Possibly the most important aspect of a painting whether in oils, acrylic, watercolor, pastels or any other medium is the quality called value. Here "value" means simply how light or dark something is. Many gifted artists have noted that of the three basic aspects of color – hue, value and chroma (intensity) – value is the most important.
Assuming that your composition is good and your drawing is accurate, the next most crucial aspect of the work is accurate values. If your values are correct, your hues (red, green, blue, orange, etc.) and your chromas (intensity or degree of brightness or grayness) can be fairly far off the mark and the painting will still be a success.
The Value of Pigments Is Affected by The Medium They Are In
Obviously, some pigments are light, some are dark and others are somewhere in between. What may be less obvious is that one pigment can have different values in different media.
Pigments in soft pastels, for example, are very much lighter than the same pigments in oil paints. The most expensive soft pastels are nearly 100% pure pigment with just enough binder to hold them together. Oil paints rarely contain more than 80% pigment, and with some pigments, ivory black, for example, the percentage will be much lower.
The drying oils that are used for oil paints has an index of refraction similar to many pigments, and so pigments are at their maximum darkness in oils. Dry pigments are light in color because air has a very different index of refraction than the pigments. This causes each tiny particle to have a bluish white highlight that acts like an added white pigment.
While the drying oils used in oil paints form solid, non-porous paint films, acrylic mediums form porous paint films. Largely because of this, pigments in acrylics are not completely surrounded by the acrylic resins. The result is that while pigments in acrylics are much darker than dry pigments, they are slightly but significantly lighter and cooler than pigments in oils.
Mediums That Produce Darker Paint Films
Generally oil-based mediums make pigments look darker than water-based or dry mediums, but this is not a completely consistent rule. Below is a list of more common mediums with a notation of whether pigments tend look relatively light, medium or dark when ground in them.
- Oils - Dark
- Alkyds - Dark
- Acrylics - Fairly Dark
- Encaustics (Wax-and-Resin-Based Paints) - Fairly Dark
- Egg Tempera - Moderately Dark
- Transparent Watercolors - Moderately Dark
- Colored Pencils - Moderately Dark
- Watercolor Pencils - Medium to Moderately Dark
- Oil Pastels - Medium to Moderately Dark
- Casein - Medium to Moderately Light
- Gouache (Opaque Watercolors or Designer's Color) - Medium to Moderately Light
- True Fresco (Wall Painting on Moist Plaster) - Moderately Light
- Soft Pastels - Light to Very Light
Ultimately, what matters for artists is the final appearance of the paint film. While the above list is fairly accurate for unvarnished or unfixed media, a coat of gloss varnish can radically darken a final paint film. Also, fixitives will darken pastels.
Water-based paints such as casein and gouache can look dramatically darker with a gloss varnish. However, in those mediums, smoothly blended areas in the unvarnished work may look "broken" (unevenly blended) after varnishing.
(See also Values of Pigments in Paints article on specific pigments and pigment classes.)
Sources:
- The Artist's Handbook of Materials and Techniques, by Ralph Mayer, Viking Press, 1982.
- The Materials & Techniques of Painting, by Kurt Wehlte, Van Nostrand Reinhold, 1982.
- The Oil Color Book by Winsor & Newton, Colart Fine Art and Graphics Limited, 2001.
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