37 



GLASS MANUFACTURE. 



GLASS MANUFACTURE. 



the manufacture ; the difficulties were at length mastered ; and among 

 other results, " Crystal Palaces " were rendered possible. The process 

 of making is from first to last very rpmarkable. The workman gathers 

 up a quantity of the semi-molten glass on the end of a tube ; rolls it 

 on a concave block of wood to give it a cylindrical form ; and then 

 swings it completely round in a vertical circle, blowing through the 

 tube repeatedly. A recess is formed in the floor of the building, to 

 afford room for this swinging. Much bodily strength and great nicety 

 are required in this operation. The hollow cylinder of glass stretches 

 out by the swinging ; but it must be so skilfully managed that the 

 stretching may occur equally in every part, bringing the glass to one 

 uniform thickness. The rotating before a fire, the blowing, and the 

 swinging, are repeated until the proper thickness is attained, and 

 the cylinder has attained the length of four, five, or even six feet. 

 By a mode of causing the heated air within the cylinder to expand, 

 the remote end becomes burst open ; the tube is detached, and 

 then the other end is expanded also. The cylinder, symmetrically 

 shaped from end to end, is then laid down ; and a diamond, fixed to 

 the end of a long handle, is drawn along the interior, making a cut. 

 The cylinder is placed in an oven, with the cut or fissure uppermost ; 

 it gradually opens, and flattens down to a sheet on a very smooth sur- 

 face. The sheet is rubbed smoothly and evenly with a piece of charred 

 wood, until as free as possible from any irregularities ; after which it 

 is annealed in another oven. Such sheets, when first made in England, 

 seldom exceeded 36 by 20 inches ; but the usual size now is 47 by 

 22 inches ; and some rare examples have been produced as much as 

 77 inches in length, requiring no less than 38 Ibe. of glass to be taken 

 up on the end of the tube. Although sheet-glass, thus made, is less 

 brilliant and more undulating than crown, its available size is so much 

 larger, that its use has in recent years become very extensive. It was 

 with sheet-glass that Messrs. Chance glazed the Crystal Palace in Hyde 

 Park in 1851 ; the glass was made in sheets 49 inches by 30, from 

 cylinders more than 4 feet long by nearly 10 inches in diameter : each 

 sheet was cut up into three panes, 49 inches by 10 ; and of these there 

 were 300,000 in number, measuring 1,000,000 square feet, and weighing 

 400 tons. The Crystal Palace at Sydenham has since been glazed with 

 the same kind of glass. Messrs Chance have recently succeeded in 

 devising a process for grinding and polishing sheet-glass, by which it 

 acquires nearly all the beauty of plate-glass with a much less weight ; 

 it is much prized by photographers. The German, or broad-glass, 

 above adverted to, is made by the cylinder process, like sheet-glass ; 

 but it is clumsily-fashioned, the cylinder is cut open by scissors, and is 

 made to spread out on a layer of sand in the oven, which does not 

 admit of the production of so much smoothness and regularity. Until 

 1836 the cylinders of sheet-glass were cut .open by a piece of red-hot 

 iron ; but now a diamond is employed. Such of the Bohemian and 

 Venetian decorated glass as does not belong to the flint-glass kind, is 

 made by the cylinder process. 



Plate Glaes. This is both blown and cast. The first-named process 

 was alone employed in England until the year 1773, when east- 

 plate began to be made. Larger pieces can be made by this than 

 by any other method. The manufacture is difficult and costly. The 

 ingredients are chosen with the greatest care, and every possible 

 amount of skill is brought to bear on the manufacture. The ingre- 

 dients used are sand of the purest and whitest quality, and soda pro- 

 duced by the decomposition of common salt and lime : manganese and 

 oxide of cobalt being added for the purpose of discharging colour. 

 Soda is preferred to potash or pearl-ash because the glass that is made 

 with it flows better when in fusion, a quality of much importance 

 where large quantities are employed for the production of the same 

 piece. The lime acts as a flux, and is used in proportions varying 

 from l-24th to l-16th of the whole materials employed. Besides these 

 ingredients, it is necessary to use a large proportion of broken plate- 

 glass or cullet. The following is one among many scales of ingre- 

 dients : Lynn sand, well washed and dried, 720 parts ; alkaline salt, 

 containing 40 per cent, of soda, 450; lime, slaked and sifted, 80; 

 nitre, 25 ; broken plate-glass, 425. It requires 40 hours' exposure to 

 the full heat of the furnace to reduce the materials to the proper state 

 of fusion and vitrification. When this is accomplished, the glass is 

 transferred from the melting-pot, by means of copper ladles, to a 

 large vessel called a curette, previously heated to a very high degree ; 

