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sand, mica, &c., is pumped into a series of troughs of wood, generally known as “laụnders;" these are long and wide; across these at intervals are placed a number of pieces of wood, called the “drags,” which serve to impede the flow of the water, and cause it to form a series of pools, in which the heavier particles of sand can collect. These launders are emptied of the sand which accumulates in them from time to tiine, so as to give
plenty of room for the sand to deposit. From the launders the water, with the clay and mica, passes on to what are called the “micas," an ingenious arrangement for promoting the deposition of the light and flaky pieces of mica. The micas are a series of troughs 20 feet long, 2 feet wide, and 6 inches deep, placed side by side in a peculiar manner. The clayey water from the launders
first passes into two of these, then from these into three, then again into four, and, finally, into six of these mica troughs; thus as it passes to the exit end the flow of water is spread over a larger surface, and becomes more feeble, a condition which facilitates the deposition of the mica. The micas get filled in about eight hours, when they are flushed of their contents with water, which carries the deposited mica through suitable channels into the waste mica pits.
Next in order to the micas is a set of settling pits. These are usually three in number, sometimes more, according to the quantity of clay which is being worked. These pits may be of any shape, but, as a rule, they are made circular (or rather cylindrical) in form, 7 feet in diameter, and 40 feet deep. Into one of these pits the clayey water from the micas is run until it is full, when the current is changed and the water run into the second one until it is full. While the filling of the second pit is proceeding, the clay in the first one is settling, and, probably, by the time that the second is quite full, has completely settled. The current of clayey water from the micas is now diverted into the third pit, while the clay in the second one is settling. The water in the first pit is now run or pumped off, and is generally used over again for washing the clay from the stopes. From a pit full of clayey water there will usually be obtained a deposit of clay about 5 feet in depth, which still contains a large proportion of water; in such a thickness of clay, and 7 feet in diameter, there will be something like 285 tons of dry clay. The clay in the first pit is dug out and thrown into what are called clay tanks, where a further settling takes place; when all the clay has been dug out the pit is ready to be filled again with water from the nicas. This alternation of filling, settling, and emptying is carried out with the three pits in succession, so that it will be seen that for continuous working a series of not less than three pits is required; if more pits are used, then the time of settling can be lengthened, which would have the advantage of giving a drier clay and shortening the subsequent operations.
From the settling pits the still wet clay passes to the settling or clay tanks; these are, at least, three in number, corresponding with the three settling pits; in some works there are more; much, of course, depends upon the quantity of clay it is desired to turn out. These settling tanks are usually rectangular in shape, about 60 feet long by 7 feet wide and 6 feet deep, and they will hold about 1000 tons of clay; in these tanks settling occurs, and the clay begins to assume the consistency of lard; when this happens no more clay is sent into it from the pits, and the clay in the
tank is allowed to settle. The water is then run off, and the clay transferred to the drying place, where it is dried ready for sale.
The clay in the clay tanks contains about 50 per cent. of water, most of which must be driven off before the clay is marketable. This drying operation is done on a series of flues, technically known as the “dry;" a usual size is 60 feet long by 13} feet wide. In this kiln or dry there will be three fireplaces, two at one end, and one at the other, each fireplace having three flues about 9 inches wide ; the sides are formed of brickwork, but the bottom is usually made of sand, partly because sand is a bad conductor of heat, and partly because any water which may drain through from the top of the flue readily sinks into it and
The tops of the flues forming the bed of the dry is made of fireclay bricks about 18 inches wide. On these fireclay bricks the wet clay from the tanks is thrown, and it remains until it is dry. It takes about 1 ton of coal to dry 10 tons of clay.
After being dried on the dry the clay is thrown on the floor of the clay linhay, which is a storage place for the dry clay, from whence it is sent out as required.
The dry and the linhay are parts of one large room, being covered over with a roof, as is seen in the drawing (Fig. 12).
