The Chemistry of Pigments and How Scientists Prevent Color Degradation

11 Aug.,2022

 

what are pigments

what are pigments

At the heart of every drop of paint, every thread of cloth, every bit of your brightly colored phone case is a pigment. Pigments are the compounds added to materials to give them color. This deceptively simple application has shaped our perception of the world via art, fashion, and even computer displays and medicine. Pigments are used in paints, inks, plastics, fabrics, cosmetics, and food.

Some of the earliest chemistry was to make and isolate pigments for paints, and pigment conservation is a focus for many modern researchers who identify and preserve artwork.

Get to know pigments

But what is a pigment, exactly? Pigments are brightly colored, insoluble powders (brightly colored liquids are called dyes). In most cases, the bright color is a result of the material absorbing light in the visible spectrum. In inorganic pigments, this absorption is the result of charge transfer between a metal (transition metals are really good at this); organic pigments tend to have conjugated double bonds that absorb visible wavelengths.

Pigments are mixed with binders to attach them to a substrate. The resulting suspension—a paint—is used to coat materials and impart color onto them. In industry, there are three pigment classes: absorption pigments (used in watercolor paints), metal effect pigments (used to create surface luster), and pearlescent pigments.

Pigments are found in nature, such as ochre (a blend of iron oxides and hydroxides) and indigo (C16H10N2O2). They can also be synthetic pigments such as mauve (an aniline derivative) or white lead. White lead, one of the earliest synthetic pigments, is made by treating sheets of lead with vinegar. They are often more robust than dyes, which dissolve in the material they are coloring. Pigments can keep their color for many centuries and withstand high heat, intense light, and exposure to weather or chemical agents.  

Cataloging colors

Because of their prominence in art, pigments have an important place in history. The Forbes Pigment Collection, housed in the Straus Center for Conservation and Technical Studies at the Harvard Art Museums, catalogs and preserves more than 2,500 pigments. Its founder, Edward Forbes, started the collection by gathering pigment samples from his travels all over the world, including colors like mummy brown, made from ground-up mummies, and carmine red (C22H15AlCaO13), obtained from cochineal insects.

The Forbes Pigment Collection is often used as a reference library to standardize colors and identify pigment samples from artworks, which can confirm or disprove the piece’s origins. For instance, in 2007, a painting supposedly by Jackson Pollock was discovered to be a forgery when chemical analysis revealed the presence of pigments that weren’t available until decades after his death (Custer, 2007).

Furthermore, many artists had personal preferences and favored certain pigments over others. Thus, knowing which pigments were used, and whether they were in character for the artist or not, can help art historians determine an art piece’s authenticity.

The Forbes Pigment Collection, which boasts more than 60 natural samples, also highlights one of the challenges with natural pigments. Natural pigments were gathered from nature, for example, ore deposits, minerals, and flowers. But tiny shifts in chemical composition or particle growth cause specific shades to vary significantly due to impurities present in the sample.

Analyzing and understanding the pigments used in paintings is also vital to artwork restoration and preservation. Many pigments chemically react with ambient light and humidity, as well as harsher substances like soot and smoke from cigars or fireplaces. Pigments may oxidize, dissolve in acid or water, undergo phase transitions, react with the binders in the paint, or degrade.

For example, eosin Y was a pigment historically favored by many artists, most notably Vincent van Gogh. Initially a vibrant red, exposure to light gradually turns eosin white as UV radiation excites the pigment molecules and leads to the production of OH radicals. This breaks down the structure of the pigment, and eventually turns it white. Knowing such information allows art historians to better conserve art.