1816.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



123 



open ; the former are used only {or flint glass, the latter for all other ilo- 

 scriptions in both shapes. The upper part is the most capacious: the 

 reason for this is, that the heat reverberates from the top of the crown of 

 the furnace directly upon the top of the pots. The pots cannot, of course, 

 be exposed cold to the heat of the furnace, but have to undergo a gradual 

 beating till they attain a white heat, and this is done in a furnace con- 

 structed for the purpose, from which all air is carefully excluded ; from 

 this furnace they are removed upon iron carriages to the glass furnace. 

 The heat required to melt glass, especially that made without lead, is very 

 great; yet, on account of the danger to the crucibles from any sudden rush 

 of air, it is impossible to make use of blast, or even fanners: the 

 proper draft is secured by the construction of an air funnel, called a cave, 

 and by having the glass-house so construcled that it can be closed from the 

 entrance of exlerual air above. Upon the arcli of the cave the furnace 

 floor or seige(from the French sitg-e, seat of tlie pots) is constructed, formed 

 of strong heavy square bricks. The round furnace is used for flint glass, 

 the flames finding vent by flues passing through the pillars of the furnace, 

 having chimneys upon the outside for carrying off the smoke. Square fur- 

 naces, agaiu, are employed for glasses without lead, a greater heat being 

 required ; which is obtained by the grate-room running llu; whole length of 

 the seige. Tlie proper construttioti of the furnace is of great importance 

 to the oper.itions of the glass-maker ; in fact, good glass cannot be made 

 without a good furnace. 



There are several distinct varieties of glass manufactured ; and so dif- 

 ferent are they, both in preparation and manipulation, that they may be 

 considered separate manufactures. There are, however, oidy two melhods 

 by which fluid or senii fluid glass is formed to shape, viz. casting and blow- 

 ing. Casting applies exclusivity to i>lale glass, and is the emplyingglass out 

 of the pot by casting it out upon a table, the casting of glass as metal is cast, 

 being yet unuraclised ; blowing applies to all other descriptions of glass. 



The tools used by the glass-maker are simple : the blov\'ing-iron — simply 

 a hollow tube ; uith this the serai-liquid glass is gathered from Ihe pot and 

 blown out into shape ; the punly, for attaching to the bottom of glass afier 

 blowing, so that the blowing-iron may be detached, and the glass, being 

 heated up, may be cut with scissars, and afterwards formed. 'J'he shears 

 or procellos, for shaping the glass whilst it is turned by the workman upon 

 the arms of his chair, or working bench. These, with the addition of a 

 pair of scissars and pincers, are the whole of the tools. 



All glass requires a'inealing or cooling ; the process is performed in a 

 furnace called a lier, from the French liei\ — figurative, perhaps, of the 

 change in stale, as well as atomic arrangement, which takes place during 

 the cooling. We know that a change dots take place, from the fact that 

 glass before cooling is of greater bulk and less specific gravity than when 

 cold ; that it parts with a portion of colour during the process, probably by 

 giving off ox J gen ; and that though, whilst in a fluid slate, glass is a good 

 conductor of electricity, when cold it is a non-conductor. The object of 

 annealing is, by a gradual diminution of the temperature, to allow of that 

 arrangement of particles necessary to the body at a low temperature, and 

 which particular arrangement alone enables the glass to support sudden 

 changes. 



The base of all glass is silica : the most convenient form in which it is 

 found is in fine sand; upon the due pi'oportion of this substance in glass 

 depends its compactness of body, brilliance and capacity to withstand sud- 

 den changes- 1( often happens, either on account of want of sufficient heat 

 in the furnace, or in order to save time in the melting or founding, that too 

 small a proportion of silica is employed, tilass which has this fault may 

 be known oy its rapidly attracting nioisiure. The diflerent descriptions of 

 glass made are known by the names of plate glass, German sheet or Ijritish 

 plate, crown or window glass, bottle glass, and flint glass ; there are 

 others, but they are merely modifications of these, and need not be noticed. 