COMPOSITION AND PROPERTIES OF CHINA CLAY.–China clay is essentially a hydrated silicate of alumina, as has been already stated; but there are some minor differences in the composition of samples from various localities, as will be seen on examining the table given on p. 81; these are, of course, primarily due to differences in the composition of the granite from which the china clay has been formed, and, secondarily, to the degree with which the decomposition has proceeded.
China clay, or kaolin, is a fine, white, amorphous powder having slight adhesive properties and adhering to the fingers when moist.
It is light, its specific gravity being about 2.2; so that it is the lightest of all the white pigments. The best qualities bave a very soft unctuous feel; the common qualities are rather rougher, but none have the slightest trace of grittiness about them. The best qualities have a pure white tint, others a more or less yellowish tint, which the china-clay makers are accustomed to disguise by adding a small quantity of ultramarine.
It is quite insoluble in water, dilute acids, and alkalies. Boiling in strong sulphuric acid for some time decomposes it
with the formation of a gelatinous residue of silica and a solution of alumina sulphate. Hydrochloric acid has little action
As a pigment it is quite permanent, resisting perfectly exposure to the atmosphere and to light for any length of
As a pigment it is not, however, much used. In oil it loses its body and becomes more or less transparent. It can be used in water-colours and in distemper work with good results, and it is used in paper-making and paper-staining. It also finds a use in the preparation of the aniline lakes, especially when these are to be used in paper-staining.
Its principal uses are for making pottery, ultramarine, finishing cotton cloths, making paper, &c.
ASSAY AND ANALYSIS OF CHINA CLAY.-China clay can be assayed for colour or tint, covering power, &c., by the methods given below. An analysis is rarely wanted, since it is never adulterated, while for all pigment purposes absolute chemical purity is not required.
WILKINSON'S WHITE LEAD AND PATTINSON'S WHITE LEAD are oxychlorides of lead prepared in various ways; neither pigment is now used.
Wilkinson's white was patented in 1799, and is made by digesting litharge with a solution of salt until it acquires a pure white colour. Unfortunately, as it is the product of various operations, there is a lack of uniformity in its composition, which is much against its use as a commercial article. Another method of making it is to precipitate acetate of lead with hydrochloric acid, and to digest the precipitate of lead chloride obtained with basic lead acetate.
Pattinson's lead was made by treating chloride of lead with lime, when it forms the basic chloride, a white insoluble body having a fair body, but wanting in uniformity of composition.
In Chapter X., dealing with the assaying of pigments, will be found some figures relating to the comparative value of the most important white pigments.
This is a fairly numerous and important class of painters' colours. They are derived from both inorganic and organic sources, and include some of the most highly valued and most used of the pigments at the disposal of the painter and artist.
Vermilion has been used for a long time as a pigment. It is a compound of mercury and sulphur in the proportion of 200 parts of the former to 32 of the latter; its chemical name is mercuric sulphide, and it has the formula, Hg S. It is found naturally in large quantities as the mineral cinnabar, especially in Spain ; but it rarely occurs naturally of sufficient brightness to be used as a pigment, and is, therefore, mostly made artificially. When a current of sulphuretted hydrogen is passed through a solution of a mercuric salt a black precipitate of the mercuric sulphide, identical in composition with vermilion, is obtained; this precipitate is characterised by being insoluble in most single acids, but soluble in a mixture of hydrochloric and nitric acids. By heat it is volatilised, and the sulphide sublimes in the form of a red powder; this transformation from black to red can also be brought about by boiling it for some time with aqueous solutions of the caustic alkalies or of alkaline sulphides. What the cause of the change may be is rather uncertain; probably there has been a re-arrangement of the atoms in the molecule of mercuric sulphide; there are many cases known of similar differences in the colour of inorganic compounds, as, for example, cadmium sulphide and basic chromate of lead. Although it is generally considered that the molecule of each of these poly-coloured bodies is always made up of the same number of atoms, yet there is no direct evidence on that point; and it is quite possible that in the different modifications of these bodies the number of atoms may vary and, therefore, be arranged dif