PIGMENTS are often classed according to their color, hue, brightness and purity. Their transparency and opacity must also be considered. The more important classification, however, is their permanency in oil or water paints.

In a general way they may be divided into mineral and organic, depending upon their origin. The mineral pigments, as a rule, are much more permanent than the organic, (vegetable or animal).

Pigments may also be soluble or insoluble, crystalline or amorphous, chemically active or inert.

The stability of pigments may be tested in various ways, either from the known physical and chemical make up of the various substances, by a study of the paints in which they have been used, and from various experimental tests for permanency. The following table indicates the various degrees of permanency as nearly as can be arrived at as applying to paints mixed with oil as a medium:

Class I

Zinc White Flake White White Lead Yellow Ochre Raw Sienna Mars Yellow Cadmium Yellow Venetian Red The Red Oxides Indian Red Burnt Sienna.

Oxide of Chromium Cobalt Green Green Ultramarine Terra Verte CobaltBlue Ultramarine Blue Raw Umber Burnt Umber Ivory Black Lamp Black Graphite.

To which list might be added the silicious earths, barytes and paris white.

Class II

Chrome Green Emerald Green.

Vandyke Brown Prussian Blue.

The various madder lakes used by artists also have fair permanence.

Class III

Chrome Yellows Asphaltum Dutch Pink Vermillion.

Carmine Crimson Lakes Artificial Vermillions Maroon Lakes.

When water is used in place of oil as a medium, white lead, flake white, cadmium yellow, artificial vermillion and chrome yellow are very treacherous pigments to use. On walls or plaster, where the lime is fresh and still caustic or strongly alkiline, the colors inadmissable in water should not be used. In this class is prussian blue also.

In the selection of pigments to be used in paint, due care should be exercised that they have no injurious influence on the linseed oil, or other media with which they are mixed; that they absorb a large amount of oil which insures greater durability; that they are not fugitive or fading and that they are finely ground.

Inert pigments are those which have no chemical action on the vehicles with which they are mixed or on other pigments with which they are combined. Such pigments are barytes, silica, gypsum and the various oxides with some few exceptions.

Chemical pigments, on the other hand, the carbonates, chromates, those containing sulphur and those of organic origin, in many cases have a chemical action on linseed oil and act or are acted upon injuriously by other pigments with which they are combined. White lead, chrome yellow, prussian blue, vermillion and the lakes are good examples. The binding quality of the linseed oil may be injured or destroyed, or darkening, fading, or deleterious changes in the colors may occur.

The most satisfactory pigments for general use, from the foregoing, would be the ochres, raw and burnt sienna, raw and burnt umber, the oxide reds, copperas reds, ultramarine blue, cobalt blue, lamp black and drop black.

Pigments, such as white lead, zinc, the oxides of iron, the blacks and chemical colors are, as a rule, purchased by the painter in paste form, ground in linseed oil, when for use as bases, or as tinting mediums, in oil paint. In paste form they are more convenient for immediate use. The process of grinding with oil has made them finer in texture and in better condition to mix readily with oil and other pigments.

The white pigments are usually ground in refined or bleached linseed oil to obtain the maximum whiteness. In grinding most other pigments, ordinary raw oil is used, with the exception of some blacks, and other colors which dry poorly, in which case boiled oil, containing dryer, is substituted.

Very cheap oil colors are sometimes ground in adulterated oils. In the purchase of such materials, one must rely largely on the reputation of the maker producing them.

Coach Colors are carefully selected pigments, ground in turpentine and japan dryer to produce quick drying flat colors. Much care is used in their preparation, and frequently the color is re-ground several times to produce the requisite degree of fineness. During the process of grinding, the mills are cooled with water so that the heat generated in the milling process will not affect the brilliancy of the color used.

Colors Ground in Water are of interest to the fresco painter, and consist of various pigments ground in that medium into which, in some cases, a little carbonate of soda is added. A little glue size or glycerine may also be combined. The size to hold up the pigment from settling and the glycerine to prevent hardening and undue evaporation of the water.