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Soda-calcium glass is called soft glass, because it is easily soft- 

 ened by heating. When potassium carbonate is substituted for 

 sodium carbonate, a less fusible substance, used in making some 

 chemical apparatus, and called hard glass, is obtained. When 

 lead oxide is employed in place of the limestone, a potassium-lead 

 silicate K 2 Si0 3 ,PbSi0 3 ,a:Si02 is formed which, on being cooled, 

 gives flint glass. This glass has a higher density and greater 

 brilliancy than soft glass, and is used in making vessels of cut glass 

 and lamp chimneys. The cutting is done with a revolving grind- 

 ing wheel. 



When glass is allowed to cool quickly, the product is very brittle 

 and apt to crumble to pieces on receiving a shock or scratch. 

 Glassware is therefore all annealed, by being passed on a slowly 

 moving frame through a long furnace, which is very hot at the 

 entrance and much cooler at the exit. 



Colored glass is made by adding oxides of metals which, with 

 the silica, give colored silicates. Oxide of chromium gives green 

 silicates, oxides of copper and of cobalt blue silicates, and oxide of 

 manganese violet. Gold oxide is reduced to the metal, which 

 goes into colloidal solution and gives ruby glass. Milky glass is 

 made by adding calcium fluoride, or stannic oxide. The green 

 color of bottle glass is due to iron (ferrous silicate) derived from 

 impure sand or limestone. 



The rough surface of ground glass is produced with a sand blast. 

 For engraved glass, the surface is covered by a stencil to pro- 

 tect it from the sand blast, and only the pattern is left exposed. 



In granite iron ware the surface is covered with a thin layer of 

 easily fusible glass (enamel, see borax). 



Pure quartz can be melted in the oxy-hydrogen blowpipe, 

 and recently chemical apparatus (silica ware) has been made 

 out of it. It has the advantage of being less soluble than glass, 

 and of not breaking even when it is heated white hot and quenched 

 in cold water. Glass breaks when chilled, because the parts first 

 cooled shrink considerably and a great strain is produced. Quartz 