Plate Glass is composed of sand, carbonate of soda and chalk, with small 

 quantities of arsenic and manganese ; the proportions vary at diflerent 

 works, but llie general proportion is — Lynn sand, 400; carbonate of soda, 

 250 ; ground chalk, 3.5, by weight. The quality of the glass depends upon 

 the quality of the alkali. Plate glass is melted in large open pots. The 

 furnaces are square, conlainiug sometimes 4, sometimes (i pots each ; when 

 the glass is melted, which takes 22 hours, it is removed to another furnace, 

 where the pots are smaller, of a cylindrical furni. Here it is fined, which 

 occupies 4 10 G hours, and when free from air bubbles and impurity the pot 

 with the glass is removed bodily from the furnace by means of a crane, 

 and hoisted to the end of the casiing table, upon which the glass isempiied; 

 a large iron roller vhich works inside the flangesof the casting table, is 

 then made lo pass over the melted glass, in order to flatten it out; it isihen 

 removed upon a wooden table on wheels to the annealing arch, which is 

 BOW at a high lemperalure, and here it is excluded from the atmosphere 

 until cold. The glass is rough and uneven, but is afterwards cut flat by 

 machinery, and then smoothed and polished; it is these processes which 

 render plate glass so cosily. 



Crown, or window glass is of much ihe same composition as plate glass, 

 except that a cheaper description of alkali is used ; Ihe ordinary mixiiire 

 is, 500 cw I. Lynn sand, 2 of ground chalk, and 1 cwt. each of sulphate 

 and carbonate of soda. The square furnace and the open pots are used, 

 there being generally G pots on each furnace. It lakes from 14 to 20 hours 

 to melt this glass, aud it then requires lo stand 4 lo S hours lo allow it to 

 become free from all air bubbles, and to cool sufliciently for working. 

 Window glass is formed by blowing : upon the blowing-iron is gathered at 

 three several times ^the fluidity of the glass never allowing fewer) the 



weight of glass necessary to produce the table, and which weighs U lb. ; 

 this is then blown oul, leaving a solid lumj) at the farthest extremity from 

 the blowing-iron, for atlaching the punty ; this is called the bullion. The 

 punty being fixed to the bullion, the blowing-iron is relieved by merely 

 touching the glass with a wet iron ; being firmly attached to the punty, it 

 is removed lo a small cylindrical furnace, called a flashing furnace, where 

 a rotatory motion being given to it, increasing as the glass becomes softened 

 by the heat, Ihe centrifugal force, together with a little sleight of hand oq 

 the part of the workman, produces a flat circular plate or table, as it is 

 then called. 



British plate, nr German sheet-glass is of the same composition as plate 

 glass, but the nuiuipulalion is diflerent. The glass is blown into open cy- 

 linders, and, when cold, these are cut open along the length with a dia- 

 mond, and placed in a flatting furnace, which is at a sufficient heat to 

 bring the glass into a semi fluid state, so that it falls quite flat. The sheets 

 thus made are afierwards cut flat and polished. The size of the sheet is 

 restricted to what can be blown and worked by one man ; it is cheaper 

 than plate glass, because all waste is avoided, and less cutting is re- 

 quireil. 



