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.