Exploring

Inherent Vice: Materials

Colorants

Colorants of various types are encountered in watercolors, pastels, prints, architectural drawings and reproductions, writing and drawing inks, and color photography.

General Characteristics

Colorants are divided into two categories—pigments and dyes. Both are colored substances that are dissolved in some type of liquid (called a vehicle). A pigment is not soluble in the vehicle, but rather is dispersed or suspended in it. A dye, on the other hand, is soluble in the vehicle and colors an object by being absorbed into it, often with the assistance of another chemical called a mordant.

The science of colorants is quite complex. In some cases, a colorant can be either a pigment or a dye, depending on the vehicle used. Many modern pigments are made from soluble dyes by chemically bonding the dyes with a metallic salt (such a pigment is called a "lake").

Pigments and dyes are customarily classified according to their origins, such as organic or inorganic and natural or synthetic (e.g., chemically manufactured).

History

Natural inorganic pigments made from minerals were the first used by humans (for example, red earths containing iron oxide) about 60,000 years ago. Natural organic pigments (made from extracts from plant and animal materials) have been used since antiquity for many purposes, including cosmetics and textile dying. The first synthetic pigments were inorganic. After 1800, technology had developed sufficiently to produce these pigments on a large scale. In 1856, the first synthetic organic dye (mauve) was developed from aniline, a chemical extracted from coal tar.

Characteristic Types of Deterioration

Colors in this artwork have faded where it was exposed to light, while areas that were protected under the frame have not faded.

The primary form of colorant deterioration is fading, although some historical pigments also deteriorate upon contact with other substances, such as heat, acidic or alkaline materials, and chemicals. Some inorganic pigments of metallic origin may also cause oxidation and acidic deterioration similar to that caused by iron-gall inks.

Fading is a chemical change in which ultraviolet light reacts with the colorant, or triggers a reaction with oxygen or moisture, causing the colorant to change to a colorless (or less colored) compound.

Natural inorganic pigments were very stable and permanent, but due to the difficulty in manufacturing them, they have been replaced in modern times with synthetic inorganic colors (many of which are also quite permanent). Natural organic pigments were very unstable and subject to light fading. The early synthetic organic colorants (aniline-based), while quite brilliant and intense, were also very unstable. Advances in organic chemistry during the 20th century have resulted in many modern synthetic organic pigments that are much less prone to fading.

Lists of historical pigments, as well as modern pigments that are ASTM-approved for artists' use, can be found in The Artist's Handbook of Materials and Techniques (see Additional Resources). These lists provide rankings for lightfastness in addition to other characteristics. The Watercolors Web site also provides information on the history and characteristics of pigments.