Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/682

Rh 660 F3 far as they involve distinct manufacturing processes, the.-e 'd.l'lt.tlt.-S of glass will be here noticezl in the above ruler. Such of the divisions as result from the application of s} c':i;1l methods of ornaincntation, and as come under the Lead of art glass, do not fall within the scope of this article. t‘i.nw_‘ (iL_‘.—Tliis, with sheet or cylinder glass, forms all .i.lin.iry blown window glass. Both varieties are prec1s_cl_y the same in composition, being a iiiixtu1'e of sodic aml caleic silicates,
 * .i.l differ only in the manner in which the sheets of_ finished

other kinds of glass vary within rat_her wide limits, and, as already explained. the lorin in which the sodic and e_al_c1e compounds are used may also be varied. The following composition of batch for window glass must tliert-fore be regarded as only one out of very many mixtures in use :— San-1, 100 parts. Chalk, or limestone. 35 to 40 ,, Suipliaic of soda ..... .. 4!’) to 4-3 ., Cullct  -50 to 150 To these materials aminute proportion of white arsenic aml peroxide of manganese, as bleaching agents, may also be added. Crown glass was, in the early part of the. present century, the only fr-rin of window glass made in Great Britain, and COllS(‘ll1lL‘Iltly it was generally recognized as English window glass, having been nia1i11— faetured only on a very limited scale in any other country. Since the introduction of slieet-glass making, the crown-glassiinlustry has steadily declined, and now its manufacture may be regarded as practically a thing of the past, not more than one or two crown fur- naces being in operation. Seeing that it possesses little more than an historical interest, it is now iiiniecessary to enter into much detail as to the processes employed in the m.1nnfacture of crown glass. The metal being brought to a proper condition for working, the “ gatherer” dips into the pot of metal an iron pipe or tube, 6 or 7 feet in length, of the shape shown in fig. 6, heated at that end which takes u p the glass, and, by turning it gently round, gathers about 1«._!;Ib I;-— 1 .,;.L.. ; '. W» FIG. 6.—Blowing Tubc. ofliquid glass on the end of it. Ifaving allowed this to cool for a little, he again dips the rod into the pot, and gathers an additional quantity of from 21; to 3 lb. This is also permitted to cool as before, when the operation of dipping is again repeated, and a sufficient quantity of metal, from 9 to 10 lb weight, is “g-atliered," to form what is teclmi- cally called a table or sheet of glass. The rod. thus loaded, is held for a few seconds in a perpendicular position, that the metal may distri- bute itself equally oii all sides, and that it may, by its ovn weight, be lengthened out beyond the rod. The operator then moulds the metal into a regular form, by rolling it on a smooth iron plate, called the “ marver," a term corrupted from the French word marbrc. He then I,-lows strongly through the tube, and thus Causes the red-hot mass of glass to swell out into a. hollow pear—shaped vessel. The tube with the elongated sphere of glass at the end of it is then handed to the ‘ ‘ blower," who heats it a second and third time at the furnace, press- ing the end, between each blowing, against the bullion bar, so ealled from the part thus pressed forming the centre of the sheet or “ biill’s eye,” and by the dexterous management of this operation, the glass is bi'ouglit into a somewhat spherical form. The blower now heats a thiid time at the “ bottominghole,” :uid blows the metal into a full- i~.iz« d ﬂattened spheroid. 'hen this part of the process has been c-oinplet--d, aii-l the glass has been allowed to cool alittle, it is rested on the “caslier box, ’ and an iron rod, called a “ pontil " or punty rod, on which a little hot metal has been previously gatliered, is applied to the. flattened side, exactly opposite the tube, which is de- t.iehc-l by toueliingit with a piece of iron, dipped beforehand in cold water, leaving a circular hole in the glass of about 2 inches diameter. Taking hold of the puiity rod, the workman presents the glass to another part of the furnace called the “ nose hole," where the aper- ture made by its separation from the tube is now pi-est-iited and kept until it has become sufficiently dtietilc to fit it for the operation of the flashing furnace. 'hilst here, it is turned dexterouslv round, slowly at first, and afterwards with inereasing rapidity; and -the glass _viel-.ling to the ceiitrifugal force, the aperture just mentioned becomes l'ff1 .i-gt l. The workman, taking great care to preserve, by a rcgii- lrii motion, the cii'culai' figure of the gl iss, proceeds to whirl it round with iii:-reasiiig velocity, until the aperture suddenly flies open with a loud ruflling noise, which has been aptly compared to the unfurl- ing of a flag in a strong breeze ; and the glass l-eeonics a circular 1 lane or sheet, of 4}, feet diameter, of equal thickness through-
 * _-'‘-'-_'s are produced. 'l‘lie raw materials employed for tlns and all
 * 'llC, ex "HID! at the point eallel the bullini or bull’s eye, where

