The Living age 1770, 18.5.1878
From the Popular Science Review.
By J. H. Collings, F. G. S., secretary of the Royal Institution of Cornwall.
* Notwithstanding the apparent opacity of the water from suspended particles of clay and mica. I have seen trout eight or nine inches long taken from these streams.Travellers by the Great Western Railway in the west of England, after crossing the great Albert Brisge at Saltash, passing over the tree-tops in the deep valleys about Likeard, and leaving the deserted copper mines of St. Blazey and Par on their right hand, usually observe truckloads of peculiar white substances occupying the disings, and are especially struck with the whiteness of the streams crossed by the railway - which closely resemble rivers of milk.*
Naturally they begin to ask what it means, and the writer has often been amused by the gravity with which some fellow-traveller, who seems to think it disgraceful to confess ignorance on any subject whatever, vaguely talks of lime, or more boldly of chalk. Perhaps some native happens to be present, who politely assures him that he is in error - that neither chalk nor lime are present in any form - and that the peculiar whiteness is caused by a refuse product from the china-blacy works in the vicinity. This of course settles the question usually; but if the original querist happens to be inquisitive or persistent, he soon finds that the native in most cases is entirely ignorant of the methods of working, the extent of the works, and the uses of the product, and if this is true of the intelligent native, still more it is of the genral public. Out of Cornwall it is a rare chance to find either commercial men, or even scientific geologists, who have any knowledge whatever of the peculiar industry which characterizes many parts of Cornwall and Devon, and especially the centre of Cornwall. Yet this industry is interesting in itself, employs a large number of men, and supplies every year more than two hundred thousand tons of the dazzling white clay, which never fails to attract the attention of tourists.
The object of the present paper is to describe the salient features of this industry. Most people know that "china" was first brought by the Portuguese from China - hence the name. It was called by them porzellano, because it was supposed to be fabricated from sea shells; hence the term "porcelain;" but no real knowledge was obtained of the materials used until the publication of the reports of the Jesuit father D'Entrecolles, in 1712, and of Count Réaumur, in 1729. These reports led to the establishment of the manufactories at Dresden, Sevres, and Plymouth - the last-named having been established in 1733. Up to 1745, the fine porcelain materials used in the Plymouth works were imported; but soon after that time, Mr. Cookworthy, the founder of the works, discvered "kaolin" (which he calls growan clay, now called china clay), and the "petuntze" (called by him growan or moorstone, and now known as china stone), similar to or identical with that used by the Chinese, in several parts of Cornwall in great abundance. In conjunction with Lord Camelford, he took out a patent for the use of these materials in 1768. How these materials are used in the manufacture of porcelain, earthenware, and more recently in manu other British manufactures, forms no part of the subject of the present paper - this is limited to a description of the modes of occurence and of preparation of the china clay and china stone.
* In a paper read before the Royal Geological Society of Cornwall in 1876, I have given my reasons for believing that the decomposition has been produced in situ by fluids circulating within the fissures, joints, and shrinkage cracks of the granite - now occupied by the solid matter of the veins referred to above - and not as commonly stated in geological works by carbonic acid acting from above.China clay is prepared by washing a peculiarly white decomposed granite, which occurs very largely in the granite dristrict, north of St. Austell, as well as in many other parts of Cornwall - and also in Devon. This natural china clay rock, which has been elsewhere called "carclazyte," is simply a granite composed of white or pale smoky quartz, white mica (lepidolite), sometimes a little greenish-yellow gilbertite, and white felspar, in which the latter is partly or completely metamorphosed into kaolin. This modification of granite occurs in areas of irregular form, generally much elongated in one direction, and extending to an unknown depth. It is in the west of England universally associated with quartzose and schorlaceous veins - evidently of later origin than the rock itself - which sometimes also contain oxide of tin. The greatest extension of the decomposed granite coincides with the "run" or "bearing" of the veins, and is more complete as the vein is followed downwards in depth.*
Many of the so-called "deposits" of clay extend for a distance of a quarter of a mile, half a mile, or even more, in the direction of the viens, while their breadth may be only a few inches, and seldom exceeds a few fathoms. It is true that very wide masses of china clay are wrought in many places, but these are invariably associated with a group of paraller veins.
* This covering closely resembles some glacial deposits; but neither organic remains, nor scratched stones, nor stones of foreign origin have been found in it, to my knowledge, although many acres have been removed in teh various clay works.
** The term "clay" is applied indiscriminately in Cornwall in the decomposed granite rock, and to the true clay washed out of it.
