Mica quarrying and processing in Scotland during the Second World War
Text from: McIntosh, R P. 2001. Mica quarrying and processing in Scotland during the Second World War. Journal of the Russell Society, v. 7 Pt. 2 pp. 71–74.
Introduction
In 1942 when the Battle of the Atlantic was at its height and Allied merchant shipping losses for the year were running at 1664 ships, the transport of strategic resources from all over the world was seriously disrupted. At home, the Geological Survey of Great Britain working with the Ministry of Supply was involved in the search for a range of strategic mineral resources. One such mineral, normally obtained from India, was mica. Used extensively in the electrical industries especially for radio parts it was in very great demand. Joint investigations began into possible economic deposits of mica between the Survey and the Non-Ferrous Minerals Development Control and the Mica Control of the Ministry of Supply. These investigations were initially focused on the mica deposit in Knoydart that had previously been discovered by the Geological Survey of Great Britain in 1938. This article is accomapnied by an image gallery with a selection of Survey photographs taken to record the mining and processing of the industry that arose out of the investigations, and recent photographs of actual samples of processed mica from the Pitlochry Depot held in the British Geological Survey rock and mineral collection.
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View of Sgurr Coire nan Gobhar mica workings, Loch Nevis, Knoydart, Inverness-shire. No. 3 Working, Upper Bench. P000156.
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View of the Sgurr Coire nan Gobhar mica workings, No. 1 Working, Loch Nevis, Knoydart, Inverness-shire. West end of Lower Bench. P000159.
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Samples of mica books after rough dressing (rough cobbed) as sent for processing to the Pitlochry Depot. (Note 0.15 m. scale at base.) P000158.
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Rough splitting of dressed 'books' by two workers at the Sorting Factory. P000163.
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Selecting sheets for grade and quality. P000164.
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Packing graded sheet mica for despatch. P000166 .
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A consignment of sheet mica ready for despatch. P000167.
Distribution of mica in Scotland
Mica is widely distributed in many common rocks but is rarely found in sufficient quantities and quality to be of economic significance. In Scotland, the only source of muscovite-rich pegmatites suitable for sheet mica are those pegmatites cutting the Moinian rocks in two areas: in a western belt extending from Knoydart southwards to Loch Shiel and Loch Sunart and an eastern group in the Strathpeffer and Garve districts of Ross-shire. Both provided a site for commercial production of sheet mica during the Second World War.
In late 1942 a mica deposit in Knoydart, that had been discovered by the Geological Survey of Great Britain in 1938, provided the focus for investigations into the possibility of working mica-bearing pegmatites. In addition, in 1943, the Survey conducted an intensive search for new mica occurrences and as a result the Eastern Mica Co. Ltd. looked at deposits at Acharacle and Loch Shiel, Ben Reisipol, Ardarie (Loch Shiel), Kinlochquoich and Glenfinnan and three sites in the Strathpeffer and Garve districts; Little Scatwell, Carn Gorm and Brae Tollie. Apart from the Knoydart deposit only the Little Scatwell deposit reached the production stage.
Knoydart mica deposit
The deposit illustrated in ( P000156) (P000159) is the quartz-mica pegmatite on Sgurr Coire nan Gobhar in Knoydart situated about 1.5 miles NNW of Kylesknoydart and 300 yards SE from Loch Coir an Lochan, at an altitude of 1700–1800 feet. It was opened up in May 1943, at first working the original pegmatite discovered by the Geological Survey but later four other quarries opened up in the vicinity. During the life of the operations the original quarry remained the most important, supplying almost 70% of the total production. The mica zone in the pegmatite was about one metre wide and was proved for a length of about 80 metres. The muscovite mica sent for processing was of a high quality, it was brownish ruby in colour, hard, substantially flat and glossy. A small proportion had light vegetable staining and mineral dots. Overall, the mica from this deposit was graded as ‘stained’ or better. The mica ‘books’ were normally between seven and eight inches in diameter; books ranging up to 20 inches to two feet were not uncommon. They were about one inch thick. Three mica books are shown in (P000158).
