Pigments, Paint and Painting: A practical book for practical men

by George Terry

When the stream of clay-water enters the pits, it contains from 1½ to 3 per cent. of clay; and what is called a good washing stream will carry about one ton of clay an hour. When the pit is full, the “hatch” is drawn, and the clay is “landed” into the tank. The upper portion is sufficiently fluid to run in of itself; but that near the bottom has to be helped out by men using “shivers” of wood or iron, which resemble large hoes; they are assisted by a small stream of water. The tanks are commonly, but not always, rectangular, built of stone, and paved with stone at bottom, often 60 feet by 30 feet by 6 feet or larger. Once in the tank, the clay is left to settle, until it has the consistency of cream cheese, the water being drawn off from time to time; it is then ready to be trammed into the “dry.”

The “dry” is a large building erected in immediate proximity to the tanks. It is always composed of two parts, the dry proper and the “linhay.” The floor or “pan” of the dry is composed of fire-clay tiles 18 inches square, 5 or 6 inches thick at the fire end, and gradually thinning off to 2 or 2½ inches at the stack end. The flues are built of fire-brick, about 15 inches wide, 2 feet deep at the fire end, and 9 inches deep at the stack end. Each flue should be supplied with a damper. The furnaces are built in and arched over with best fire-brick; the fire bars run longitudinally, and are about 6 feet long. The grate surface is about 2 feet 6 inches wide in front, and 4 feet 6 inches to 6 feet at back, according as each furnace supplies three or four flues.

The clay, brought in from the tanks in tram-waggons holding about half a ton, is tipped on to the tiles, and spread in a layer from 9 inches thick at the fire end to 6 inches thick at the stack end. The fire end is loaded and cleared every day; the other end perhaps twice or thrice a week, according to the length of the dry, thickness of tiles, perfection of draught, &c. An average size for a first-class dry is perhaps 15 feet wide and 120 feet long; but some have been constructed considerably larger than this. The pan of the dry should be 6 or 8 feet above the linhay whenever possible, so as to afford storage space for the dry clay, without expending labour in piling. The tiles should be as porous as possible, for very much more water passes through the tiles and into the flues than is driven upwards in the state of steam. The temperature should never be allowed to rise so high that the workmen cannot walk on the tiles, otherwise the clay may become baked and damaged.

In cases where there are no means of artificial drying, as at some old-fashioned works, the thick clay is at once transferred from the original settling pit to shallow depressions in the ground, called “pans.” Ten or twelve of these, each holding from 40 to 50 tons, should be provided for each settling pit; they measure from 20 to 40 feet square, and 2 feet deep, and are enclosed by granite walls, the interstices of which are rendered impervious by plugging with moss. The clay, filling two-thirds of their depth, is here left exposed to the sun and wind, by which it is partially deprived of its moisture.

In order to complete its desiccation, the clay is removed from the pans after three or four months’ exposure. A number of parallel incisions are made lengthwise in the clay, by means of a knife attached to a long handle; the strips are next divided transversely, by men with spades, who throw the blocks on to a board, upon which they are borne by women and children to the sandy drying yard, where, in fine summer weather, they soon become dry. They are then collected, and piled away in sheds, under a number of thatched gates or “reeders,” or are placed in some sheltered position where air can circulate around them without their becoming wet from rain.

When required, the blocks are scraped by women armed with hoes, before being despatched from the works. The transport is often effected in small casks, holding about half a ton. A few years since, a machine for drying china-clay was invented by a mechanical engineer named Leopoldo Henrion, of Sampierdacena, near Genoa. It is said that, by its use, the operation can be effected in a few hours, at a relatively small cost.

Collins was first led to adopt his arrangement in consequence of the formation of the ground; but he is inclined to recommend it in most cases if practicable. Very large quantities of stone are required in the dry pits, tanks, &c. Very often this is got, in part or entirely, in the process of excavating the pits, &c.; but if it cannot be so obtained, a very serious expense will be incurred, in some instances amounting to several thousand pounds. The total cost of the works may even be doubled from this cause, if stone has to be fetched from a distance of several miles.

Two modes of building with rough stone are adopted; they are known as “lime building,” and “dry stone walling.” The first needs no special remark, but the second is very ingenious and very effectual. The wall is built up double, with a batter of about ¾ inch or 1 inch to the foot. Moss is placed between the joints of the wall, and the space between is filled in with sharp sand, the refuse of that or some other clay works. A small stream of water is then made to flow over the sand, which is well beaten in with rammers, or by treading with the feet. This process is continued, a foot at a time, till the wall reaches the required height, when it is either paved with rough stones set on edge, or turfed. A wall properly built, in the manner just described, is quite impervious to moisture, and will stand for fifty years or more. It is, where the proper kind of sand is abundant, much cheaper than lime walling, and is always preferred for the walls of pits and tanks.

Where the bed of clay is situated on a hill-side, with plenty of space below, a tunnel is driven in from the hill-side or from the valley to the required depth, and a rise is put up as before. This rise is then divided off into two parts. In the smaller, a button-hole launder is placed as before, and packed around with clay; but the larger is left open. A stream of water, obtained by pumping or otherwise, is made to run over the stope, and down the button-hole launder. It then flows along a launder placed in the bottom of the level, until it makes its exit in the valley. It may then be purified, settled, and dried exactly as already described—the works being laid out at a lower level than the adit; or, if the clear water is wanted to flow over the stope, or it is, for any reason, necessary to place the pits and tanks at a higher level than the stopes, the water is pumped up after partial or complete purification.

The main difference in this mode of working is that instead of pulling the sand and rubbish up over an incline, it may be tipped down the pass into waggons, run out through the level, and tipped over the hill-sides. In cases where waste water is abundant, it may even be washed out at night, thus saving the expense of tramming. Of course, when the workings have reached their full depth, the rise and the launder are dispensed with, and the adit level communicates directly with the “bottoms.” By this mode of working a considerable economy may be effected, especially when it is not necessary to pump the clay water for settling or repeating.