GLASS. 



475 



od with the art of painting on glass. See a subsequent 

 division of this article. 



The Venetians were long celebrated for their 

 glass manufacture. France succeeded to the pre- 

 eminence. Britain did not become distinguished 

 {'or her glass until about the commencement of the 

 sixteenth century. The British plate-glass company 

 was established in 1773, at Ravenshead, in Lanca- 

 shire. Before this time, all our plate glass was im- 

 ported from France, but we now manufacture it 

 superior to any other nation in Europe. The value 

 of the annual produce of glass in Britain, may be 

 estimated at 2,000,000 ; giving employment to 

 about 50,000 individuals. The excise laws relative 

 to the glass manufacture, are complicated in the ex- 

 treme, enforced under heavy penalties, very nu- 

 merous and eminently qualified, to check the im- 

 provement of this highly useful branch of industry in 

 this kingdom. So injurious has their influence 

 already been, that, notwithstanding the very heavy 

 duty on imported glass, large quantities are brought 

 to this country from the continent. 



Glass is formed by the fusion of silicious matter, 

 such as powder-flint, or fine sand, together with some 

 alkali, alkaline earth, salt, or metallic oxide. The 

 nature of the glass will depend upon the quality and 

 proportion of the ingredients of which it is formed ; 

 and thus, an infinite variety of kinds of glass may be 

 inade,but in commerce, five kinds only are recognised, 

 i.e. 1st. Bottle or coarse green glass. 2nd. Broad or 

 coarse window glass. 3rd. Crown-glass, or the best 

 window glass. 4lh. Plate-glass, or glass of pure 

 soda. 5th. Flint-glass or glass of lead. 



The physical properties of glass are of the highest 

 importance. One of these is that of preserving its 

 transparency in a considerable heat, and remaining 

 almost entirely without extension. Its expansibility 

 is less affected by heat and cold than that of any 

 other solid substance which has been accurately ex- 

 amined. On this account, it is especially fit for 

 pendulums. Its great ductility, when heated, is 

 also a remarkable property. It can, in this state, be 

 drawn into all shapes, and even be spun into the 

 finest threads. It may be cut by the diamond, and 

 also by a hot iron, although the last manner is rather 

 unsafe. 



Before describing the process of manufacturing 

 the different species of glass, it will be necessary to 

 premise a few particulars, relative to the construc- 

 tion of glass-houses, furnaces, and the pots or cruci- 

 Jiles in which the materials are fused. 



Glass houses are commonly constructed of brick, 

 and made of a conical form, the diameter of whose 

 base varies from eighty to 100 feet, the perpendicu- 

 lar height being nearly the same. A large vault is 

 made in the interior of the cone, extending from side 

 to side, and of sufficient height to allow work- 

 men to wheel in and out rubbish from beneath the 

 furnace, which is placed over the vault, and se- 

 parated from it by means of an iron grating, on 

 the upper side of which, the fuel is laid. The 

 construction of furnaces, is a matter of the great- 

 est moment, and much judgment is required in 

 the selection of such materials as shall best resist 

 the destructive effects of the high temperature, to 

 which they are subjected, as likewise in determining 

 the form best fitted for the conduct of the operation, 

 and the economy of fuel. Three different kinds of 

 furnace are employed by the glass maker ; 1st, The 

 calcar or fritting furnace, is that in which the ma- 

 terials used for the formation of the glass, are calcin- 

 ed, and a chemical union between the ingredients 

 commenced, forming what the workmen call a frit. 

 This furnace is in fact an oven, the fuel being placed 

 at one side, and the flame made to reverberate from 



the crown or roof back to the frit, care being taken 

 to keep the heat at first below that point at which the 

 alkali would volatilize, and throughout below that 

 point at which the frit would be fused. 2nd, The work- 

 ing furnace is that in which the frit, placed in earthen 

 pots or crucibles, is melted and formed into glass. 

 These crucibles are formed of Stourbridge clay, care- 

 fully freed from impurities, powdered and worked 

 with warm water, and steeped in a cistern until 

 brought to the consistence of a paste. It has been 

 found highly conducive to the durability of these 

 crucibles to form them of a mixture of the Stourbridge 

 clay, with the powder of old crucibles in the propor- 

 tion of about five parts of the former to one of the 

 latter, and this practice is now universally adopted. 

 After the crucibles have been formed of the required 

 shape and dimensions, they are left in the apartment 

 where they have been made for at least a year, in 

 order that they may be completely dried, and then 

 they are subjected to the action of a furnace, the heat 

 of which is made gradually to increase until the tem- 

 perature has nearly reached that of the working fur- 

 nace. The size and shape of the crucible will vary 

 according to the purpose for which it is intended. 

 The usual dimensions of these crucibles is, depth 40 

 inches, diameter at top 40, and at bottom 30. Those 

 for bottle and crown glass are open at top, as repre- 

 sented infig.6,PlateXLlL, and are made from three to 

 four inches in thickness ; those for flint glass, shown 

 in fig. 5, are covered at the top and are made from two 

 to three inches in thickness. Twelve crucibles are 

 commonly placed in one furnace at equal distances 

 from each other, round the circumference of the fur- 

 nace, each pot being opposite to an opening in the 

 wall of the furnace in order that the crucibles may be 

 charged or discharged by the workman from without. 

 The working furnace is of a circular form externally 

 and made to terminate in a chimney, the interior being 

 an arched dome. The fire on the grating at the cen- 

 tre of the furnace is supplied with air from the vault 

 below, and the flame and hot air having acted upon 

 the crucibles placed round the fire, pass out by the 

 chimney and escape through the top of the glass- 

 louse. Very lately an improvement has been made in 

 the working furnace of Messrs Pellatt & Co. of Lon- 

 don, who have effected a great saving of fuel by con- 

 structing two furnaces each of half the diameter of a 

 twelve crucible furnace, and terminating in one chim- 

 ney by means of flues, as may be seen in fig. 3. By 

 this contrivance the crucibles are brought nearer to 

 ;he fire, for the area of one of these small furnaces is 

 only one-fourth of the area of the twelve crucible 

 7 urnace, while its circumference is only the half of 

 t, and will therefore contain six crucibles. When 

 ;he glass has been fused hi the working furnace, and 

 'ashioned into the required form by means which will 

 3e afterwards described, it is carried to another fur- 

 nace or oven, the temperature of which is somewhat 

 ower than that necessary for fusion, the glass is suf- 

 ered there to remain for several days, the heat of the 

 oven being allowed to diminish by slow degrees until 

 ;he fire be quite extinguished. This furnace is called 

 ,he annealing furnace, and the process of allowing 

 ,iie glass to cool there is called annealing. Unless 

 ,his process be carefully managed, the articles formed 

 ii the glass house can be of no use from their liability 

 ,o break by the slightest scratch or change of tem- 

 perature. The annealing furnace is shown in fig. 2. 

 one of the doors of the arch being shown open and 

 the oilier shut. 



Common green or bottle glass is formed of the 

 coarsest materials, such as coarse sea or river sand, 

 ime, and clay, and the most inferior and cheap alkalies 

 such as soap boilers' waste, and the slog obtained in 

 the smelting of iron ore. When soap boilers' waste 



