A pigment is a substance that appears a certain color because it selectively absorbs wavelength of light. While many materials possess this property, pigments with practical applications are stable at normal temperatures and have a high tinting strength so only a small amount is needed to see the color when it's used on objects or mixed with a carrier. Pigments that either fade or else blacken over time or with extended exposure to light are called fugitive pigments.
Historic and Prehistoric Pigments
The earliest pigments came from natural sources, such as charcoal and ground minerals. Paleolithic and Neolithic cave paintings indicate carbon black, red ochre (iron oxide, Fe2O3), and yellow ochre (hydrated iron oxide, Fe2O3·H2O) were known to prehistoric man. Synthetic pigments came into use as early as B.C.E. 2000. White lead was made by mixing lead and vinegar in the presence of carbon dioxide. Egyptian blue (calcium copper silicate) came from glass colored using malachite or another copper ore. As more and more pigments were developed, it became impossible to keep track of their composition.
In the 20th century, the International Organization for Standardization (ISO) developed standards for characteristics and testing of pigments. The Colour Index International (CII) is a published standard index that identifies each pigment according to its chemical composition. Over 27,000 pigments are indexed in the CII schema.
Dye and Luminescence
A pigment is a substance that is either dry or else insoluble in its liquid carrier. A pigment in liquid forms a suspension. In contrast, a dye is either a liquid colorant or else dissolves in a liquid to form a solution. Sometimes a soluble dye may be precipitated into a metal salt pigment. A pigment made from a dye in this manner is called a lake pigment (e.g., aluminum lake, indigo lake).
Both pigments and dyes absorb light to appear a certain color. In contrast, luminescence is a process by which a material emits light. examples of luminescence include phosphorescence, fluorescence, chemiluminescence, and bioluminescence.
Pigment Definition in the Life Sciences
In biology, the term "pigment" is defined somewhat differently, where a pigment refers to any colored molecule found in a cell, regardless of whether or not it is soluble. So, although hemoglobin, chlorophyll, melanin, and bilirubin (as examples) don't fit the narrow definition of pigment in science, they are biological pigments.
In animal and plant cells, structural color also occurs. An example may be seen in butterfly wings or peacock feathers. Pigments are the same color no matter how they are viewed, while structural color depends on the viewing angle. While pigments are colored by selective absorption, structural color results from selective reflection.
How Pigments Work
Pigments selectively absorb wavelengths of light. When white light strikes a pigment molecule, there are different processes that can lead to absorption. Conjugated systems of double bonds absorb light in some organic pigments. Inorganic pigments may absorb light by electron transfer. For example, vermilion absorbs light, transferring an electron from the sulfur anion (S2-) to a metal cation (Hg2+). The charge-transfer complexes remove most colors of white light, reflecting or scattering back the remainder to appear as a certain color. Pigments absorb or subtract wavelengths and do not add to them like luminescent materials do.
The spectrum of the incident light affects the appearance of a pigment. So, for example, a pigment won't appear quite the same color under sunlight as it would under fluorescent lighting because a different range of wavelengths are left to be reflected or scattered. When the color of a pigment is represented, the lab light color used to take the measurement must be stated. Usually this is 6500 K (D65), which corresponds to the color temperature of sunlight.
The hue, saturation, and other properties of a pigment depend on other compounds that accompany it in products, such as binders or fillers. For example, if you purchase a color of paint, it will appear different depending on the formulation of the mixture. A pigment will look different depending on whether its final surface is glossy, matte, etc.
List of Significant Pigments
Pigments may be classified according to whether they are organic or inorganic. Inorganic pigments may or may not be metal-based. Here is a list of some key pigments:
Other Inorganic Pigments