Bo(</<' ^/as's is composed of the cheapest materials which can be pro- 

 cured — ordinary pit sand, refuse alkaline waste from soap works, refuse 

 lime from gas works, &:c. The proportions of the materials vary according 

 to quallly. Hollies are blown in moulds : the glass having been blown in 

 the mould, nothing remains but to form the mouth; this is done, the bottom 

 being attached to an iron punty, by holding tiic extreme edge of the neck 

 to Ihe heat for a short period, and, having collected a small quantity of 

 liquid glass upon the end of a small iron, called a ring iron, a ring of glass 

 is allowed to cover this extreme end, and this is afterwards worked into 

 shape by a machine which forms the inside and outside of the mouth at 

 the same time, merely by Ihe workman turning the bottle on Ihe iron upoa 

 his knee once or twice. The rapidity with which bottles are made is 

 almost incredible ; a workman, with Ihe assistance of a gatherer and 

 blower, will begin and finish 120 dozen of quart bottles in 10 hours, which 

 averages nearly 2^ per minute, and this is ordinarily done; and in some 

 works the men are restricted to 2 per minute, to prevent the work being 

 slighted. It may not be uninleresting lo observe the low price at which 

 this description of glass can be procured, now that Ihe duly has been re- 

 moved : quart bottles can be produced at ihe works at about 14s. per 

 gross : each gross weighs 2 cwt., which is equal to 7s. per cwt., or £7 per 

 ton, for manufactured bottles ; if from this we deduct for workmen and 

 incidental expenses, £2 per ton, it would leave the price of bottle glass 

 £5 per ton. 



Flint glass h thus designated from calcined flints having been formerly 

 used in its composition ; this is now replaced by fine sand. The term flint 

 glass is now applied lo all glass into Ihe mixture of which lead enters, and 

 is used in Ihe manufacture of table glass, &c. In the manufacture of flint 

 glass the circular furnace is used, the pots surrounding the grate room ; ou 

 either side of the pots are flue-holes, which pass through the pillars, the 

 smoke being carried up by flues outside these. The heat thus reverberates 

 from the crown of the furnace, and is drawn round Ihe pots previous lo 

 passing through the flue-hole. The melting pots are covered in, to protect 

 Ihe glass from dust, v\hich would i^iTect the colour. The materials used in 

 flint glass are sand, red lead and litharge, carbonate and niirate of potash, 

 arsenic, and manganese ; and the greatest care is taken iu the selection of 

 them — the beauty of the glass dependii]g mainly upon the qualily of the 

 materials. 'J he best sand comes from Alum Bay, Isle of Wight; this is 

 carefully washed aud dried previous to using. Ked lead, or lilharge: this 

 assists as a flux, and gives density, brilliancy, and duclilily,- — the latter 

 quality being particularly required in flint glass; it is, perhaps, owing to 

 the superior quality of Ihe oxides of lead prepared in England ihat we are 

 iuad\auceol oiher nations in Ihe manufaclure of fiue flint glass. The 

 carbonate and nitrate of potash are used wholly as fluxes ; soda, though 

 more active, is never used where quality is required, as it aflects the colour. 

 For almost every purpose, Ihe best glass of every descripiiou is that which 

 contains the greatest amount of silica. If the sand, lead, and alkali, 

 even though ihe quallly were never so pure, were melted, the glass 

 which would be produced would not be colourless, but of a pale green; 

 and this, in all probability, is not so much the result of impurity, as ihe 

 deoxidating efl'ect of the fusion. To obviate this, it is necessary to use 

 oxide of manganese, which, by supplying oxygen, retains the diflerent 

 subslances in Ihat stale of oxygenation necessary to a colourless glass; if 

 too much manganese he used Ihe colour is slighlly purple, designated by 

 the glassniakeis " high ; the green lint, again, is called ''low:" in other 

 words, the glass is high when it contains more than sutlicient oxygen, and 

 low when too litlle. ftiinute quanliiies only are necessary ; from a quarter 

 to half an ounce per cwt. is sufficient. Arsenious acid is sometimes used 

 in flint glass, its use being to expel the carbonic acid gas present in Ihe 

 materials ; if loo much is used it gives opacity. 



Glass must be considered, unfortunately for science, an imperfect body. 

 The principal imperfection, more especially of Uint glass, arises from 

 what are called cords, or stria; in the body of the glass, which give it the 

 appearance of alcohol and water imperfectly mixed; through ihese stria' 

 the rays of light will not pass, but are diverged and broken. This detect 

 is altnbuled to the difference in specific gravity, or want of homogeneity 

 of the particles: this, no doubt, is true; but the question is, lo what 

 cause is this attributable? I would suggest, that it may arise from the 

 unequal distribuliun of heat to the materials during fusioa and whilst in a 



