ance in dealing with such cheap natural pigments as china clay, umber, &c.

The strength of the current of water is a matter that requires attention; if too strong, it will carry over some of the coarse material from the buddle to the settling tanks, and will prevent the fine material from settling in the end tanks; on the other hand, too gentle a current will not extract the whole of the valuable material from the crude stuff in the buddle; this is a detail which the operator can easily arrange.

If only small tanks are required they may be made of wood; large tanks may be built of stone flags, or of brick, if flagstones of sufficient size are not available. If bricks are used the inside of the tank should have a smooth surface, so as to facilitate the ready removal of the colour which has settled out. In any case arrangements should be provided for running off the clear top liquor from the settled pigment; this may be done by providing in each tank a set of holes kept stopped by plugs, which are removed when it is desired to run the water away. Or the water may be syphoned off by means of syphons provided for that purpose.

The amount of water required to levigate a pigment is a variable amount, depending on the nature of the sample of colour under treatment and on the plant used, so that no definite rules can be laid down.

The size of the tanks can be varied to suit the required output of colour, and is a point which every colour-maker must settle for himself, remembering, first, the deposited colour will contain about half its weight of water, and will therefore be heavier than the dry material; second, that the total volume of the tanks must be much larger than that of the material which settles out from them. Another point is to make the tanks sufficiently strong to bear the pressure of the water, &c., they contain, which is great; thus a tank, 20 feet long x 5 feet broad and 4 feet deep will hold 20 × 5 × 4 = 400 cubic feet of water, or 400 × 62.35 lbs. = 11.13 tons, which is the pressure exerted by the water on the bottom of the tank.

In some cases, before levigating, the material is ground, and in such cases the grinding is usually done under water; for this purpose special mills are made, descriptions of which are given further on.

DRYING OF PIGMENTS.

After a colour has been prepared for use as a pigment by the process of levigation, as just described, or by that of precipitation,

described below, and also by other processes, it is in a wet condition, probably containing from 25 to 50 per cent. of water, according to its nature. If required in what is known as the pulp state, in which condition it is used by paper-makers and stainers, no further treatment is necessary; but, if required to be used in the preparation of paint, it is absolutely necessary that it be dried, otherwise it will not mix with the oil used in the manufacture of the paint.

The drying of pigments is carried on in what are called "drying-stoves;" these are usually nothing more than brick chambers with solid walls on three sides, and a door on the other, covered with a roof; round the bottom of three sides runs a horizontal flue belonging to a furnace which can be fed from the outside. The wet colours are usually placed in shallow, flat, earthenware pans, which are placed in piles one above another, and then left in until they are dry. This is by no means a satisfactory method, the piling of the pans, one above another, and the absence of any system of ventilation beyond accidental cracks in the door and walls, tend to keep the atmosphere of the stove saturated with steam, and to check the drying operation.

A better plan is shown in Fig. 29; it consists of a brick chamber built of any convenient size; as before, the flue, F, of a furnace runs round the bottom; the sides of the flue are built of brick, the top of flagstone, and the fireplace, E, is placed outside the chamber. Instead of such a flue, steam pipes may be used for heating it. Above the flue or steam pipes, is a staging, 8, forming a false floor, on which is erected a framework, C, C, C, C, of iron or wood forming skeleton shelves on which the pans of wet colour are placed. These shelves support the pans a small distance apart from one another, and so allow free egress for the water-vapour which comes from the colour. A constant current of warm air, generated by a fan or air propeller, is continually flowing over the pans of colour and out through the ventilator, V, in the roof of the stove, thereby carrying off the water-vapour as fast as it is given off from the wet colour. It should be borne in mind that the colour, just as it comes from the filters or presses, may contain from 25 to 50 per cent. of water; if, by any means, this water is prevented from escaping from the colour, then the drying is retarded; or if it is prevented from readily escaping from the stove, it is liable to condense on the inside of the roof, and to fall down in drops on to the colour below. In some cases, e.g., chrome-yellows, these drops are apt to produce spots on, and discolouration of, the pigment which is being dried. The more freely the water-vapour can escape into the atmosphere

the less chance there is of such mishaps occurring. D is a door for filling the stove, and G, G skylights.

In dealing with barytes and china clay, special forms of drying stoves have been described.

Some pigments, like the two just mentioned, the oxide reds, burnt umbers, burnt siennas, ultramarine, Guignet's green, are capable of standing a high temperature without being altered in shade; these may be dried in a stove heated to a high temperature, in which case the drying is done quickly. On the other hand, certain colours, such as the chromes, Prussian blue, emerald green, &c., must be dried slowly; for such colours the stove shown in Fig. 30 would be very useful. The two sides not represented in the drawing are of brick, and support the roof.

Fig. 30.-Drying stove for pigments.

Stretching from side to side are a number of iron shelves just far enough apart to take an earthenware pan and leave a little space between it and the shelf above. These shelves do not stretch completely from back to front, but, as shown in the drawing, they are arranged to come alternately flush with the front and back, the side of the shelf nearest the front and back of each shelf being turned up to form a flange. The front and back of the stove are made of a number of iron plates, which form a series of doors to the shelves, the top of the plates being bent over to catch on the flange of the shelf above, as shown in the

drawing; it is not necessary that the doors should fit air-tight. A fan at the top of the stove creates a current of air through it, a chamber at the bottom is kept hot by steam pipes, or flue from a furnace; through this chamber passes all the air that is allowed to go into the stove; this hot air passing over and under the colours dries them, and, being hot, absorbs and carries away the water vapour liberated from the wet colours. This stove is effective and economical, and is so constructed that the pans of colour can be readily removed and the shelves quickly refilled.

A drying stove has been constructed in the following manner. A cylindrical vessel was constructed of iron plates of any convenient size. This was divided into three chambers by two perforated iron plates; in the central chamber, which is the largest, is placed the material to be dried; the bottom chamber is kept hot by means of steam pipes, and is provided with an opening to admit air. The upper chamber is fitted with an exhaust fan, so arranged as to draw the air out of the central chamber; the perforations in the plate dividing the central from the top chamber are larger than those in the plate dividing the bottom from the central chamber, the consequence being that the air is drawn away from the central chamber faster than it enters from the bottom hot air chamber, so that a partial vacuum is created in the central chamber which is beneficial to effective drying.

In any stove the colours are best placed in earthenware pans of about 12 to 16 inches in diameter, and 3 to 6 inches in depth; smaller pans may be used, but it is not advisable to exceed the sizes just given. Pans made of galvanised iron have been used, but these are liable to rust and so lead to discolouration of the pigments dried in them; enamelled iron pans, which can now be bought at a reasonable figure, are well worth a trial as being lighter and less liable to break than earthenware pans.

PREPARING PIGMENTS OR COLOURS BY
PRECIPITATION.

Many colours-the chrome-yellows, Prussian blues, Brunswick greens, lakes, &c.—are prepared by a process of precipitation, the principle of which is that when two or more substances in the state of solution are mixed together a reaction sets in-what the chemist calls double decomposition occurs-and new products are formed; one of these being insoluble in the liquid used is thrown down or precipitated out of the solution, usually in the form of