MANUFACTURE OF GLASS. l6l 



SFCT1ON III. 

 Manufacture of Glass. 



Drinking, Watch , Windois, and Plate.Glass. 



Glass is a combination of sand, flint, spar, or some other sili. 

 ceous substances, with one or other of the fixed alkalies, and in 

 some cases with a metallic oxyd. Of the alkalies, soda is com- 

 monly preferred : and of the siliceous substances, white sand is 

 roost in repute at present, as it requires no preparation for coarse 

 goods, while mere washing in wat--r is sufficient for those of a finer 

 quality. The metallic oxyd usually employed is litharge, or some 

 other preparation of lead, as being the cheapest metal we can have 

 recourse to. 



It is also necessary that the siliceous matter should be fused in 

 contact with something called a flux. The substances proper for 

 this purpose are lead, borax, arsenic, nitre, or any alkaline matter. 

 The lead is used in the state of red-lead ; and the alkalies are soda, 

 pearl-ashes, sea-salt, and wood-ashes. When red-lead is used 

 alone, it gives the glass a yellow cast, and requires the addition of 

 nitre to correct it. Arsenic, in the same manner, if used in excess, 

 is apt to render the glass milky. For a perfectly transparent 

 glass, the pearl. ashes are found much superior to lead ; perhaps 

 better than any other flax, except it be borax, which is too expen. 

 sive to be used, except for experiments, or for the best looking, 

 glasses. 



The materials for making glass must first be reduced to powder, 

 which is done in mortars or by horse-mills. After sifting out the 

 coarse parts, the proper proportions of silex and flux are mixed 

 together and put into the calcining furnace, where they are kept in 

 a moderate heat for five or six hours, being frequently stirred about 

 during the process. When taken out, the matter is called frit. 

 Frit is easily converted into glass by only pounding it, and vitrify, 

 ing it in the melting pots of the glass furnace : but in making fine 

 glass, it will sometimes require a small addition of flux to the frit to 

 correct any fault. For, as the flux is the most expensive article, 

 tlie manufacturer will rather put too little at first than otherwise, 

 as he can remedy this defect in the melting pot. The heat in the 

 furnace must be kept up until the glass is brought to a state of per. 



VOL. VI. Id 



