Old King Coal Part 4: Coal Colours

Martin Pitt continues his series on coal, exploring how tar gave rise to the world’s first synthetic dyes and a global chemical industry

IN 1832 Karl von Reichenbach (1788–1869) painted his doorposts with his newly discovered creosote, hoping the smell would deter dogs from urinating on them. It didn’t – but the reaction produced an intense blue colour. He developed a chemical route (without dogs or urine) and sold the product as Pittakall (Greek for beautiful tar), the first synthetic dye. It sold locally but did not change the world

Popular purple

In 1843 celebrated German chemist August Wilhelm von Hofmann (1818–1892) showed that a chemical from coal tar was the same as one which could be made from decomposing the natural dye indigo, and named it aniline, after the Arabic word for indigo, anil.

In 1856, at what is now Imperial College London, Hofmann’s 18-year-old assistant William Henry Perkin (1838–1937) tried to follow a paper synthesis for quinine but instead produced a black powder that gave a lilac tint in alcohol. Hofmann dismissed it as impure and told him to discard it, but Perkin noticed it stained silk, resisted washing and didn’t fade in light. He refined the method, patented it and – with his father, brother and a friend – set up a company to manufacture the dye. He was just 20.

Perkin thought his mixture was simply aniline with potassium dichromate, but the reaction also involved p-toluidine and o-toluidine. The product was a mix of isomers, not fully identified until 1994 and yields were limited to about 5%.

Perkin called it aniline purple, then mauveine, which he produced at a price vastly less than the natural dye Tyrian purple, extracted from sea snails. It created a sensation in Britain and France when Queen Victoria and the Empress Eugenie wore dresses of this colour, and other chemists and companies rushed to try to create other dyes by oxidation of aniline.

One of these was Hoffmann, who in 1858 described a purple-red one. At almost the same time, so did French chemist Francois-Emmanuel Verguin (1814–1864) who patented it on 8 April 1859, and called it fuchsine, after the flower fuchsia.

On 4 June 1859, the French army defeated the Austrians at the Battle of Magenta. Soon after, the company that had acquired Verguin’s patent marketed the dye under the trade name magenta in France – a hugely successful move. Today, magenta remains one of the four standard inks used in colour printing.

The same year, the chemical equipment company Simpson, Maule and Nicholson became the second in Britain selling artificial dye, also fuchsine, which they called roseine.

Their chemist, Edward Nicholson (1827–1890), had devised a cheaper process that achieved yields of up to 42%, though the product contained as much as 6% arsenic from the oxidiser As₂O₅. Subsequent company products were Nicholson’s blue and Hoffmann’s violet (1862), and aniline yellow (1863). Perkin and others modified fuchsine to give aniline blue, crystal violet, Britannia violet and Perkin green.

By throwing different chemicals at aniline (of variable composition) and existing dyes, researchers produced a wide range of dyes with differing colours and applications – sparking a wave of patent disputes. In the early days, many court cases failed because the companies concerned could not exactly specify the chemicals they were using or producing.

A major boost to this early industry came not from the experimenters but the German theoretical chemist August Kekulé (1829–1896) who in 1865 published his understanding of the benzene ring’s shape and bonding. There were rapid developments in the understanding of shape and reactivity of aromatic molecules, leading to better experimental plans.

Meanwhile in Germany, Friedrich Bayer (1825–1880), a trained dyer who had built a trading business across Europe, was impressed by the new synthetic colours he imported. Working with his chief chemist, Johann Weskott (1821–1876), he began experimenting and in 1863 founded “Friedr. Bayer et comp.” to produce fuchsine, capitalising on the weakly enforced patent laws in the German states at the time. Bayer is now one of the largest pharmaceutical and biomedical companies.

Friedrich Engelhorn (1821–1902), a former goldsmith, was the manager of a public gasworks in Mannheim, Germany. Reading about Perkin and Verguin, he saw an opportunity to make use of his works tar and set up a small factory nearby which began producing fuchsine in 1861. In 1865, with industrial partners he founded the “Badische Anilin- & Soda-Fabrik” (Bavarian Aniline and Soda Factory) now known as BASF, the largest chemical company in the world.

The natural dye indigo, used for over 6,000 years, produced an intense, long-lasting colour and was the world’s dominant dye. Hoffmann’s identification of aniline in indigo naturally led to attempts to synthesise it.

The first successful industrial method was patented in 1890 by German chemist Karl Heumann (1850–1894) and it came into production in 1897. It was initially only slightly cheaper but easier to apply and more consistent in quality. World annual production of the natural dye was 19,000 t in 1900, but this had dropped to 1,000 by 1914, with the loss of livelihood for many farmers and processing workers in several countries. Britain also lost huge profits from its Indian plantations and factories. It proved a problem for the UK in World War One when it could no longer get the German dye for blue uniforms and Indian supplies had all but ceased.

An improved process was jointly developed by BASF and Hoechst in 1904 and became the industrial standard from 1926. Current world production is about 50,000 t, much of it for jeans and denim. Natural indigo is still produced for artisan and home dyeing to the extent of about 500 t, as it is very labour intensive.

An improved process was jointly developed by BASF and Hoechst in 1904 and became the industrial standard from 1926. Current world production is about 50,000 t, much of it for jeans and denim. Natural indigo is still produced for artisan and home dyeing to the extent of about 500 t, as it is very labour intensive.

