GLASS 



243 



of materials, but tin* following may be taken its an 

 average: Fine sand, loolli. ; refined sulphate of soda, 

 42 Ib. ; carbon in powder, .".IK. ; carlnmate of lime, 

 20 to 23 IK. ; arsenic, 8 oz. ; cullet, or broken plate- 

 . ail lib. Ui-line.l sulphate has coiii|iletely 



taken tin- |I|;K f carbonate of soda. When the 



materials have been melted, and the glass is ready 

 for use, the pot is lifted out of the furnace ( fig. 

 13) by HUMUS of the forceps, and wheeled up to the 

 r.i-iinj.' table (fig. 13) : here it in seized by a crane 

 and tackle, by which it is lifted, and so nicely poised 

 over the tabfe that it can lie easily tilted so as to 

 pour nut its contents. All this requires so much 

 care and steadiness that the men, impressed with 

 the great danger of carelessness, usually preserve 

 I MM feet silence during their work. The table is of 

 large size usually about 30 feet in length, by 10 to 

 Jo feet in width. When the red-hot glass, which 

 is not in a very liquid state, is poured on, it im- 

 mediately begins to spread ; two strips of iron, a 

 little thicker than the plate is intended to lie. are 

 placed on each side of tne table, and a steel or cast- 

 iron roller is laid across, resting on these strips, 



DMCir/wcl I 

 rilltHAClS\ 



DM urine I I 

 fUHNACt^ 



Fig. 13. 



Fig. 14. 



which regulate the thickness of the plate, and also, 

 by their distance apart, determine its width. The 

 roller, passing backwards and forwards at a uniform 

 speed over the table, spreads the glass into a plate 

 of the size required. In some works the casting- 

 table with its apparatus is run on rails from kiln to 

 kiln, and in this case the plate is pushed direct 

 from the table into the kiln. In other works (and 

 this is the more modern f)lan ) the casting- table is 

 fixed, and the plate is pushed from it on to a 

 movable table, and thence into the kiln. The 

 annealing ovens or kilns are large shallow brick 

 chambers, in which the plates lie during the process 

 of annealing, and which are heated to a suitable 

 temperature prior to receiving the glass. 



It is obvious that in the arrangements of the 

 casting^ hall considerable variety is possible. In 

 the older works the furnaces were in the middle, 

 and the annealing kilns on either side. Fig. 13 

 represents a hall of this kind. Fig. 14 is a more 

 modern arrangement, and, of course, other com- 

 binations can be adopted. When the plates are 

 sufficiently cool to le removed from the kilns they 

 are carefully examined, and such as are sufficiently 

 free from defects are taken to the grinding-room. 

 Formerly the grinding process was accomplished by 

 rubbing one plate upon another, with sand inter- 

 posed, both plates being liedded in plaster. At the 

 present time for the upper glass is substituted a 

 rubbing plate of cast-iron, both the lower plate of 

 glass and the upper one of iron being set in motion 

 while in contact by machinery adapted for the pur- 

 pose. In the preliminary stage or grinding, sand 

 and water are used ; but, when the greater portion 

 of the rough surface of the glass has been removed, 

 the process is completed by using powdered emery 

 of the coarser sorts. When one surface of the glass 



has been thus treated, the o|>eration in repeated on 

 the other. The next proce** in that of amoothing, 

 for which a separate machine in required. Impend 

 of cast-iron plates, one sheet of glaw* is uiied to rub 

 upon another, tin- upp-r -h'-<-ts which are movable 

 being weighted. Kmerv of the liner description in 

 used in this process, the final touches l>eing given 

 by hand, with the aid of the very finest emery 

 powder. After both sides have received this 

 smoothing, the plates are removed to another room, 

 where they are again embedded on tables which are 

 movable by machinery, so that the whole surface of 

 the plate may be brought under the action of the 

 polishers. Tiiese are padded iron buffers attached 

 to short iron rods passing through holes in a beam 

 acted on by springs or weights. The buffers are 

 covered with felt, and nib the glass as it pa*e 

 from side to side ; the surface or the glass leing 

 supplied with oxide of iron, in a very fine state of 

 division and mixed with water. When any in- 

 equalities are encountered, the springs yield and 

 allow the buffers to pass freely over them. An 

 older plan of polishing is to use wooden rubber- 

 blocks covered with felt. 



Rolled Plate. Mr Hartley, of Sunderland, intro- 

 duced about 1850 a method of making rough plate- 

 glass suitable for roofs and other purposes where 

 light only is required without transparency. The 

 casting-table has generally a series of fine grooves 

 upon it, but it can be marked with any required 

 pattern. Very large pots or continuous tanks can 

 be used for this process, as only a ladleful of glass 

 is required for each sheet. Quite recently rough 

 plate has been made by passing the contents of the 

 ladle between two rollers revolving in opposite 

 directions, but the glass thus made has not as yet 

 superseded that made by Mr Hartley's process. 



Patent Plate. Sheet-glass made by the cylinder 

 process, when free from flaws and of good colour, 

 is to a limited extent ground and polished much in 

 the same way as ordinary plate-glass, but it is 

 rather higher" in price. However, its lightness is 

 an advantage for some purposes, such as tjie glazing 

 of picture frames and for photographic negatives. 

 It is called patent plate to distinguish it from cast 

 plate-glass. 



Qualities of Plate-glass. It is of importance to 

 know that there is a great difference in the quality 

 of plate-glass supplied by different manufacturers. 

 W*e do not refer to air-cavities or other imperfec- 

 tions which can be readily seen, and from which 

 the highest-priced glass, such as that used for 

 mirrors, is almost entirely free ; but to a defect bj 

 which the transparency of the whole surface of the 

 glass is impaired. It will often be found, for ex- 

 ample, that, of two plates apparently equally pure 

 and free from flaws, one win take on a peculiar 

 dimness a few days after being cleaned, while the 

 other will remain quite clear and transparent for 

 as many weeks. Plate-glass which does not keep 

 long clean to use a homely phrase may often 

 pass unnoticed for a long time until experience of 

 a better quality calls attention to it. Some inferior 

 qualities of ' pressed glass,' noticed below, also take 

 on a kind of scum even when newly cleaned, a fault 

 most readily noticed in plain articles. 



Flint-glass. M. Bontempsin his Guide du Verricr 

 gives the following mixture for ordinary flint-glass : 

 Sand, 100; red lead, 66'66; carbonate of potash, 

 33 '33 i.e. one part of carl>onate of potash, two of 

 red lead, and three of sand. Sometimes a little 

 peroxide of manganese is added, and a portion of 

 the carlx>nate of potash may lie replaced by refined 

 nitrate. Cullet is usually added to the extent of 

 alnmt one-fourth part of the whole mixture. The 

 pots for flint-glass (fig. 6) are covered or hooded, w> 

 as to protect the melted glass from any impurities in 

 the flames of the furnace. The materials used are 



