ALABAMA MICA DEPOSITS 35 



properly prepared, with resultant loss to the producer, 

 being either "rough-trimmed," or "thumb-trimmed," and 

 only partially edged. 



"Thumb-trimmed" mica is the term used to describe 

 mica blocks or books having the rock edging in part 

 crudely removed by hand separation or breaking off, 

 "Rough-trimmed" mica, represents a slightly better stage 

 of preparation, and "knife-trimmed," its competent and 

 proper form, all edging imperfections being removed, 

 and the physical condition of the sheet for subsequent 

 handling and shipment being as far as possible perfected. 



*Sheet mica is also classified according to its uses. Con- 

 denser mica must be clear ruby, colorless or greenish, 

 must be split easily and smoothly into films as thin as 

 one-thousandth of an inch, must be free from cTacks, 

 holes, foreign substances, stains, spots, smoke, and sheets 

 must be plain and smooth, free from waves, wrinkles, 

 reeves, rulings, or knots. Uncut condenser mica, to cut 

 any particular size, must not be larger than one and one- 

 half times that size each way. Cut but not split con- 

 denser mica is cut into regular sizes, 2 by 2 or 2 by & 

 inches, which are slightly larger than the thin sheet act- 

 ually used in the condensers. From this cut mica the 

 condenser film is then punched out in the shape and size 

 used. If cut and split, the individual pieces ^are :"rom 

 O.C01 to 0.003 inch thick, with an allowable variation of 

 only O.OC05 inch. If cut or punched into the required 

 shape, split to the necessary thinness, and carefully in- 

 spected, the mica is called condenser film mica. The cut 

 edges of the films must be smooth, and the finished film 

 must of course be free from all defects and cracks. For 

 wireless outfits, each film must withstand 20,000 volts. 

 For magneto condensers, no high-voltage resisting test 

 has to be made, but each film must be free from cracks 

 or other defects. Such "film mica" is very different 

 from the thin splittings used in the manufacture of mica 

 board. These irregular-shaped splittings are sometimes 

 also called films. It has been estimated that on an aver- 

 age it takes from 2 to 3 pounds of trimmed Indian or cut 



*U. S. Geolopical Survey Mica in 1918. 



