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Colorful Chemistry: Pigments and Dyes Brighten Up Our World
by Amy Sillup, September 2, 2012
 
Humans love and can see many colors and color is important in the animal and plant world for survival, seeking mates and even serving as a disguise or camouflage. Learn about the chemical world of color here.
 

Colorful Blanket Flower with Organic Pigments of Color. Photo Credit: U.S. Army
 


A Prism Separates the Different Colors and Displays the Rainbow of Colors from Red to Violet. Photo Credit: GFSC of NASA.

 The World of Color, Some Simple Thoughts
 
Painter Jackson Pollock once went through a “black period,” when he used only black, gray, taupe, and white in his artworks. Fellow artist Tony Smith gave him some excellent advice: “You will never get back to any objectivity until you go for color.” Just about any other word could be substituted for “objectivity” in that sentence (“joy,” “beauty,” and “pleasure” certainly spring to mind) and it would still make a lot of sense. Without color, the world would be a drab and depressing place indeed.

From the bold yet utilitarian hues of pill coatings, paints, and plastics to oh-so-tempting M&M candy shells to the subtle or shocking tints of the fashion and cosmetic world’s latest offerings, color chemistry is all around us, all the time. It may be taken for granted by most, but those interested in the molecular world quickly learn to appreciate the structure, functionality, and history of both natural and synthetic dyestuffs.

General Information on Pigments and Dyes

Solid dyestuffs are called pigments, while liquids are known as dyes. Organic or “carbon-based” pigments are sometimes combined with inorganic substances like alumina (aluminum hydroxide) to create “lakes;” this makes the color insoluble and is useful to create pill and candy coatings that won’t rub off on consumers’ hands. “Mordants” are substances that are absorbed onto cloth. When the dye is then applied, it reacts to form a complex with the mordant; the final color result depends on the structure of both reactants.

Classifications of Color Chemicals

Dyes and pigments are often organized by reference to specific groups of atoms in their molecular structures, such as “azo dyes.” They may also be named according to how they work or the way they are employed: Reactive dyes react and bond chemically with the substance they color by forming covalent linkages or bonds with it, whereas direct dyes are applied directly from a bath of sodium salt.

Color chemicals may also be simply classified as organic (carbon-based) or inorganic. Note that some dyes have an organic portion but also contain a metal ion. Ferrous gluconate, used to color black olives, is composed of iron in the +2 oxidation state along with two molecules of a negatively- charged form of glucose, for instance.

Inorganic Pigments: From Pretty Poisons to Non-toxic Tints

The earliest-known inorganic pigments were probably black manganese oxide and red iron oxide, which were used to decorate cave walls over 17 millennia ago. Iron oxides and salts are still used in many color cosmetics, but some widely-used inorganic pigments eventually turned out to be extremely dangerous: Examples include not only notorious white lead paint, but also emerald-hued “Paris green,” made from copper acetoarsenite, and vermillion, which contained mercuric sulfide. Barium sulfate makes a much safer white pigment than lead, and chromium oxide green avoids the noxious drawbacks of the old-time arsenic compounds. Mercury in vermillion is also a heavy metal which is toxic.

Some glazed dishware and glass produced before 1942 actually contain enough uranium to activate Geiger counters. They only release alpha particles, so they’re not exactly deadly; and are hugely sought-after collector’s items today. The uranium in them can leach into acidic foods, though, so it’s still not such a good idea to ingest food placed on them.

Pre-1942 Fiestaware and Geiger Test Kit

Photo by: Jeff Keyzer  
Inorganic compounds like white titanium dioxide are not only indispensable to the paint industry, but are also used to lighten vixen-red makeup products to demure shades of pink. Titanium dioxide was used as early as 1926 by the famous designer Elsa Schiaparelli to remove “vulgar” shine from synthetic fabrics like rayon. It is present in everything from lacquer to so-called “non-chemical” sunscreens to toothpaste.  
 
 

Photo by: D. Sharon Pruitt (Pink Sherbet Photography)

 

The seldom-discussed element praseodymium is used in many glazes. It forms a striking green oxide by itself, and a cheerfully sunny yellow when combined with zirconium oxide.

Bright red, yellow, or brown cadmium pigments are beautiful, and have been used enthusiastically for many years. The yellow/brown shades are actually due to the presence of selenium, sulfur or both sulfur and selenium (sulfoselenides). Cadmium is stored in the liver and its half-life in the body may be as long as thirty years.  Cadmium is bound by a protein in the kidneys called metallothionein, and can damage the kidneys if levels exceed 200 parts per million (think 200 peanuts in a pile weighing about 2200 pounds). Cadmium also remains a suspected human carcinogen, so cadmium selenium-sulfide pigments may gradually be phased out of use completely.

The luxuriously blue-purple cobalt-aluminum oxide is an example of an inorganic color that has stood the test of time. Samples of dishware with cobalt blue glazing that date back to ancient Egypt have been discovered. Copper (II) phtalocyanines also create a stunning blue pigment.

Inorganic pigments made an undeniable contribution to the world, but they weren’t suitable for all uses. In order to dye textiles, make pill-coatings and food colorings, effectively alter the color of their hair, or simply figure out the pH level of a solution, people often had to turn to the trusty world of organic chemistry (carbon-based chemistry). Scientists started out with dyes derived from animals and plants, and eventually moved into the lab to save money and time by improving Nature’s handiwork with a chemist’s creative touch through a series of thoughtful, manipulative, chemical steps and reactions.
 
Sources

Brock, William H., The Norton History of Chemistry, New York: W.W. Norton & Company, 1992.

Daintith, John, editor, A Dictionary of Chemistry, Third ed., Oxford University Press, 1996.

Emsley, John, Molecules at an Exhibition, Oxford University Press, 1998.

Field, Simon Quellan, Why There’s Antifreeze in Your Toothpaste: The Chemistry of Household Ingredients, Chicago, IL: Chicago Review Press, 2008.

Fenichell, Stephen, Plastic: The Making of a Synthetic Century, New York, HarperBusiness, 1996.

Gray, Theodore, The Elements, New York: Black Dog & Leventhal Publishers, 2009.

Quadbeck-Seeger, Hans-Jü rgen, World of the Elements Elements of the World, Federal Republic of Germany: Wiley-VCH, 2007.

Tony Smith’s quote taken from Solomon, Debra, Jackson Pollock, New York: Simon & Schuster, 1987, p. 232.