OPTICAL GLASS FOR WAR NEEDS 119 



glass of the country was made in this fashion during the war 

 and it has become the established procedure for that type of 

 glass. Much optical glass has also been* made in this manner. 

 After casting, the glass is passed into lehrs where it stays six 

 hours or more to cool. Grinding and polishing can be done 

 on the large pieces before cutting for inspection. 



A large number of batch formulae have been developed and 

 the production of newer and better types of glass is the subject 

 for continued research. Some of the limitations may be 

 mentioned. If silica is used in quantities above 75 per cent, 

 the glass cannot be properly melted. Alkali must be below 20 

 per cent, or the glass is hygroscopic. Lime must be less than 

 13 per cent, or crystallization will result. Lead above 70 per 

 cent, also will cause crystallization. Barium oxide may 

 be used up to 50 per cent., but great care must be exercised 

 or the pot will be attacked. Boron oxide jnay be used up to 15 

 per cent, or 20 per cent., but zinc above 12 per cent, causes 

 crystallization. Alumina above 5 per cent, gives a glass that 

 is too viscous, but alumina toughens glass and serves to counter- 

 act the tendency to crystallize. Arsenic increases transparency 

 by setting up an oxidizing action at the high temperature of 

 the furnace, thus reducing the color which arises from the pres- 

 ence of iron. Nitrates alone are too active. Carbonates alone 

 do not give the necessary oxidizing agents, while if too much 

 alkali is used with too little nitrate, the glass will not " fine " 

 well. 



From importing exclusively in 1914, the United States rapidly 

 developed the industry until late in the war we were in position 

 to become exporters and served Italy's requirements during the 

 last months, taking a considerable burden off the shoulders 

 of the English and French makers. Of the 675,000 pounds 

 of ordinary crown, boro-silicate crown, barium crown, ordinary 

 hard crown, light flint, medium flint, and dense flint produced 

 for war purposes, 95 per cent, was made under the direction 

 of the Geophysical Laboratory, with ten men in the field and 

 thirteen at work concurrently in the laboratory. About 2.8^ 