 when filled, it remains some hours in the furnace, to disperse the air 

 that may have been introduced into the mass by the operation of 

 ladling. When this effect has been produced, the cuvette is withdrawn 

 from the furnace and taken to the casting-table, over the upper end 

 of which it is raised and suspended by means of a crane. It is then 

 thrown into an inclined position, by which the contents are allowed 

 to flow out upon the table. The liquid glass is distributed by means 

 of a roller over the whole surface of the table, bars of metal being 

 placed at each side along its entire length, and across the bottom, in 

 order to prevent the glass from running upon the floor. The casting of 

 laige plates of glass is one of the most beautiful processes in the arts : 

 the large mass of melted glass, rendered in a high" degree luminous by 

 beat, exhibiting changing colours in the sheet after the roller has been 

 passed OTer it. Previous to the casting, the table is placed with one 

 end against the mouth of an annealing oven ; and as soon as the plate 

 ix set, it is carefully slipped from the surface of the table to the 

 floor of the annealing oven. When the oven has received as many 



plates as it will contain upon its floor, the door is closed and its 

 crevices are stopped with mortar or clay, to insure the gradual cooling 

 of the plates. The plates remain in the oven during a fortnight, 

 after which the ovens are opened and their contents are withdrawn. 

 The plates are then squared by means of a glazier's diamond, then 

 ground and polished, and when intended for mirrors they are silvered. 

 In order to their being ground they are imbedded in plaster of Paris. 

 To commence the grinding, powdered flint is rubbed steadily and 

 evenly over the surface by machinery worked by steam power, the 

 two sides of the plate being ground in succession. Emery powder 

 is then substituted for ground flint, coarse at first, but finer after- 

 wards, as the rougher inequalities of the surfaces are removed. The 

 polishing is also performed by steam-machinery. The plates are 

 firmly fixed upon large tables, and the polishing instruments, which 

 are of wood covered with many folds of woollen cloth, having carded 

 wool between each fold, are passed to and fro over the surface. The 

 polishing substance used is colcothal, and oxide of iron which remains 

 in the retorts after the distillation of acid from sulphate of iron ; the 

 two surfaces are polished in succession. 



The novelties introduced within the last few years in the plate- 

 glass manufacture are as numerous as those which bear relation to 

 sheet-glass ; they have had the effect of cheapening the price and 

 increasing the attainable size of the plates. Messrs. Swinburne, a 

 leading firm in the glass trade, have it recorded in their books that 

 their charge, in 1771, for a plate measuring 50 inches by 40, was 

 611. 3., whereas the price in 1856 was 4i. 4s. Some of the recent 

 inventions relate to the form of the furnace and melting pots ; some 

 to the mode of tilting over the molten metal ; some to the form and 

 construction of the casting-table ; and some to the mode of effecting 

 the difficult operation of transferring the plate of glass from the 

 table to the annealing oven. A few years ago Messrs. Hawks & 

 Crawshay made for Messrs. Swinburne a casting-table of extraordinary 

 size ; it was a solid mass of iron 18 feet 4 inches long, 10 feet 10 inches 

 wide, and 7j inches thick, weighing 26 tons. A planing-machine was 

 expressly constructed for planing and smoothing this iron surface. 

 When the large plates of glass are annealed, the grinding and polish- 

 ing begin : these processes, like all else, have been recently made 

 the subjects of much inventive skill ; but their details need not be 

 noticed here. It is an interesting fact, connected with the history of 

 the plate-glass manufacture, that the machinery constructed by James 

 Watt for the Company in 1788 was still in existence, at the Ravenhcad 

 Works, and even in use, in 1858. 



Bottle Glass. This is the commonest kind manufactured, the alkali 

 employed being the cheapest that can be procured, with the addition of 

 a portion of lime to assist fusion. Considerable manufactures of bottle- 

 glass are carried on at Newcastle-upon-Tyne, encouraged by the low price 

 of the fuel (small coal) which is used in the furnaces. The ingredients 

 are usually nothing more than lime and sea-sand, the latter article having 

 been frequently wetted with sea-water, and allowed to dry, in order 

 that the salt may be allowed to deposit itself in the sand ; the soda 

 contained in the salt is the only alkali, properly so called, that is used. 

 Or, another mixture is, Tyne-saud, lime, and the refuse of the soap 

 and alkali works. Bottle-glass is fashioned by blowing, much in the 



Bottle-glass makin. 



same manner as flint-glass, but with the addition of a mo'ulding procesx. 

 The description given in a former paragraph of making a flint-glass 

 perfumery bottle will suffice to convey a notion of the mode of making 

 wine and beer bottles, pickle and oil bottles, &c. ; except that the 

 latter are made with much less nicety. Four men can make about a 

 hundred beer bottles in an hour. There is a greater weight of coarse 

 green bottle-glass made in England every year than of all other kinds 

 of glass combined. 



Stained or Coloured Glass. Coloured glass is more easily produced 

 than colourless, seeing that many niceties in the choice and manage- 

 ment of ingredients are necessary to remove all tint from the glass. 

 Nevertheless, to produce a particular colour is an art requiring much 

 skill. As in the instances of enamels and artificial gems, the colours 

 are mostly produced by adding metallic oxides or chlorides to the 



