Pagina-afbeeldingen
PDF
ePub

whiting, its specific gravity being about 3:05. Its colour is good, and it forms a dense white powder; in body and covering power it is quite equal to barytes. It works well in both oil and water, and mixes fairly well with other pigments, although it has a slight tendency to possess alkaline properties. It is quite permanent, and resists exposure to light and air.

Magnesite may be distinguished by the following tests : It is soluble in dilute hydrochloric acid, or in sulphuric acid with effervescence. This solution gives no precipitate with ammonium chloride and ammonia, ammonium sulphide or ammonium oxalate, but gives a copious white, somewhat crystalline, precipitate of ammonium magnesium phosphate on addition of sodium phosphate to an ammoniacal solution; this precipitate, on ignition, is converted into magnesium pyrophosphate.

CHINA CLAY. China clay or, as it is sometimes called, kaolin, which is its Chinese name, is a natural product, and only requires levigating and drying to prepare it for use as a pigment. It owes its name to its use in the manufacture of the white and finer varieties of pottery. Prior to its discovery in this country the pottery in common use had a brown colour, due to the use of clays highly contaminated with iron, and much of the best kinds of wbite pottery were imported from China; hence the use of the term,

china," for white pottery. When the existence of the deposits of china clay in Cornwall was found out, and its use in making pottery became general, the goods so made were denominated “china,” and the material “china clay.” As a pigment it is not used to the extent its many good qualities entitle it to.

ORIGIN AND COMPOSITION OF CHINA CLAY.In England and other countries where china clay is found it is invariably associated with granite rocks, and is evidently a decomposition product from them. The exact cause which has led to the decomposition of the granite rocks in those places where china clay is found, and to its not undergoing this decomposition in others, where it is equally well developed, is a matter which at the present moment is one of uncertainty.

Granitic rocks are formed by the aggregation of three minerals in various proportions; these three minerals are quartz, felspar, and mica. , Quartz is the special form of silica (oxide of silicon, Si 0,) which occurs in these granitic rocks; it is absolutely unchangeable by any amount of exposure to the atmosphere.

In granites it forms the base mass throughout which the other minerals are distributed.

Mica is a double silicate of potassium and alumina, having usually the composition shown in the following analysis Silica, Si 02, .

46:3 per cent. Alumina, Al, 03,

36.8 Potash, K,O,

13:7 but the proportions vary in micas from different districts; in some varieties of mica the potash is replaced by magnesium, in others by sodium. Mica is characterised by crystallising in thin transparent flakes or plates, which are sometimes found of large size; these flakes have the property of cleavage very highly developed, so that mica can be easily split up into very thin leaves. It is a very refractory substance, and will resist a considerable amount of exposure to atmospheric influences without being decomposed; it will resist heat to a great extent. During the natural decomposition of the granitic rocks, which gives rise to the formation of china clay, the mica and quartz undergo no change whatever.

The third mineral which is found in granite is felspar; this occurs in the rock in the form of white or slightly reddish, opaque rhombic crystals. In some granites the crystalline form of the felspar is well developed, while in others it assumes an amorphous appearance. There are many varieties of felspar, but they are all of them double silicates of alumina and of the alkali-metals, while in some calcium replaces to a greater or less extent the alkali-metal. The most important and most common felspar is the potash variety, known as orthoclase; this has the composition shown in the following analysis :

Silica, Si 02,

64.20 per cent. Alumina, Al, 03,

18:45 Potash, K20,

17:35 On comparing this analysis with that of the mica given above it will be seen that the felspars contain a larger proportion of silica and of the alkaline constituent, while the proportion of alumina is only about one-half that found in the micas.

The formula of felspar is 6 Si 0,, Al, 03, K, O.

The soda-felspar has a similar composition ; in some felspars both alkali-metals are present.

Felspar is capable of undergoing decomposition when exposed to the destructive action of the atmosphere, which decomposition is probably chiefly brought about by the action of the carbonic

acid and water which are always present in the atmosphere;
probably the alkali is eliminated in the form of carbonate, which,
being soluble in water, passes into solution in the springs which
rise in the granite, while the silica and alumina are left behind
in an insoluble form, the kaolin or china clay of the potter. The
average composition of china clay is :-
Silica, Si 02,

47 per cent.
Alumina, Al, 03,
Water, H, 0,

13 which corresponds to the formula, 2 Si 0,, A1, 02, 2 H, O. But, as a rule, the composition of the china clay found in various localities varies somewhat from this, as will be seen in the following analyses of some china clays from several widelyseparated localities, which are quoted from several sources.

