32 GEOLOGICAL SURVEY OF ALABAMA 



recoverable with proportional values based on size, con- 

 dition, and properties. 



The chief use and largest consumption of sheet mica 

 is for electrical insulation. For use in high-potential 

 machinery or appliances, only high-grade sheet mica, 

 wholly free from imperfections in the form of cracks, 

 pin-holes or inclusions is rigidly specified. 



Originally, sheet mica found its principal and almost 

 sole use in glazing, for stove windows and for certain 

 forms of lamp chimneys. Although still necessarily in 

 use for these purposes and commanding increasingly high 

 prices for the grade and sizes of sheet required, the 

 amount so used at the present time is hardly one-tenth of 

 the sum total of consumption, which has been estimated 

 to average : for electrical insulation, splittings, 40 per 

 cent, and sheet 46 per cent., or a total for electrical in- 

 sulation of 86 per cent. ; stove glazing 10 per cent., phono- 

 graph diaphragms 2 per cent., the remaining 2 per cent, 

 covering all other purposes. 



The following table indicates, as to size, the various 

 grades upon which market quotations usually base : 



Trade Number Size in Sq. Inches. 



Extra Special 60 and up 



A.A ; 48 to 60 



Al : 36 to 48 



1 24 to 36 



2 , 15 to 24 



3 : : : 10 to 15 



4 6 to 10 



5 : 3 to 6 



6. ... ... to 3 



During the late war, an entirely new use developed for 

 sheet mica in surgery as a covering for open wounds un- 

 der treatment, by reason of its transparency, its anti- 

 septic quality, because of absolute freedom from corro- 

 sion, and its value as a non-conductor of heat and of cold, 

 plates of mica being included to some extent in first aid 

 kits used in the field. 



Ground mica, aside from its former chief uses in patent 

 roofing, in the annealing of steel, in various forms of lub- 

 rication and for fancy paints, tiles, and concretes, for 



