Categories: Accidental discoveries |  Comments
Guest post by Rowena Fletcher-Wood
When I am an old woman I shall wear purple
– Jenny Joseph
Colour is a powerful thing. It communicates emotions, beauty and status, unleashes our creativity and draws our attention. Wearing purple may have seemed slightly eccentric to poet Jenny Joseph, but in ancient Rome, it was the colour of power – and no surprise: their Tyrian purple was not only a lasting dye that seemed to become more rather than less vibrant under the sun, but it reportably took as many as 12,000 snails to produce enough dye to colour the trim of a toga!1
— Purple clothing – ©iStock
Up until the mid 19th century, all dyes were naturally produced – from minerals (yellow ochre), plants (indigo and turmeric), or animals (cochineal and Tyrian purple). Of course, some of these natural dyes had their problems. Making Tyrian purple required thousands of snails and a laborious process, other colours were inclined to fade or run, and some even broke down the fabrics they were used to colour.
Despite all of this, synthetic alternatives were not sought at the time: organic chemistry was not sufficiently understood to guess at a link between structure and properties, and in fact, by the mid 1800s, chemistry was still very much a private practice.
One chemical practitioner was August Wilhelm von Hofmann of the Royal Society. Hofmann was interested in quinine, an extract from the bark of the cinchona tree used to treat malaria. In the mid 1800s, he published a hypothesis for the synthetic production of quinine, and set one of his students the challenge of producing it.
The student, William Perkin, was only 18 years old, and had been taught at the Royal Society since he was 15. Impressed by the importance of this challenge, but finding himself on the Easter break of 1856, Perkin took the project home with him. Unlike book work, Perkin’s home project required setting up a makeshift home laboratory – not an unknown practice in the 1850s. he set about experimenting with benzene and aniline in his east end quarters. These kinds of experiments were extremely dangerous, especially because they involved two substances of which one was explosive and the other toxic. Only a year earlier, in 1855, Charles Mansfield, another student of Hofmann’s, was severely burnt whilst performing an experiment to distil benzene from naphtha – an injury that led to his slow and excruciating death.
Perkin, however, was lucky. Although his experiment to synthesise quinine failed – hardly surprising given the differences in structures that were not appreciated at the time – he produced instead a disappointing black clump. This clump was mauveine, or aniline purple. Transformed from aniline, it was much less toxic (although it could be broken down by the liver into nasty precursors), and ran a vivid purple colour when he tried to wash it away with ethanol.
Son of a carpenter, and keen painter himself, Perkin decided to abandon his pursuit of quinine (without telling Hofmann, of course) and invest his time exploring his new discovery. With the secret assistance of his friend Arthur Church and brother Thomas, Perkin produced enough mauveine to dye a piece of silk, test it under sunlight, and send it off to a dye factory in Perth. Some of his original samples are still around today. Perth was excited, and encouraged Perkin to patent his discovery later the same year.
An enthusiastic and surprisingly acute businessman, Perkin delivered his dye into production and onto the shelves with the help of a loan from his father. His was one of the first commercial chemical discoveries, and marked a turning point in our understanding of organic chemistry.
Although the structure was slow to be understood, Perkin revealed his new dye was a substance made of 27 carbon atoms, 24 hydrogen atoms, and 4 nitrogen atoms only, and began experimenting with similar compounds, creating such marvels as Perkin’s Green and Britannia Violet. At the peak of his colourful activity, it was famed that the Grand Union Canal changed its tone every week. And he wasn’t the only one: the synthetic dye industry erupted into technicolour, and with it came new understanding of structure and key functional groups such as azo dyes, triphenyls and anthraquinones that form so many of the shades we know today.
Because of the way they are thought to bind to fabric – via hydrogen bonds to cationic sites common in fibres – this group of organic compounds are now known as acid dyes. Solid, soluble, and anionic in solution, they are easy to get onto the fabric, and hard to get back off. Dyes are forever.
1 Jacoby, Silk Economics and Cross-Cultural Artistic Interaction: Byzantium, the Muslim World, and the Christian West Dumbarton Oaks Papers 58 (2004:197–240) p. 210