*** The holes are - except the top one - temporarily covered with pieces of board nailed over them. The whole contrivance is called a "button-hole lavander."The granite rock is usually covered by a layer, from four to thirty feet thick, of brown or yellow sandy earth, ofter full of angular pieces of hard granite, schorl rock, tourmaline schist, with sometimes a little tin ore, etc. This layer is called by the workmen "overburden," and it must be removed before the clay can ge got at.* The process of working is usually as followes: let us suppose that a patch or band of suitable decomposed granite, called by the workmen a "bed of clay," has been discovered in a hillside. The first thing to be done is to drive an "adit-level" horizontally right into the hill beneath the bed of clay, the position and extent of which has been more or less accurately determined by systematic "pitting" through the overburden. This adit-level is a sort of tunnel - from six to nine feet high, and from three to six feet wide. While this level is being driven, a large piece of the overburden is removed so as to expose a considerable are of the bed of clay.** A vertical opening or shaft is then made from the inner end of the adit, to the surface of the uncovered clay bed - partl by digging downrards from above ("sinking", partly by digging upwards from below ("putting up a rise"). A square wooden pipe, having holes at regular distances of a few feet in one of its sides, is then placed in the vertical opening, so as to keep open a communication with the level below;*** the remainder of the shaft is then either filled in or kept open for the removal of the coarse sand or stones produced in working; and the regular washing of clay may be proceeded with. Of course the arrangements for obtaining the clay vary very much in different works. These different arrangements were described by the author in the "Journal of the Society of Arts," 1875.
Granite, as is generally known, consists mainly of three distinct minerals, quartz, felspar, and mica. IN the decomposed granite it is the felspar only which is decomposed, or converted into kaolin: but this renders the whole mass so soft that a pick or shovel may be readily driven into it to a considerable depth. The mode of working is to break up a portion with a pic to a depth of several feet, in a kind of slope, around the mouth of the pipe or launder which passes down into the adit below. This is called a "stope." A stream of water is then made to flow over the broken lumps, which are kept well stirred up y a workman, called a "washer," whose duty it is to keep breaking and stirring them up. The water, clear at first, speedily becomes white and milky by washingout the soft decomposed felspar, and runs down to the bottom of the stope, carrying with it the quartz grains and flakes of mica. The quartz and the coarser mica flakes, called "sand" by the workmen, settle in a shallow pit, called sandpit, wfrom whence they are constantly shovelled out by a man placed there for that purpose - while the stream of clay water, carrying with it many minute flakes of white mica, passes on down the vertical launder and through the adit-level to be further treated.
* Occasionally these pits are left filled with day water, and uodtsturised for several weeks, when the intense blueuess of the water equals that of some Alpine lakes, and is due to the same cause — the suspension of some minute particles of solid matter. The stream of clay water, if thick, contains usually about two per cent. of clay, and perhaps one-half per cent. of mica in suspension. This is made to flow slowly through a succession of narrow channels, called "drags " and "micas." in which the fine mica and a little clay are gradually deposited, while the bulk of the clay passes on with the water, and falls into a circular pit from twenty to thirty feet in diameter, and eight or ten feet deep, lined usually with granite blocks. Here It gradually settles to the bottom, while the clear water passes off at a little depression in the rim of the pit, and may either be pumped up to be used over again, or allowed to flow into the nearest river. This effluent water is often clear enough to drink.*
Once or twice a day it becomes necessary to clean ant the long channels, called "drags" and "micas," in which case the fine mica and clay, which has settled at the bottom, is washed out into the nearest watercourse by a strearnoi water, and this it is which fouls the streams.
In some works, as at the celebrated Cardale mine, worked as an open quarry for tin for four centuries, not only is the mica washed away in this manner, but the large quantity of sand produced is also washed away at night, and has very large accumulations of sand have collected in the valleys below St. Austell, St. Blazey, and elsewhere.
The clay having settled in the pits to a consistency somewhat thicker titan cream, is in old-fashioned works run out into shallow excavations called " pans." These are about two feet deep —the clay is put into them during the winter months, and is allowed to remain until nearly dry, when it is cut out in square blocks and piled up under sheds, or in the open air, till completely dry. By this mode of working a good deal of loss is experienced, as every block of clay is more or less injured at the surface by exposure for months to the vicissitudes of the weather, the invasions of sheep, cattle, and geese - which latter abound on the moors - and the mischief of stone-throwing boys. When thoroughly dry, therefore, every block has to be separately scraped before it is in a fit state to be sold.
The scraping operation is a peculiar and somewhat ghastly sight to those who see it for the first time. Gangs of tali women, white-aproned — every vestige of complexion hidden with white clay—stand at tables scraping the blocks all day long, with a little three-cornered scraper like a miniature Dutch hoe, and often dismally singing hymns which sound like dirges.
The advantage of air-drying is that no expense is incurred for fuel — the disad-vantage is the loss incurred by scraping, the expense of scraping, and the large area required for the drying-pans.
