CLASS 



ami l>\ K-igei Kacnii in tin- l.'itli century; towards 



tl lose ui which Siilviim d'AiniHto invented 



' UHSCH, which wen- subsequently improved by 



.mdro Spina. Glass reflector.- I'm tele-copes, 



of great si*e &nd accuracy, have IM-CII made in 



(MM- TKI.I.srol'K). 



regards proce e- of making, that called the 

 c\ lindi ic.il was used by the. ancient-, and is nn-n 

 ti-'iicd liy Th"'., phi I u- at iln- end of the I'Jih <-<-n 

 tiny. The rotatory process wan first introduced in 

 Imliemia, subsequently into France in 1730, but 

 not into England till 1832. Pressed glass was 

 invented in America. In England there were 

 twenty-four window-glass factories in 1847, and 

 only seven in 1886. In 1889 there were in the 

 1'nitt'd Kingdom 43 manufacturers of flint-glass, 

 4 of sheet, 4 of polished plate glass, 7 of rolled 

 or rough plate-glass. The value of the export of 

 glass from Britain increased from 26,694 \n 1848 

 to about 500,000 in 1855; in 1887 it was 

 1,021,029, and in 1888, 1,109,341. The value of 

 the foreign glass of all kinds imported in 1887 was 

 1,674,268, and in 1888 was 1,906,770. In 1880 the 

 glass manufacture of the United States gave work 

 t" _'! 1 establishments, employing 24,177 hands. Of 

 the total product, with a value of $21,154,571, over 

 two- fifths were made irt Pennsylvania, and nearly 

 an eighth in New Jersey. The export of glass and 

 glassware had in 1886 a value of $773,878, in 1887 

 of $883,504. The imports had a value of $7,301,340 

 in 1887. 



Manufacture. In its ordinary state, glass is a 

 solid Ixnly with a characteristic lustre called vitre- 

 ous, and a conchoidal or shell-like fracture when 

 broken, best seen in pieces of some thickness; 

 further, it is more or less brittle, a property which 

 arises from its outer and inner molecules cooling 

 from a state of fusion at a very unequal rate. It 

 is usually said to be amorphous, but perhaps it 

 rather represents a stage between the perfectly 

 amorphous and the -crystalline states. A tendency 

 in his glass to crystallise in cooling is one of the 

 things a glass- maker dreads. Glass is commonly 

 transparent, although this property is not an essen- 

 tial one, since a true glass may be almost opaque, 

 or at most translucent, even when very thin. 

 Glass when softened by heat is highly tenacious, 

 anil may be easily moulded into all conceivable 

 shapes ; it welds when red-hot ; at a lower heat it 

 is plastic, and may be cut with knives and scissors; 

 when cooled it is usually quite brittle. But molten 



glass can be rapidly drawn out into long threads 

 undreds of feet in length, and such threads retain 

 when cooled sufficient flexibility to be woven into 

 a beautiful silky fabric. 



The chemical composition of glass differs with 

 the different kinds. It is essentially a silicate of 

 soda or of potash combined with a silicate of some 

 alkaline earth or other basic body, such as the 

 oxide of lead. Silica with potash or soda alone, or 

 with both, forms a soluble glass unfit for windows 

 or vessels of any kind. The following table gives 

 the composition of the chief kinds of glass : 



( 1 ) Window-glass, including crown, sheet, and plate : 



silicate of soda and lime. 



(2) Bohemian Crystal-glass : silicate of potash and lime. 



(3) Flint-glass, of ten called crystal-glass or simply crystal : 



silicate of potash and lead. 



(4) Bottle-glass that is, of the common kinds : silicate of 



lime and alumina ; with smaller quantities of the 

 silicates of potash or soda, iron and manganese; 

 tJe silicates of baryta and magnesia being also 

 frequently present. 



There are some other kinds made on a more 

 limited scale, such as optical glass, strass, and 

 enamel glass. Any of tlie above kinds of glass 

 may be coloured by the use of certain metallic 

 oxides. 



Haw Material*. I'm- the letter kind* of Am 

 the-e arc tin- following : Silirn, employed chiefly in 

 (In- form of sand, of which an abundant MUpply, 

 sulliciently free from iron for ordinary window- 

 glass, is to be found in Kngland. 1 <n the beat 

 qualities of plate and flint glass, in which purity of 

 colour i.-e eiitial, manufacturers have recourse to 

 the sands of France and Belgium. Potuxh, an 

 pearl-ash, or wood-ashen, or tin- sulphate of potaftli. 

 Smlii, in the form of carltonate or sulphate of Hoda. 

 /.mil-, in tin- state of caustic lime, chalk, or ordinary 

 limestone, if sufficiently pure. Baryta, from heavy 

 spar or witherite ; but barium eompOHBdi are ax 

 yet only to a limited extent employed. Lead in 

 safest used in the fonn of red-lead (peroxide), a 

 quality free from copper, which would impait 

 colour, being specially made for glass-makers. 

 I'ln-lxni, in the form of charcoal or powdered anthra- 

 cite coal, for the decomposition of the alkaline sul- 

 phates. Cutlet or broken glass of the kind intended 

 to be made. All the above materials must lie as 

 free as possible from .iron or other impurities 

 when colourless glass is required ; and, in older to 

 prevent any iron or carbon present from tinging the 

 glass, small quantities of oxidising agents, as nitre, 

 arsenious acid, and peroxide of manganese, are also 

 employed. Bottle-glass is made of comparatively 

 coarse materials, as will be presently seen. 



Glass Pots, or Melting-vessels. T^iese require to 

 be very carefully made of some very refractory 

 clay, since the cracking of one in the furnace, 

 which sometimes happens when it is newly put in, 

 is a considerable loss to the manufacturer. In 

 Great Britain the famous Stourbridge fireclay is 

 nearly always used for them. It is almost wholly 

 composed of silica and alumina with water, and is 

 nearly free from oxide of iron or other easily fusible 

 ingredient. Much attention is given to the pre- 

 liminary preparation of the clay, called tempering. 

 It is then put into large cisterns, mixed with water, 

 and kneaded with the naked feet, which render* 

 the clay of a uniform consistency and free from air 

 cavities ; but it requires to be turned over and 

 kneaded repeatedly. After a week or two, it is 

 removed to large tables, where it is mixed with the 

 ground fragments of old pots, and carefully worked 

 into a plastic mass. This prepared clay is next 

 made up into small rolled pieces, with which the 



Eot- maker slowly builds up the pot, adding only a 

 iw inches to its height in a day. Foreign pots are 

 made in moulds of thick wood strongly hooped with 

 iron, but in England entirely by hand. The pots 

 are usually kept several months in stock, after 

 which they are 

 annealed by being 

 kept for a few 

 days at a red heat, 

 in which state 

 they are trans- 

 ferred to the glass- 

 furnace for use. 

 New pots require 

 to be ' glazed ' 

 by throwing in 

 a quantity of 



broken glass, which protects them from the further 

 action of the materials used in glass-making. Fig. 

 5 represents a pot for window or bottle glass, and 

 fig. 6 a flint-glass pot, which has always a covered 

 top. 



furnaces. The furnaces which have been long 

 in use for its different kinds will be noticed in 

 turn as we describe the processes of making glass ; 

 as, however, the Siemens furnace has come into 

 extensive use in various departments of the glass 

 manufacture, it is necessary to give an idea of it- 

 construction. A general description of this furnace 

 is given under IRON ; but we give here a plan ( tig. 7 ). 



Fig. 5. 



Fig. 6. 



