308 ECONOMIC GEOLOGY 



heat and practically unaffected by molten alkalis and strong 

 acids. Starch is sometimes used as a binder. The crucibles are 

 dried for several days and fired in a Hempel electric furnace at a 

 temperature of 2000 to 3000 C. 



Owing to the low coefficient of expansion of zirconia these wares 

 can be plunged red-hot into water without risk of fracture. 



A small amount of zirconium is manufactured into ferro-zircon- 

 ium which is used in the refining of steel. 



COLUMBIUM 



The production of columbite in the United States is limited to 

 the mining of a few hundred pounds annually for museum and 

 laboratory material. Such a production was produced in 1911 

 by E. E. Hesnard, Custer, South Dakota. 



TANTALUM 



The production of tantalum in the United States is likewise 

 small. It is derived largely for domestic consumption from the 

 mineral tantalite. The one use which has brought tantalum 

 into prominence has been the making of filaments for incandes- 

 cent electric lamps. The toughness of the metal made its use 

 popular. Within the last few years the process by which tungsten 

 wires can be drawn has been so far improved that tantalum lamps 

 can show little advantage in toughness over tungsten lamps. 



As wire for incandescent electric lamps tantalum is valued at 

 approximately $500 per avoirdupois pound. 



A small quantity of tantalum is annually imported. 



SELENIUM 



The production of selenium in the United States is not large. 

 It now averages about 10,000 Ib. The product is obtained as a 

 by-product in the electrolytic refining of copper. The price 

 ranges from $3 to $5 per pound. Selenium is used in the manufac- 

 ture of enamels, glazes and red glass. 



TELLURIUM 



The actual production of tellurium in the United States is 

 small. It can be recovered in considerable quantities in the elec- 