it is attached to the iron rod. The she t of glass, new full." ex- panded, is moved round with a moderate x cloeity until it is sufficiently cool to retainits form. It is carried to the mouth of the kiln or aiiiiealiiig arch, where it is l'L5l.t:d on a bed of sand and dc- GLASS [IA.'L'F.CTL'I:E. tacfied from the puiity rod by a shears. The sheet or table is then lifted on a wide prongcd fork, called a faucet, and put into the arch to be tempered, where it is ranged with many others set up edge- wise, and supported by iron frames to pre.vent their bending. l“i'oni 400 to (300 tables are placed in one kiln. A sketch of the interior of a crown-glass house, during the progress of these opcr-.1tions, has been given in Plate 'f., fig :2. The kiln having been clayed up, the fii'e is permitted to die out, and the heat diniinislied as gradually as possible. 'hcn the glass is properly annealed, and siillieiciitly cold to admit of its being handled, it is withdrawn from the oven after the removal of the wall built into the front of the arch, and is then quite ready for use. The largest sized tables of crown glass made will cut into slabs 30 inches across, fi'oin which sipiarcd pieces measuring 38 by '24 or 35 by 25 inches may be obtained. b'ni-:1-3'1‘ Gi._tss, as already mentioned, is the same in composition as crown glass, which it has now entirely supplanted. The success of sheet glass is due principally to the fact that it can be produced in sheets of much greater dimensions than is possible in the case of crown glass; it is free from the sharp distorting Stl'lz.l3 and waves common in crownclass ; there is no loss of glass as there is with the bull’s eye of crown ; and modern improvements effected in the iiiain1- facturing process leave little distinction in brilliancy of surface be- tween the two qualities. Sheet glass is made o1i the greatest scale in Austria, Germany, and Belgium, and it was long distiiignislied in the_l5ritish market; as German sheet glass. III 1832 Chance of flir- mingham and subsequently Hartley & Co. of Sundcrlaiid introduced the manuﬁicture into England, and in the hands of these firnis, as well as of others who followed in their footsteps, the industry prospered and developed, till it has now attained dimensions equal to those it has reached in most of the Continental nations, where the art was long established before it came into use in England. Slicet-glass making involves two principal operations,—-—(1) the blowing of the cylinder, and (2) the opening, flattening, or spread- ing of the glass. The structure and internal arrangenients of the niclting furnace is practically the same as in the case of crown glass. The ordinary type of oblong furnace usually contains 10 pots—5 in each side of the fire-gr-.1te—eaeh pot being of a capacity of about 1 ton or 22 cwt. of metal. Radiating from the work-holes, and raised about 7 feet above the floor level, or a correspondingly deep sunk pit. are ten long stages with an open space between each sufficient to allow the workman to swing about his long tube freely in forming the elongated cylinder of glass. Fig. 7 is a ground pl-.in of a coimnon sheet-glass furnace and staging of planks c, at the extremities cl of which are placed a tul) of water and a wooden nioulding-block. In- stead, however, of having these stages erected in front of the melting furnace, it is now a connnon practice to gather and block the glass at the melting furnace, and to blow it in front of a separate oblong reheating or blowing furnace, fi'oii1 cach opening of which the wooden stage runs out over a pit excavated to the depth of 7 feet or thereby. Common bricks may be used for the construction of this reheating furnace, as the heat I'l'(1llll'ctl in it is by no means intense. Ijlowi/i_r/. The charge or batch requires about 16 hours to melt, and other 8 hours are C°“5‘"“°‘1 _i“ 9°01-‘"8 it to FIG. 7.—Plan of Sheet-Glass I-‘uritaee. the working consistency. When the metal is ready for working, the workmen take their stations, each having his own pot and stage and also an_ assist- ant, aiid commence making the eyliiiders. After gathering the quantity of metal required (which on an ayerage amounts to '30 ll‘-). the workman places it in a horizontal position in the large hollow of it wooden block (fig. 8), which has been hollowed so that. 'll('ll the workman turns the metal, it shall foi'in it into a solid cyliiidrieal mass. In the meantime, the assistant, with a sponge in his hand, and a bucket of water by his side, lets a fine stream of water run into the block, which keeps the wood from burning, and also gives a brilliancy to the surface of the glass. The water, the moment it comes in contact with the glass, is raised to the boiling point, and in that state does no injury to the metal ; but it is only when the metal is at a high temperature that such is the case; for, whenever the glass is cooled to a certain degree, it immediately cracks upon coming in contact with water. When the workman perceives that the mass of metal is sufficiently formed and cooled (fig. 9), he raises the pipe to his mouth at an angle of about 75 degrees, and