : A workman, by dipping his 



V, 



Fig. 9. 



long iron blowpipe two or three times into the 

 >ot, takes up on the end of it a gathering of about 

 Ib. of metal, which, when the pipe is held 

 upright, lengthens by its own weight into a bulb 

 -liape (n, iig. ). Rolling this on the marver, the 

 workman makes the outer portion conical, and 

 then, by blowing, forms it into a pear shape (b, fig. 

 9). Further heating and blowing brings it into 

 the shape of a flattened sphere, and to a much 

 increased size (c, fig. 9), with a point c' called the 

 bullion point. 



At this stage the glass is transferred from the 

 blowing-pipe to an iron rod ( ponty ), on the end of 

 which a lump of hot iron metal has l>een placed. 

 This lump is made to assume the form of a little cup 

 by pressing it on an iron point, and is then pressed 

 against the bullion point of the flattened sphere, to 

 which it becomes firmly attached. The pipe is 

 detached by means of a piece of iron dipped in 

 224 



coli I water (tl, fig. 9). The globe of glowt in now 

 helil with the jioiity. The ojM-rator next carrier it 

 to the nose hole, ami jneseni - the opening formed 

 bv the detachment of the blowpipe to the action 

 of the furnace ; this again softens the glaHH, which 

 i- i hen taken to the Hashing furnace, and kept con- 

 tinually revolving, by turning the ponty on a rest 

 in front of the furnace opening. The revolutions 

 are at first slow, but are gradually accelerated as 

 the softening of the glass goes on, and the cen- 

 trifugal force so produced throws the edges of the 

 orifice outwards, as in e, fig. 9. As the glass 

 flattens, it is revolved with greater rapidity, and 

 advanced so near to the mouth of the furnace as 

 to draw the flames outward, by contracting the 

 draught. This completes the softening of the 

 glass ; it then opens suddenly, with a rushing 

 noise like the unfurling of a flag in the wind, 

 caused by the rapid flying outward of the softened 

 glass and the rush of the flames outwards. It 

 becomes perfectly flat, and of equal thickness, 

 except at the bullion or centre (/, fig. 9). The 

 flashing is now complete ; and after l*eing detached 

 from the ponty, it is taken to the annealing oven, 

 into which it is passed through a long hori/ontal 

 slit which forms tne opening, and when fairly in, it 

 is dexterously turned on its edge. Here it remains 

 at a temperature somewhat below that required to 

 soften glass, until the oven is filled with these so- 

 called tables of glass, when the heat is suffered to 

 decline, until tlie whole is cold, when they are 

 removed to the packing-room, to he packed in 

 crates for sale. 



Sheet or cylinder glass, as already stated, has now 

 almost entirely displaced crown-glass for windows. 

 The Bohemian process, at present practised, was 

 introduced from France in 1832, although a very 

 rude kind of sheet-glass had been previously made 

 in England. Sheet is made in a quite different 

 way from crown glass, inasmuch as a long and 

 perfect cylinder is sought to be produced by the 

 olower instead of a sphere of glass. Very much 

 larger sheets can be obtained by this than by the 

 crown-glass process, as the form is rectangular and 

 there is no lump in the 

 centre. In some works the 

 largest sizes are made with 

 the aid of a mechanical 

 apparatus for swinging the 

 cylinders, called an 'iron 

 man.' Fig. 10 shows a 

 ground-plan of an eight- 

 pot furnace heated by gas. 

 The gas and air are sup- 

 plied through the five aper- 

 tures, called ' ports ' ( three c 

 for gas and two for air, or 

 vice versa), which are placed 

 at either end of the furnace, 

 below or on a level with 

 its bed, each end forming 

 the entrance and exit alter- 

 nately (vide description of 

 the reversing system, fig^ 8). 

 This is the furnace origin- 

 ally designed by Messrs 

 Siemens, and adopted by 

 Messrs Chance in 1861. 

 Since that period the paten- 

 tees have introduced various modifications, bat 

 it is questionable whether they have improved 

 upon their original design. In very long furnaces 

 it is better to place the ports between the pots, 

 and in a line parallel to tne sides of the furnace. 

 In this system the pots are heated partly by the 

 direct action of the flame, and partly by radiation 

 from the crown of the furnace. In his latest 

 specifications Mr F. Siemens has taken a 



Fig. 10. 



o, apertures for entrance 

 of gas ; b, apertures tor 

 entrance of air ; c, work 

 ing holes over pots. 