40

[ocr errors]
[blocks in formation]

China clay occurs in large deposits along with the other constituents of undecomposed granite, the china clay usually forming from 15 to 20 per cent. of the whole deposit. The deposit does not usually occur in layers, but in basins in the surrounding granite ; over the deposit there is usually a layer of soil known locally as the “overburden,” which varies in thickness from one to several feet. Fig. 11 is a drawing of a china-clay deposit taken from Mr David Cook's Treatise on China Clay. The deposits of china clay are often of great depth.

MANUFACTURE OF CHINA CLAY,—The process of extracting the china clay from the undecomposed quartz and mica is essentially one of levigation and is comparatively simple.

When a deposit of china clay has been found, the overburden is removed and usually two shafts are sunk; one of these is as close as possible to the edge of the deposit and is known as the permanent shaft, the other is sunk in the centre of the deposit and is known as the “rise,” partly because it is often not sunk from the top; but the permanent shaft is first sunk and then a

[ocr errors][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small]

drift is dug to the centre of the deposit and the centre shaft dug out from the bottom ; in any case, whether the rise is sunk from above or dug out from below, a drift or horizontal shaft connects it with the permanent shaft. The size of these shafts varies in different places, but it is not necessary that they should be of large dimensions; the rise in particular need not be any larger than can comfortably be excavated.

The permanent shaft is fitted with pumps, the use of which will be seen presently.

The rise is fitted with a square wooden pipe of 9-inch side and long enough to reach from the top of the rise to the horizontal drift at the bottom, In one side of this tube is a number of holes about 4 inches in diameter and at a distance of 1 foot from centre to centre; before placing in position these holes are stopped up by plugs known as " buttons," and the tube itself is known as the "button-hole launder"_“launder” being a local term to signify any tube or trough through which liquors or materials can flow. After the button-hole launder is placed

in position in the rise all the space between it and the sides of the rise is filled up with clay pressed down rather tightly.

One essential feature of china-clay works must be a plentiful supply of water; without this it cannot be profitably carried on, and the source of supply should be, if possible, above the works, so that it may be supplied to any part by the aid of gravity alone; if it be on a level or below the works, then pumping becomes a necessity, and this should be avoided whenever possible, as it is a source of expense which reduces the profit of working the deposit.

A china-clay works is shown in Fig. 11, in which the relative position of the clay deposit with the surrounding rocks is shown, with the overburden, the permanent shaft, the button-hole launder, and the pumping house.

The deposit of china clay is worked in the following manner:The overburden is removed and then a current of water is directed against the sides, known at the works as the “stopes” of the deposit; naturally these currents of water will wash the china clay, &c., to the lowest part, which is the centre of the deposit. At the bottom of the stopes is arranged a hollow or sand pit; in this the sand or quartz collects, and is removed from time to time by means of a sand waggon which is hauled up the sides of the clay pit on an inclined tramway by means of a windlass driven by the engine. The clay along with the mica flows through the topmost hole in the button-hole launder into the horizontal drift at the bottom, from thence it passes into the permanent shaft, from which it is again pumped to the surface to be treated in the manner presently to be described. As the level of the china-clay deposit descends, the buttons in the launder are removed until, finally, when all the clay is worked out, the bottom of the deposit is reached, and with it the last hole in the launder. The length of time required to work out a deposit depends entirely on its extent, which may be small or large; the deposits, as a rule, are large and take some years to work out.

The crude clay as it comes from the pits or stopes is still impure; it contains much fine sand or quartz, most of the mica (which does not separate so readily out of the clay as the sand does), together with undecomposed particles of felspar; from these it requires to be separated; this is done by a process of levigation by means of water. In early times when the clay was first worked this was done in a crude form, usually in a series of tubs, but now a better system is adopted, which is shown in Fig. 12.

From the button-hole launder the water containing the clay,

« VorigeDoorgaan »