G58 



GLASS 



1099 



the object being to obtain a pot full of glass, to facilitate the skimming off the impurities 

 and saudiver. The mouth of the pot is now shut, by applying clay-lute round the 



stopper, with the exception of a 

 small orifice below, for the escape 

 of the liquid saline matter. Flint- 

 glass requires about 48 hours for 

 its complete vitrification, though 

 tho materials are more fusible than 

 those of crown-glass ; in conse- 

 quence of the contents of the pot 

 being partially screened by its 

 cover from the action of the fire, as 

 also from the lower intensity of tho 

 heat. 



Fig. 1099 represents a fljnt-glass 

 house for 6 pots with the arch or 

 leer on one side for annealing the 

 crystal ware. In fig. 1100, the 

 base of the cone is seen, and the 

 glass pots in situ on their platform 

 ranged round the central fire-grate. 

 The dotted line denotes the con- 

 tour of the furnace,^. 1099. 



Whenever the glass appears fine, 

 and is freed from its air-bubbles, 

 which it usually is in about 36 

 hours, the heat is suffered to fall a 

 little by closing the bottom valves, 

 &c., that the pot may settle ; but 

 prior to working the metal, the 

 heat is somewhat raised again. 



It would be useless to describe 

 the manual operations of fashioning 

 the various articles of the flint- 

 glass manufacture, because they 

 are indefinitely varied to suit the conveniences and caprices of human society. 



Every different flint-house has a peculiar proportion of glass materials. The fol- 

 lowing have been offered as good practical mixtures : 



1. Fine white sand 300 parts. 



Eed lead or litharge 200 



Refined pearl-ashes 80 



Nitre 20 



Arsenic and manganese, a minute quantity. 



2. Fine sand 50'5 



Litharge 27'2 



Refined pearl-ashes (carbonate of potash, with 5 per cent, of water) 17'5 



Nitre 4-8 



100-0 



To these quantities from 30 to 50 parts of broken glass, or cullet, are added, with 

 about a two-thousandth part of manganese and a three-thousandth part of arsenic. 

 But manganese varies so extremely in its purity, and contains often so much oxide of 

 iron, that nothing can be predicated as to its quantity previously to trial. 



M. Payon, an eminent manufacturing chemist in France, says that tho composition 

 of ' crystal ' (tho namo given in Franco to their finest flint-glass) does not deviate 

 much from the following proportions: 



Wood fire. Coal fire. 



Siliceous sand 3 



]\liiiiuni 2 



Carboofttd of potuh . . 1J 



The flint-glass lerr for annonlinfr glass is nn arched gallery or large flue, about 36 

 feet long, 3 feet high, 4 \vi<le : Invin^ its floor raised above 2 feet above the ground of 

 the glass-house. Tho hot air arid smoke of a fire-place at one end pass along this gal- 

 lery, and are discharged by a chimney 8 or 10 feet short of the other end. On the 