Its more modern works the air-drying is mostly replaced by kiln-drying. Long kilns are built of fire-clay tiles covering brickwork flues from sixty to one hundred or even one hundred and fifty feet long. The clay from the circular pits is first run into large tanks, where it remains for a month or two, till pretty stiff. It is then taken into the kiln or "dry" on tramwaggons, and dried by throwing it upon the hot tiles. When dry it is cut up into convenient-sized blocks and piled up ready for market. On the whole, the smaller amount of waste in kiln-dried clay, and the saving of expense for scraping, more than makes up for the expense of fuel, and few modern works are without kilns for drying.
The preparation of the china stone for the market is a much more simple operation. China stone is also a kind of granite, which is, however, only partially decomposed, but it is only valuable when it happens to be free from mica and all other minerals except the quartz and partially decomposed felspar. It occurs chiefly in the parishes of St. Stephens, St. Dennis, and Breage, and is often associated with china clay. The rock is simply quarried down, and the joints dressed over where discolored with oxide of iron, when it is at once ready for shipment, chiefly to Runcorn. From Runcorn it is sent to the potteries, where it is used with china clay in the manufacture of porcelain and earthenware.
It is not only in the potteries, however, that china clay is used. As stated above, more than two hundred thousand tons per annum are now exported from the two western counties, the value of which at the present exceptionally low prices may average, perhaps, 1l. per ton at the shipping port. Of this large quantity probably little more than one-third is used in the English potteries. Fully as much is used by the piper-makers of the United Kingdom, and probably twenty thousand tons are used by makers of alum and sulphate of alumina, and at least as much by "bleachers" of calico and yarn. Many thousands of tons are shipped to the Continent for all the above purposes, and also for the manufacture of ultramarine. So cheap and convenient an article is also no doubt used to a considerable extent by adulterators and sophisticators of various kinds, but it probably only needs to be more widely known to be used legitimately for many purposes to which hitherto it has not been applied.
We have spoken above of the "sand" of the "mica." Nowadays much of the fine mica is re-washed and sold as an inferior "mica clay" for making inferior kinds of paper and pasteboard, but the bulk of it must still be regarded as a waste product. The sand is also largely a waste product, and as there are from three to eight tons of sand yielded for each ton of clay, large accumulations exist at most of the principal works. Of late years some of this — mixed with refuse clay— has been made into excellent fire bricks, for which there is a considerable demand, especially for the copper-smelting furnaces of Chili and Mexico; but there are still millions of tons available when a proper use can be discovered.
Very good building-bricks have also been made from the discolored clay and mica, but the heavy cost of carriage usually prevents the use of building-bricks at great distances from the place of manufacture, so that, although there is a fair local demand. millions of tons of material are still available for any suitable purpose which may be hereafter discovered.
Some of the sand is very coarse, some very fine — but in all cases it consists mainly of angular fragments of quartz, admirably adapted from their extreme sharpness for mixing with cement or lime for making concrete blocks, or with asphalt for pavements.
The coarser varieties also make excellent gravel walks, as the small quantities of clay and mica usually present serve to bind the grains together under foot to form a firm roadway.
The following analyses, selected from the writer's paper read to the Royal Geological Society of Cornwall in 1876, fairly represent the composition of average specimens of the materials described above.
[TABLE IS MISSING / TAULUKKO PUUTTUU]
All the older writers speak of "talc," " talcose granite," proto,,rine," etc., as being abundant in Cornwall and elsewhere. Even Mr. J. A. Phillips, writing in 1875, says: In some districts mica is replaced by a talc-like mineral, and the granite rock itself passes into protogine." What there may be elsewhere I am unable to see, but there is certainly nothing of the kind known at present in Cornwall. From the whole of the granite districts of Cornwall and Devon talc seems to be entirely absent, and magnesia generally is an extremely scarce substance.
The number of china-clay works at present in operation in Cornwall and West Devon is little short of two hundred, and of course the conditions of working vary considerably in different localities and under different conditions. In some, a little washing only is done during the winter months by means of a small natural stream of water, the total annual produce being only a few hundred tons; in others, valuable machinery and extensive buildings enable the proprietors to turn out nine Or ten thousand tons in the same time; but in the main the description given above as fairly describes the occurrence and preparation of china clay and china stone as is possible within the limits of such an article as the present.
The trade at present is much depressed — mainly, I believe, owing to the slackness on the port of those (potters, paper-makers, bleachers, and others) who use clay; but partly, perhaps, owing to the over-production of a few years since. In consequence of this the prices have fallen more than thirty per cent. during the last five years, and the quantities produced have also shown a considerable falling off. With the general revival of English trade - whenever that may take place - no doubt this branch will also revive.
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