294 CHAPTER XI. COLOUR AND PAINT MACHINERY. In making colours or pigments, and in preparing them for use in painting, and in making paint, there are a good many mechanical operations which are common to all pigments and all paints. Upon the care with which the various mechanical operations, such as precipitating, drying, grinding, &c., are carried out, depends much of the quality of the pigment or paint, especially as regards its brilliance and covering power. The machinery for carrying out these various mechanical operations forms an important part of the outfit of a colour- or paint-shop. In the present chapter it is proposed to discuss the various machines which are required to carry out the various processes involved in making a pigment and its conversion into a paint. LEVIGATION. In the preparation of such natural pigments as the ochres, siennas, umbers, china clay, and barytes for use in paint-making, levigation plays an important part. These bodies as they are found in nature contain a good deal of gritty matter and other impurities, from which they must be freed before they are of use in paint-making; there is no better process for this purpose than levigation. The principle of the process of levigation depends upon the fact that when fine particles of a comparatively light material mixed with coarser particles of the same material or with particles of a heavier material are agitated with water and then allowed to stand, the coarser and heavier particles will fall first, while the lighter particles will form a layer on the top of the coarse particles, which can thus be separated from the fine particles. A modification involving the same principle is where the mass of material is subjected to the sifting action of a current of water, the strength of which suffices to carry the fine particles only into a tank, where they are allowed to subside. China clay is an example of the preparation of a pigment in this way (see p. 84). Should the raw material be made up of several distinct kinds of particles-very fine, fine, medium, and coarse, as in the case of some ochres-it is possible, by means of levigation, to separate them into their various constituents. By so arranging the current of water that it runs through a number of tanks with varying rates of speed, the coarse particles will be left in the first tank, the medium particles in the second tank, the fine particles in the third tank, and the very fine in the fourth or last tank. It will be seen that levigation, while effective, is a very cheap process; for it only requires a cheap material, water, and the cheapest kind of colour plant, tanks, for carrying it out. The details of the plant required or used in levigating at any particular works depends upon many factors, such as the position of the works, whether situated in the centre of a town, in a wide valley, or on a hill side. The facilities for obtaining the requisite supply of water is also a factor in determining the arrangement of the plant. In Fig. 28 is shown in plan and elevation a plant suitable for A 2 3 4 Fig. 28.-Levigating plant. levigating ochres, umbers, &c. It consists of 9 tanks, 8 of which are arranged in 2 series of 4, while the ninth is an odd tank. Another good arrangement would be one of 10 tanks, 9 arranged in 3 sets of 3, the tenth being an odd one. In the odd tank, A, the crude material is thoroughly mixed with water; in Cornwall, Derbyshire, and a few other mining districts, this tank is known as the "buddle;" in this the very heavy stuff remains while the current of water, which is continually passing through, washes away the finer particles. From the buddle the water flows into the first settling tank 1; this being large, the current becomes retarded, and some of the material it contains settles out; from tank No. 1 the water flows into tank No. 2; this is made, or should be made, rather larger than tank No. 1, so that the current being spread over a larger surface becomes slower, and, therefore, has less force, thus allowing the finer particles to settle out. From No. 2 the water flows to No. 3 tank, which is larger still; and, finally, to No. 4 tank, which is yet larger, so that very fine particles of pigment can settle out; when tank No. 1 is full the current from the buddle is diverted into the second series of tanks, while the colour or pigment in the tanks of the first series is settling out; when this is completed the water in these tanks is run off, and the pigment dug out, when the tanks are ready to be filled again. By having a set of four settling tanks, four qualities of ochres, or siennas, or umbers, may be obtained. When the second series of tanks are full, the current is again sent through the first series. By having three series a more perfect system can be adopted; the current of water is sent through the first series until these are full, then through the second series while the material in the first is settling out; when the second is full, the current is diverted to the third series; by this time the colour in the first will have settled out, and can, as explained above, be collected; when the first lot of tanks are emptied of their contents they are ready to be refilled by diverting the current from the third set of tanks. Thus the three operations of filling, settling, and emptying can go on concurrently in a complete manner. The tanks should be arranged, as shown in the drawing, one above the other, so that the water can run from one to the other; and the last tank of the series should be of such a size that it will take a day to fill it. In ultramarine-making, where levigation forms an important part of the finishing process, the last tank is either made very large or a large number of small ones are provided, as the fine ultramarine takes a week to settle. When space is available it is a good plan to have a set of large storage tanks; into these is thrown the wet pigment taken out of the settling tanks, and here it remains for some time; a further settling takes place, and the pigment becomes drier; this effects an economy of both time and fuel in the complete drying during the final stage. This saving of fuel is a matter of some import 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, s, 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 |