Cut and trimmed muscovite mica from the Loch Nevis Mica Prospect, Knoydart, Invernessshire

BGS image ID: P527602

Muscovite mica from the Loch Nevis Mica Prospect, Knoydart, Invernessshire.

Specimens of graded mica classed as 'Graded No 5 1/2 good stained'. The specimens of muscovite mica would have been worked at the Pitlochry Sorting Factory during the Second World War. 

After the mica 'books' were extracted from the quarry the first process they underwent was rough dressing. This was initially done near the quarry at Knoydart but soon transferred to the Pitlochry Sorting Factory. It consisted of splitting the books into sheets and the removal by cutting of the flaws, incrustations and striations. The mica would then be passed to the cutters who, using skill and great care would remove the remaining flaws and trim the edges leaving block mica of irregular shape with a curved and indented outline. Further fine splitting to remove stains and spots required great judgement to balance loss in weight with the possibility of improving the quality of the block. The final process was the grading for size and quality. Size was defined by the area of the largest rectangle that could be cut from it, while quality was based on clearness, hardness and flatness. Typical remaining imperfections such as air spots, mineral or vegetable spots or lines, softness or waviness would affect the electrical and/or mechanical properties of the mica. Finally, mica to the weight of around 50 lbs. would be placed into wooden packing cases before despatch to London.

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.

British Geological Survey Petrology Collection sample number MC 7382.

Bob McIntosh

Mica processing at the Pitlochry Depot, Perthshire. 1945.

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.

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