Blues for a Thousand Years

Indigo dye, the blue in blue jeans, is one of the oldest known naturally occurring plant dyes. Vat dyes—a range of dyes based on indigo—are used in cotton dyeing where high wash and boil fastness is required. Because of the high alkali concentration in the dye bath, vat dyes cannot be used on animal fibers because they would dissolve.


The German chemist Adolf von Baeyer began working on the first synthesis of indigo in 1878 and developed a second pathway in 1880. However, his approach remained impractical for industrial application and a search for alternative starting materials was undertaken by BASF and Hoechst. The first synthetic version of indigo was introduced to the textile industry in 1897 and completely replaced the natural dye.

But this more recent history is a bit of a wash when compared with the Mayans who developed an extremely fast version of the indigo pigment some 1700 years ago. Using synchrotron X-rays, physicists have now revealed the secrets behind this remarkable longevity and durability.

The Mayan pigment was made by burning incense made from tree resin and using the heat to cook a mixture of indigo plants and a special type of clay. As the pigment mixture was heated, indigo molecules filled a network of tiny channels inside the clay. Some of these bits of indigo plugged the pores on the surface, preventing the color from escaping over time. In other words, the clay acted as a binder similar to that used for slow delivery of modern medicinal drugs. A modern replication of the clay uses a porous substance called zeolite, which is widely used in commercial products like laundry detergent and cat litter.