Building Materials, Their Use and Origin 423 



Materials and process. Of what is glass made? A visit 

 to a glass factory reveals that sand, more or less white, is the 

 chief constituent, and mixed with sand one or more of these 

 substances : limestone, or chalk, sal soda (sodium carbonate), 

 glauber's salt (sodium sulfate), ' potassium carbonate called 

 potash, red lead (lead tetroxid), carbon, and arsenic. The 

 mixture is put into large melting pots of fire-clay, and heated 

 to a temperature at which it melts or fuses. It is boiled long 

 enough to secure thorough mixing and the elimination of all 

 gases. The molten mass unlike water has no definite freezing 

 or melting point, and hardens very slowly as it cools. As it 

 cools, it hardens sufficiently to become a plastic, pasty mass, 

 which may be molded or blown into any desired form. It may 

 be kept pasty for an indefinite time in a furnace, as is necessary 

 when it is being made into articles by blowing. 



Bottles, tumblers, lamp chimneys, vases, and many other 

 useful objects are made by blowing and molding combined. 

 The glass-blower takes up on the tip of his blowpipe enough of 

 the plastic mass, blows it into a bubble which is then put into 

 a mold and blown until it fills all parts of the mold. It is 

 trimmed and finished and transferred to the annealing furnace. 

 Glass which cools too rapidly is very brittle ; when it cools too 

 slowly, it crystallizes much in the fashion of sugar sirup. It 

 is therefore annealed, or gradually cooled in an annealing 

 furnace in which the temperature is lowered as required by the 

 product. 



Window glass is made by blowing the molten glass into a 

 large globe which by skillful manipulation is gradually length- 

 ened into a cylinder. The cylinder is then cut open, heated, 

 and spread out flat. Or window glass is made by machinery 

 which gives it any desired thickness and size. Many common 

 and cheap glass articles are pressed into shape by dies. 



For plate glass the molten mass is poured upon molding 

 tables and rolled smooth by hot rollers operated usually by 

 hand. The form is about one inch thick. After being an- 



