Painting with water colors is very similar to printing with colored inks as described in the previous topic. However, painting with oils is different and much more complicated. The reason for this complexity is that most oil paints reflect a significant fraction of the light that is incident on them, and the reflected light often has a different spectrum from the light that is reflected from the underlying substrate after having passed through the paint twice. These two beams combine again, so that what we see is a combination of the spectrum of the light reflected from the surface and the spectrum of the light reflected from the substrate. To make matters more complicated, the amplitude of the reflected light from a layer of paint depends on the difference between the index of refraction of the layer and the index of refraction of whatever is below it. Thus a layer of some paint may look different if is applied over another layer of paint instead of being applied over the bare underlying substrate (paper, canvas, …). Since all paints reflect light to some extent, it is hard to produce a true black using the subtractive primaries, and a separate black paint is usually used – just as for printing.
To make matters worse, pigments in oil paints are usually made from readily available, cheap materials, so that they often do not coincide with the ideal subtractive primaries we have been discussing. For example, we recognize that the artist’s simple rule, that
Yellow + Blue = Green
is not consistent with our descriptions. Yellow and Blue are complementary colors by our definition, so that the additive sum would be white and the subtractive sum would be black. Using our subtractive rules, we would have said that
Yellow + Cyan= Green
While Cyan is certainly a shade of blue, it is not the monochromatic primary blue that we talked about above. The difference is that Cyan is a difficult color to make in oil paints, so that the artist’s rule is based on what is available in oil pigments, so that the artist’s Green will not be quite the same as the green we have talked about. The same thing is true of magenta – it is hard to come by and expensive, so that it is not used as a subtractive primary in oil colors. Since red is easier to come by, both artists and printers often use “red” in place of what we would call “magenta.” In fact, artist’s red usually does have some blue as well, and it is not the same as the monochromatic red that we think of as a primary color.
The pigment – the material that gives oil paint its color, is usually a fine powder that has been suspended in an oil base. The base is often linseed oil, which is chosen because it is a polyunsaturated oil. Polyunsaturated oils are used as carriers of the pigment because they are liquids at room temperature but harden slowly when they are exposed to the oxygen of the air. The hardening produces a matrix of oil molecules that traps the powdered pigment. However, the reflectivity of the matrix changes as the oil cures so that the perceived color also changes as the paint dries. Many oil based paints take a very long time to dry completely at room temperature because the surface of the paint cures first and prevents the lower layers from curing. In some cases, the painted surface must be heated to completely cure the paint.
House paints and paints used for other non-artistic purposes are often very different from the oil-based paints used by artists. They often use latex as a base rather than an unsaturated oil, and the curing process is quite different.