MIXABILITY.—This is a term which the author has introduced in connection with pigments to express the power of pigments to mix more or less readily with oil or other vehicles or other pigments. It is a most important property, and much of the value of a body when used as a pigment depends on it; some pigments do not readily mix with oil, while some of the modern pigments made from coal-tar colours seem to have the property of retarding the drying of the oil, therefore they cannot be said to mix well with it. Then, again, in mixing pigments together to produce compound tints or shades, there are some pigments which can be mixed with all others without any ill effect being observed; on the other hand, some pigments can be mixed with a few others without any change occurring, but when mixed with others some action of a deteriorative character will take place.

To test pigments for the property of mixability, the best method is to provide a sheet of cardboard of not too porous a character, and to rule this into squares of about 2 inches each way. A little of a pigment is rubbed with a small quantity of linseed oil in a white basin, during which operation its behaviour with the vehicle will be noticed; it should be observed whether it mixes freely with oil or shows a tendency to separate out; this latter effect may be due in most cases to the pigment not being thoroughly dry. A little of the mixed pigment is now rubbed in one of the squares of the card. Then prepare mixtures of the pigment with other pigments-white lead, whiting, Prussian blue, emerald green, yellow chrome, vermilion, lamp-black-rubbing the pigments together with a little gum-water; these mixtures are rubbed on the card. When all the mixtures are ready the card is exposed to the air, and to diffused daylight only, for some time, say two or three weeks. After two or three weeks' exposure, the card may be examined and any effect of change of colour noted; during the interval observations should be made as to the drying of the oil in the first square to see whether the pigment has any influence of any kind, either in retarding or in facilitating the drying; the former case will show that the pigment is not suitable to be used as an oil-colour, although it may be perfectly suitable as a water-colour. The other squares will show the action of one pigment on another; those which exhibit no alteration in shade or tint beyond, perhaps, a little fading will show the pigments which may be mixed together without any effect upon one another; while those which have altered will show the observer what mixtures to avoid.

FINENESS. The quality of a pigment is a feature which is more or less dependent upon the size of its particles, the smaller

these are (or, in other words, the finer the pigment has been ground or produced in the process of manufacture) the better will it be as a pigment; its body or covering power will be increased, its colouring power will also be improved and its tone brightened very considerably; therefore, it may be laid down that the finer a pigment is the better it will be for use in painting.

It is by no means easy to make a practical test for the fineness of a sample of pigment. By rubbing between the fingers it is possible to make a rough comparative examination, but no accurate results can be arrived at by this means; when the quality of two samples is very similar this rough test cannot be relied on. By spreading a little on a plain microscope slide and examining it through a powerful microscope, using, say, a 1-inch objective, some idea of the relative fineness of two samples may be obtained. Another method of testing which will give more reliable information and better comparative figures than the tests just noted is the following:--Weigh out about 5 grammes into a mortar, and grind, without much rubbing action, into a smooth paste with water; then transfer this to a tall cylindrical graduated measuring glass, and rinse out the mortar with water, so as to get the whole of the material into the glass; fill this up with water to the top mark, and, putting in the stopper, shake well for a few minutes; then place on one side; the particles will gradually settle, and the time it takes for the water to become clear up to, say, the half-mark should be noted. If this be done with a number of samples a series of figures will be obtained which may be taken as showing the comparative fineness of the various samples, for the rate at which the material settles depends. upon the fineness of its particles; the finer these are the slower is the action, while the best samples are characterised by subsiding most slowly. Then this test will also show whether the sample is of uniform quality or whether it contains both coarse and fine particles; in the former case the rate of deposition will be uniform, while in the latter case the larger particles will settle out very rapidly, leaving the finer particles to subside more slowly. For example, the author tested three samples of china clay by this method. Sample A took 60 minutes to settle out; sample B 44 minutes; while the coarse part, forming the great bulk of sample C, settled out in 30 minutes, and the finer remainder in 90 minutes. A and B were uniform in quality, but sample A was superior to B, being composed of the finer particles. Sample C was of mixed quality, containing both coarse and fine particles; the former settling out rapidly, the latter more slowly. would not be so good as B for many purposes.

This test cannot be used for examining the fineness of samples of different pigments: thus, for example, the comparative fineness of a sample of white lead and of barytes cannot be ascertained by its means; only different samples of the same pigment can be compared together. This arises from the fact that the specific gravity of a body has a great influence on the speed with which its particles will fall when placed in water, and as there is a great difference in this respect between different pigments, it follows that a sample of white lead will settle out much quicker than a sample of china clay, although both samples may be equal in fineness of powder.

The method of testing the fineness of the particles of a pigment here given is possibly not a perfectly accurate one; but still some very useful information as to the quality of a pigment may be obtained by its means, and it does not need any elaborate apparatus to carry it out.

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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. 91).

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

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