The Redcoats are coming, the Redcoats are coming!” Well, Paul Revere could hardly have missed them. And if he were observant, he could even have distinguished the officers from the privates. The privates’ coats were coloured red with a dye derived from the root of the madder plant. The officers’ jackets’ were a stunning red extracted from the cochineal insect. A royal decree actually required privates’ uniforms to be dyed red in order to support British agricultural interests, specifically the cultivation of the madder plant, or Rubia tinctorum.
The dying process was not straight forward, involving multiple soakings of the fabric in lye, olive oil and sheep’s dung. The actual chemical responsible for the colour was eventually determined to be a compound known as alizarin. Since madder root was not grown in the colonies, American soldiers had to settle for blue uniforms, dyed with indigo. “Blue-bellied Yankees,” the British smirked. We know who had the last laugh on that one. As far as cochineal red goes, it was extracted from the female cochineal insect that infests certain species of cactus. It was far more expensive, but more brilliant than alizarin.
While the expression ‘redcoat’ conjures up an image of the British Revolutionary War soldier, the use of madder root as a dye was not a British invention. Thousands of years before Revere’s famous ride, madder was being used to colour the wrappings of ancient mummies. The stunning red togas of Roman centurions also owed the colour to madder juice. But the most ingenious use of madder extract can be attributed to Alexander the Great. Myth tells that, before a critical battle with the Persians, Alexander had his soldiers splash themselves with madder juice and stagger onto the battlefield. Thinking they could easily defeat the wounded soldiers, the Persian’s quickly attacked, falling into Alexander’s clever trap.
The madder industry was huge in Europe up to 1870, with about 70,000 pounds being produced annually. Then William Henry Perkin and Heinrich Caro put the farmers out of business. Perkin had already made a name for himself by synthesizing mauve, hailed as the world’s first synthetic dye. Perkin had hit upon the dye by accident while trying to make quinine, a much-needed anti-malarial drug. One day, frustrated with his lack of progress, he discarded his chemicals in the sink and was rewarded with the appearance of a brilliant new colour.
During the 1860s, Perkin turned his attention to alizarin and wondered if this substance could also be produced synthetically. By 1869 he had solved the problem and was making the red dye in the laboratory. The importance of this endeavor, however, far surpassed the importance of alizarin as a colourant. This was the first time that a dye was synthesized based upon the knowledge of the molecular structure of the desired product. Just the year before, Adolf Baeyer had shown that complicated organic molecules could be broken into simpler compounds by heating with zinc. Alizarin, when treated in this fashion, yielded anthracene, a well-known component of coal tar.
Perkin converted anthracene to alizarin in a process he patented on June 26, 1869. Unfortunately for him, Caro had filed a patent for a very similar process in Germany the day before. Perkin had the rights to alizarin in England, but lost out on the lucrative German market by 24 hours. Perkin’s and Caro’s alizarin syntheses turned out to be so practical that within a few years the price of the dye dropped to 55 cents a pound from $15 a pound. Today alizarin is still used to dye wool, but of course it has been joined by a host of other synthetic dyes.
Joe Schwarcz is the director of McGill University’s Office for Science and Society.
Read his blog at chemicallyspeaking.com.