Every material has color. One might say that the whole world consists of coloring materials. The problem is to what extent they can be used for coloring or painting. First we must draw the distinction between coloring and painting, each a different activity using different coloring materials.
Whether you are going to paint a garden fence or a picture, you use color powders, which feel like flour to the touch. These are mixed with suitable binding material to make paints. Alternatively you buy the coloring material ready prepared in tins, boxes, or tubes, or in blocks which will dissolve in water, and which contain water-soluble glue. In every case the color can be seen as it will look after it has been applied.
If you intend to dye cloth or stain wood you are again given a powder or pieces of crystal, but it generally looks almost black. It dissolves completely in water, possibly with the addition of other chemicals, but even in solution it does not show its ultimate color. This it will not do until it has combined chemically, and without needing a binder, with the cloth or wood fibers.
Unlike paints, these dyes remain in true solution; they cannot be removed. Filtered infusions of tea or coffee are "inks" of this kind. But if a can of oil paint is left standing for some time the
color powder sinks to the bottom in a thick ooze which has to be stirred vigorously to make it combine again with the oil above it into a homogeneous paint.
The same is true of a glue solution, and it is even more apparent in powder which is mixed only with water for painting purposes. These are all pigment or solid colors, which form a more or less opaque layer or skin over the object treated. Pigment colors differ from inks, dyes, or stains both in being indissoluble solids and in their chemical composition: inks, .dyes, and stains are organic chemical substances, while pigment colors on the whole are inorganic in nature. There is one exception among the materials used in worthwhile painting: alizarin red has an organic basis.
substance gives this color; the paint is prepared by giving an organic dye to white day.
The inorganic pigment colors are for the most part metal oxides and hydroxides, either by themselves or combined with clay. They exist in nature as colored earths and are thus usually rather dull, "impure" colors. Artificially produced pigments are on the whole purer; some approximate to the absolute purity of spectrum colors, but never attain it completely. The effect of complementary colors can make them seem so in a picture.
The nature of pigment can be understood from the following example: bricks are made of loam, a very yellow clay, and baked; yet when they come out of the oven they are red. While the heat of the oven fuses the clay into one piece, it causes the yellow in the clay, which is iron hydroxide (rust), to give off its water content, thereby becoming a red iron oxide.
If the unbaked loam is dried and crumbled fine, or if baked bricks are ground to a fine dust, they can be used in that condition for painting. Bocklin did this, though it is much easier to purchase yellow and red Clay, technically pure and uniformly ground, as ochre. If yellow ochre is scattered on a hot plate it will soon become red ochre. The extraction of the water content makes the powder specifically heavier and denser; it loses some transparency, the characteristic of the so-called glaze colors, which allows the ground to shine through. Opaque colors, which are often burnt pigments, cover the surface completely with a relatively thin application, so that the color effect is constant whether the ground is light or dark.
Most metal hydroxides can be turned into darker and more opaque colors. Of synthetic paints the most striking example is the hydrous oxide of chromium, hydrous green chromic oxide, or viridian, as it is usually called, an extremely pure, transparent color. By burning or strong heating it becomes anhydrous green chromic oxide, chrome green, a dull color and very opaque. The specific weight alters from 2.74 to 5.21, and the granulation is finer. Not all pigments can be altered by burning, and some are destroyed by strong heat.
There are also pigments which make a chemical reaction when mixed together or with a calcareous binder. All pigments affect the drying time of oil binders. Oil with white lead dries to the touch in about 30 hours, whereas with umber it is still sticky after 100. To avoid these difficulties attempts have been made to use absolutely neutral pigments like ground glass pastes, but these are not satisfactory because they break up the light in undesirable ways. Modern painting technique has therefore to resign itself to using pigments which, like natural ochre and carbon, were used 30,000 years ago for the cave paintings, and which have thus proved their durability.
Both old and new pigments are chosen primarily for their tolerance to light. They are classified according to their ability to combine with binders, principally with lime.
A number of pigments are similar in color but have quite different properties and prices; for example, the brightest and most light-resistant yellow is cadmium, but it is destroyed by lime. When painting on lime it can be replaced by uranium yellow, uranium oxide, which gives almost the same color; if it need not be quite so pure ard bright, Mars yellow, yellow iron oxide, is usually preferred by painters, since uranium yellow is the most expensive of all colors.