GLASS 



Long experience at St. Gol>nin has proved that ono part, of dry carbonate of soda 

 is adequate to vitrify perfectly three parts of fine siliceous s:md, as dial, of the mound 

 of Aumont near Senlis, of Alum Bay in the Isle of Wight, or of Lynn in Norfolk. It 

 is also known that the degree of heat has a great influence upon \\\<- vit rifirntion, and 

 that increase of temperature will compensate for a certain deficiency of alkali ; for it is 

 certain that a very strong fire always dissipates a good deal of the soda, and yet the 

 glass is not less beautiful. The most perfect mirror-plate has constantly afforded to 

 M. Vauquelin, on analysis, a portion of soda inferior to what had been employed in its 

 formation. Hence, it has become the practice to add, for every 100 parts of cullot, or 

 broken plate, that is mixed with the glass-composition, one part of alkali, to make up 

 for the loss that the old glass must have experienced. 



To the above-mentioned proportions of sand and alkali, independently of the cullet 

 which may be used, dry slaked lime carefully sifted is to be added to the amount of 

 one seventh of the sand ; or the proportion will be, sand, 7 cwts. ; quicklime, 1 cwt. ; 

 dry carbonate of soda, 2 cwts. and 37 Ibs. ; besides cullet. The lime improves the qual ity 

 of the glass, rendering it less brittle and less liable to change. The preceding quan- 

 tities of materials, suitably blended, have been uniformly found to afford most advan- 

 tageous results. The practice formerly was to dry that mixture, as soon as it was 

 made, in the arch for the materials, but it has been ascertained that this step may Le 

 dispensed with, and the small portion of humidity present is dissipated almost instantly 

 after they are thrown into the furnace. The coat of glaze previously applied to the 

 inside of the pot prevents the moisture from doing them any harm. For this reason, 

 when the demand for glass at St. Gobain is very great, the materials are neither 

 fritted nor even dried, but shovelled directly into the pot ; this is called founding raw. 

 Six workmen are employed in shovelling in the materials either fritted or otherwise, 

 for the sake of expedition, and to prevent the furnace getting cooled. One-third of 

 the mixture is introduced at first ; whenever this is melted, the second third is thrown 

 in, and then the last. These three stages are called the first, second, and third fusion 

 or founding. 



According to the ancient practice, the founding and refining were both executed in 

 the pots, and it was not till the glass was refined that it was laded into the cuvettes, 

 where it remained only 3 hours, the time necessary for the disengagement of the air- 

 bubbles introduced by the transvasion, and for giving the metal the proper consistence 

 for casting. At present, the period requisite for founding and refining is equally 

 divided between the pots and the cuvettes. The materials are left 16 hours in the pots, 

 and as many in the cuvettes ; so that in 32 hours the glass is ready to be cast. During 

 the last two or three hours the fireman or tiseur censes to add fuel ; all the openings 

 are shut, and the glass is allowed to assume the requisite fluidity; an operation called 

 stopping the glass, or performing the ceremony. 



The transfer of the glass into the cuvettes is called lading (trejctage). Before this 

 is done, the cuvettes are cleared out. that is, the glass remaining on their bottom is 

 removed, and the ashes of the firing. They are lifted red hot out of the furnace by 

 the method presently to be described, and placed on an iron plate, near a tub filled 

 with water. The workmen, by means of iron paddles 6 feet long, flattened at ono end 

 and hammered to an edge, scoop out the fluid glass expeditiously, and throw it into 

 water; the cuvettes are now returned to the furnace, and a few minutes afterwards the 

 lading begins. 



In this operation ladles of wrought iron are employed, furnished with long handles, 

 which are plunged into the pots through the Tipper openings or lading holes, and 

 immediately transfer their charge of glass into the buckets. Each workman dips his 

 ladle only three times, and empties its contents into the cuvette. By these three 

 immersions (whence the term trejctcr is derived) the large iron spoon is heated so 

 much that when plunged into a tub full of water, it makes a noise like the roaring 

 of a lion, which may be heard at a very great distance. 



The founding, refining, and ceremony being finished, the workmen next try whether 

 the glass be ready for casting. With this view the end of a rod is dipped into the 

 bucket, which is called drawing ili<' <il<i** \ the portion taken up being allowed to run 

 off, naturally assumes a pear-shape, from the appearance of which they can judge if 

 the consistence be proper, and if any air-bubbles remain. If all bo right, the < 

 are taken out of the furnace, and conveyed to the part of the hal/c where their contents 

 are to be poured out. This process requires peculiar instruments and manipulations. 



Casting. While the glass is refining, that is, coming to its highest point of ; 

 tion. preparation is made for the most important process, the casting of the plati-. 

 whose success crowns all the preliminary labours and cares. The oven or <wv/?/"/> 

 destined to receive and anneal the plate is now heated by its small fire or tifar to such 

 a pitch that its sole may have the same temperature as that of the plates, being nearly 

 red hot at the moment of their being introduced. An unequal degree of heat in the. 



