|BGS image ID: P527602|
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.