Tuesday, 9 April 2013

Mica processing at the Pitlochry Depot, Perthshire. 1945.


Processing of Scottish mica. Pitlochry Depot (January 1945). Perthshire. Cutting and trimming mica at the Sorting Factory.
BGS image ID: P000163
Processing of Scottish mica. Pitlochry Depot (January 1945). Perthshire. Cutting and trimming mica at the Sorting Factory.

The split mica sheets are trimmed of all flaws, structural imperfections or, less serious, inclusions. The cutting knife is held at a low angle to the cleavage so the plates are consequently bevelled. The workers can clearly be seen using their knives. Processing of mica involved three stages, 1. rough cobbing, 2. splitting, 3. trimming. It does not undergo any other process of treatment or concentration before it is ready for use.


Processing of Scottish mica. Pitlochry Depot (January 1945). Perthshire. Plates of mica illustrating the various commercial grades.
BGS image ID: P000185
Plates of mica illustrating the various commercial grades. 

The photograph shows eight grades of mica. One to four are grades of staining. Five to seven are grades of spotting. Eight is a mica showing zoning, a structural imperfection. Note the re-entrant angles on three and four where imperfections have been cut out. Mica is graded for commercial purposes according to type, colour and freedom from staining. The primary division is into Muscovite or White Mica and Phlogopite or Amber Mica. Muscovite is subdivided into two types 'Ruby' and 'Green' types. Both are colourless when less than 0.5 mm. but when greater than this size they have reddish and greenish tinges respectively.

Production from the Pitlochry factory totalled 7,122 lbs. of block mica in addition 85 tons of scrap mica, resulting from the dressing operations, was sold. Mica, during wartime, became a mineral of high strategic value, an essential war material which due to its unique physical properties rendered it indispensable as an insulating medium in the manufacture of many types of electrical, radio and telephone equipment.

Bob McIntosh

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