A pigment is a substance that gives color to another material, whether paint, plastic, or fabric. Unlike a dye, which is a soluble substance that chemically bonds with a medium, a pigment consists of insoluble, microscopic particles that are mechanically suspended within a liquid or solid binder. The chemical composition of these insoluble particles determines the hue, durability, and lightfastness of the resulting colorant, ranging from simple earth minerals to complex, synthesized organic molecules.
How Pigments Create Color
The perception of color from a pigment relies on the physics of selective light absorption and reflection. When white light strikes a pigmented surface, certain wavelengths are absorbed by the pigment’s molecules. The remaining, unabsorbed light is reflected back to the observer, determining the perceived color.
For instance, a yellow pigment absorbs blue and violet light, reflecting the combination of red and green that our brain interprets as yellow. The specific part of the molecule responsible for this interaction is the chromophore, or “color carrier.”
Chromophores are regions within the molecule that contain a system of alternating single and double chemical bonds, known as conjugated pi-bonds. This electronic structure allows electrons to be excited by the energy of visible light, absorbing that energy and removing the corresponding wavelength from the reflected spectrum. By chemically altering the chromophore’s structure, scientists can tune the molecule to absorb different wavelengths, thereby shifting the perceived color.
Pigments Derived from Minerals and Earth
The oldest and most durable pigments are inorganic compounds sourced directly from the earth, primarily composed of metal oxides. The common earth pigments—ochre, sienna, and umber—all derive their color from iron oxides, which are highly stable mineral structures.
Yellow ochre, for example, is composed mainly of goethite (\(\alpha\)-FeOOH), a hydrated iron oxide. When this pigment is heated, the water is driven off, transforming it into hematite (\(\alpha\)-Fe₂O₃), an anhydrous iron oxide that produces a deep, warm red hue. Umber differs from ochre in that it contains a significant proportion of manganese dioxide in addition to iron oxide, contributing to its darker, brownish tones.
Other historically prized mineral pigments include cinnabar, a brilliant red derived from naturally occurring mercuric sulfide (HgS). Lapis lazuli, the source of natural ultramarine blue, is a complex sodium aluminosilicate mineral known as lazurite. Its deep color is caused by the presence of a unique sulfur radical ion within its crystal lattice. These inorganic pigments are renowned for their exceptional permanence.
Pigments Derived from Living Systems
Organic pigments from living systems are complex, carbon-based molecules produced by plants, animals, or fungi for biological functions. The most widespread natural pigment is chlorophyll, a green compound built around a cyclic tetrapyrrole ring structure that binds a central magnesium ion. This structure makes it efficient at absorbing light for energy production.
Carotenoids are another major class, responsible for yellow, orange, and red colors in carrots, tomatoes, and autumn leaves. These are long, linear molecules featuring extensive conjugated double-bond systems that absorb blue light. Historically significant animal-derived pigments include carmine, a vibrant red pigment extracted from the cochineal insect.
Carmine’s primary coloring agent is carminic acid, a complex anthraquinone derivative that is precipitated with a metal salt, typically aluminum, to form the insoluble pigment. Madder root yields alizarin, another anthraquinone-based molecule, which creates durable red and crimson lake pigments when combined with a metal mordant. These organic pigments are less light-stable than their mineral counterparts, but they often possess greater tinting strength.
Modern Synthetic and Manufactured Pigments
The development of modern chemistry in the 19th and 20th centuries led to the creation of synthetic pigments, allowing for unprecedented color purity, stability, and mass production. These can be categorized as synthetic inorganic or synthetic organic, often replicating or surpassing the qualities of natural sources.
A major synthetic inorganic success was the industrial production of ultramarine, which is chemically identical to natural lapis lazuli but far more affordable. This sodium aluminosilicate blue pigment is manufactured by heating kaolin clay, silica, soda ash, and sulfur to high temperatures.
The most vibrant modern pigments are often synthetic organic compounds, such as the phthalocyanines and azo pigments. Phthalocyanine blue and green are based on a flat, macrocyclic ring structure that coordinates a central copper atom, resulting in colors of exceptional lightfastness and intensity. Azo pigments are characterized by the presence of an azo group (\(-\text{N}=\text{N}-\)) in their structure. This group forms the chromophore, allowing chemists to generate a vast spectrum of yellows, oranges, and reds with high tinting power.