(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
In oil paint each pigment, by its own chemical nature, will tend to be either transparent, semitransparent, or opaque. Most blacks, blues, violets, and purples are transparent, as are colors where the pigment is a like a dye-pigment, such as alizarin crimson and madder. Earth colors range from semitransparent to opaque, depending on their source. All cadmium and chrome colors, all whites, or colors that are made with white (such as Naples yellow and flesh tint), and genuine vermilion are opaque.
Of all the whites, zinc white is most transparent, followed by lead white and then titanium white, the most opaque. You can make a transparent color semitransparent or opaque by the addition of white. Many colors will take up to 10 percent white without any appreciable change in the surface appearance. Opaque colors can often be made semitransparent by thinning them with a painting medium and applying in thin glazes.
Virtually all manufacturers’ color charts are encoded with information regarding the transparency and opacity of the colors in their unmixed state. Some companies, such as Schmincke, even label their paint tubes. It is important to note that in lesser grades of paint, it is more common to have expensive colors replaced with blends of other less costly colors, which in turn can affect the transparency and opacity. A mixture of white and phthalocyanine blue, for example, is sometimes sold as cobalt blue, or cobalt blue hue. This blend is opaque and does not produce the same kinds of tints when mixed with other colors. Genuine cobalt blue is prized for its transparency as well the delicate tints it can produce. Another example is cadmium red, the better grades of which are made from c.p cadmium sulfo-selenide, and which is prized for its opacity and brilliance. It is sometimes replaced with lesser grades, which are often mixed with barium sulfate, and are therefore less opaque and not as brilliant.
It used to be common practice simply to paint over undesirable areas with opaque colors until it was discovered, after many years of aging, that areas which were hidden began to show through the increasingly transparent surface. This phenomenon is called pentimento. Therefore, if an area needs to be repainted, the area should first be prepared with a new ground of white paint.
To an artist who paints very large, these differences may seem insignificant for, if one has to stand twenty to thirty feet back from the surface of the painting to see it, it is unlikely that small details will be noticed. However, if you are a master at the craft of painting, or paint in a style where details must and will be seen, these differences are very important.