The rock was blasted and quarried and the mica transported by pony down a track constructed by troops who were training in the area. The mica was rough-dressed in a shed and transported from a jetty by boat to Mallaig and from there by rail to Rannoch and then by bus to the sorting factory at Pitlochry.
Processing at the Pitlochry Depot
The main sorting factory at Pitlochry was opened in August 1943. It had a staff of six, for the first few weeks under the direction of Mr A B Mudie of the Eastern Mica Company and later under Mrs D G Readdie (the wife of Mr D G Readdie, of the Ministry of Supply, Mica Control department). The staff complement increased to 36 in November 1943.
The mica was sourced from two main areas, Sgurr Coire nan Gobhar in Knoydart and Little Scatwell in the Strathpeffer – Garve district. Small quantities of block mica were also obtained from Carn Gorm at the east end of Loch Garve and Brae Tollie, five miles north-west of Alness, both of which could be regarded as little more than extensive trials. The preparation of block mica for industrial use was a highly specialized task and the workers employed, young girls recruited locally, had to undergo many weeks of training though it was possible to judge after only a few days if a girl had the aptitude for the work. The trainees’ first task was to master the technique of cutting the mica quickly and cleanly and from there they learnt how to extract the largest possible plate with the minimum of waste, how and when to remove interlaminar stains by splitting, and how to remove damaged films from the surface of the block as thinly as possible. Once trained a worker could produce daily from 4.5 lbs. for the smaller grades to 16 lbs. for the larger grades.
The equipment used consisted of various types of knives, each of which had a distinct and separate purpose. Ancillary equipment consisted of nothing more than grading charts, white cardboard used for inspection purposes, overalls, protective clothing and gloves along with the shallow wooden trays for holding the mica as it passed through the different processes.
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 depot. 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 (P000160), (P000163). 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 (P000164). 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 (P000166), (P000167) To prevent transit damage the mica blocks would be packed in layers with overlapping edges.
The mica was used for a wide range of radio, electrical and scientific purposes. Typical items include condenser plates, valve bridges, cathode ray plates and discs, commutator separators, wrappings and washers, heater plates and stove plates. In addition to the block mica the depot produced a large amount of scrap mica. This consisted of material from the mine which was found unsuitable for processing, waste material from the rough dressing or trimmings from the cutting process. It was ground for use as a filler in the manufacture of rubber and insulating boards.
Summary of production at the Pitlochry Depot
Production at the Pitlochry Depot totalled 7122 lbs. of block mica and 85 tons of scrap mica. The quality was stated to be equal to that from India and other sources but the production costs per lb. of block mica were many times that of India. The production data from the two areas are as follows:
| Rock quarried | Crude mica produced | Crude mica yield | ||
| Knoydart | 3606 tons | 74 606 lbs. | 20.69 lbs./ton | 0.92% |
| Little Scatwell | 7079 tons | 117 778 lbs. | 16.6 lbs./ton | 0.74% |
| Total | 10 685 tons | 192 384 lbs. | 18 lbs./ton | 0.8% |
The British Geological Survey holds many mica specimens from most of the deposits that were under investigation or worked during World War 2 as well as a small collection of processed mica from the Pitlochry Depot.
Bibliography
Kennedy, W Q and Lawrie, T R M. (1943). Commercial mica in Scotland. Part 2. Preliminary description of some occurrences north of the Great Glen. Wartime Pamphlet No. 34. Geological Survey of Great Britain: Scotland.
Rose, W C C. (1946). War-time development of Scottish mica deposits.Bulletin Imperial Institute, London44, 325–329.
Crockett, R N. (1975). Mica. Mineral Resources Consultative Committee, Mineral Dossier No. 15. London: HMSO.
Mica. Mineral Resources File. Unpublished records. British Geological Survey.