248 METHODS OF GLASS MA"NUFACTUEI]SrG. 



which was much purer than any that could be obtained 

 near Venice. 



In the 17th century, England made rapid progress in 

 the development of the industry, and it was due to 

 chance that she soon led the world in this branch of 

 manufacturing. The furnaces used at that time for 

 melting glass were heated with wood, and on account of 

 the growing scarcity of this fuel, it was decreed that 

 only two of the fifteen glass houses then in operation, 

 would be permitted to continue. Soon after coal was 

 used as fuel, but this presented new difficulties to over- 

 come. The glass being melted in uncovered crucibles or 

 pots, was colored by smoke and soot, thereby largely 

 destroying the value of the product ; to overcome this, 

 covered pots were employed, but the amount of heat 

 which had formerly melted the batch, was found to be 

 insufficient now, and as no greater amount of heat could 

 be obtained the only relief was to use more powerful 

 fluxes. Amongst many which were tried was litharge or 

 oxide of lead. The result was that glass so produced was 

 far superior in color, transparency and brilliancy to any 

 manufactured in ancient Rome or modern Venice and 

 Bohemia, and was called flint glass or crystal. 



During the 18th and 19th centuries there was little 

 progress made in improving the quality of the glass pro- 

 duced, the principal advancement in the industry having 

 been made in developing cheaper methods of manufac- 

 turing. 



Glass is a transparent compound, produced by the 

 fusion, at a high temperature, of silica and various 

 metallic bases. The bases employed, regulating the 

 solubility, density, color, &c. 



True glass is a mixture of soda or potassium silicate, 

 and one or more insoluble silicates such as barium, 

 stronsium, calcium, magnesium, aluminium, manganese, 

 iron or lead. While the silicate of soda or potassium, 



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