The main product from Perkin’s manufacture of mauveine was a black insoluble powder. He sold this to printers for use in ink. It was a polymer of aniline, now called polyaniline or aniline black. In 1863, Lancashire chemist John Lightfoot (1832–1872) obtained a patent for soaking cloth in an aniline salt and treating with dichromate and copper sulfate, so that aniline black particles were generated within the fibres, giving a strong and hardwearing black colour. This was one of the most successful dyes ever, eventually becoming the standard for clerical garb and umbrellas as well as printing ink.

Other black dyes started to take over in the 1930s, but aniline black is now used as a pigment for laser and inkjet printers, and being electrically conductive can act as an electrode in lithium batteries.

Amazing alizarin

Perkin took anthracene (three benzene rings fused together) from coal tar, oxidised it to yellow crystals of anthroquinone and eventually produced alizarin, the red colour in the natural dye madder, which he patented in 1869. Sadly, three German chemists had submitted their patent a day earlier.
However, Perkin did some additional work and in 1870 agreed a collaboration for the European market. He and his brother built a factory for the UK, while BASF marketed elsewhere. A German dyeworks at Hoechst, near Frankfurt was producing and selling natural alizarin and had also discovered the process.

While the Germans – among them Hoffmann, now at the University of Berlin and acting as a consultant to the companies, and Kekulé – likely held the advantage in chemical understanding, Perkin’s standout achievement lay in designing and constructing large-scale process equipment, a significant feat of chemical engineering that he shared with his German counterparts.

Together with his brother Thomas, who left a career in architecture, Perkin also developed a method for producing anthracene and presented it to suppliers. They then distilled the crude anthracene over potash in batches of up to 1,800 lb and produced 450 t a year of alizarin paste in a complex process and works.

Numerous variants and colours were produced by companies in the UK, France, Germany and Switzerland. Perkin became rich enough to retire at 36 to concentrate on research and was knighted in 1906.

Azo dyes

This new class of materials were created with cold nitrous acid and an aryl amine such as aniline and discovered in 1858 by a German industrial chemist Peter Greiss (1829–1888) who had gone to the University of Marburg as a research student after his chemical works had been destroyed by fire (it was a new method but no one knew about the azo linkage at the time). The discovery so impressed Hoffmann that Greiss was invited to join him in London. His aniline yellow was produced by Simpson, Maule and Nicholson.

He also met two other German scientists, Heinrich Caro (1834–1910) and Carl Alexander von Martius (1838–1920), working for the Manchester dye company Roberts, Dale & Co. It was there that two more azo dyes, Manchester yellow and Bismark brown, were produced in 1867 by collaboration between the three.

The characteristic of these compounds was a -N=N- bridge only proposed in 1866 by Kekulé and more generally accepted after 1875. This is an example of conjugation – alternating single and double bonds which is characteristic of coloured organic compounds, moving the electronic spectrum from the ultraviolet to the visible.

Science and industry

In 1868, Heinrich Caro was back in Germany and became co-managing director of BASF along with Engelhorn. His first major project was industrial production of alizarin and in 1869 he shared the patent along with the two researchers who had developed the laboratory synthesis. With his knowledge of English and England, he negotiated a successful collaboration with Perkin. However, the greater investment in research by BASF meant they were able to control both the colours produced and the cost better than the company to which Perkin eventually sold his factory and process. In fact, Perkin’s process only produced alizarin as a minor component, with a related dye as the principal one. Caro knew that different degrees of sulfonation gave colours ranging from violet to red, which they produced at will, and further improvements followed. The Germans gained control of the market by a cartel of companies, including the British one.

German academic chemist Adolf von Baeyer (1835–1917) produced a novel compound (but not a dye) called fluorescein (he also discovered phenolphthalein and later received the Nobel prize for his work on dye chemistry). It was a condensation product of resorcinol (benzene diol) and phthalic acid (from naphthalene). Caro obtained it from Baeyer and brominated it to produce eosin in 1874, which was fluorescent red or pink on silk and considered so beautiful it sold for US$100 a pound in the US (about US$3,000 today). Eos is the Greek goddess of dawn. This was the first phthalein dye.

There was a new scientific twist when the German-Belgium firm AGFA “Aktiengesellschaft für Anilinfabrikation” (Corporation for Aniline Production) bought a sample of the BASF eosin and funded scientific study. Eventually Hoffmann published its formula and a method of synthesis, revealing the industrial secret. It had not been patented, since that provided little protection, and others began to produce it. However, Hoffmann’s publication gave an insight into the nature of these chemicals, enabling industrial chemists such as Caro to develop new colours and products.

In 1876, Swiss chemist Otto N Witt (1853–1915), working at Williams Thomas & Dower in Brentford near London, produced the first deliberately designed synthetic dye. He formulated the theory of dyeing, introducing the concepts of chromophores (the groups responsible for colour) and auxochromes (which make a coloured compound capable of binding as a dye). Witt was able to predict both the colour and dyeing properties before carrying out the synthesis. Marketed as London yellow, it was followed by further variations, and his theory went on to have a profound impact on both the dye industry and the wider field of colour chemistry.

Caro had actually recommended Witt to his company and was working on something similar so came to an agreement. The BASF version was marketed as chrysoidine. AGFA again obtained a sample and Hoffmann published the details.

It was clear companies could no longer rely on secrecy for this technology. In 1871 the various German states had unified as one federal country, so in 1877 a new patent system started for the German empire and the German Society of Chemical Industry Interests was formed. Companies also routinely took out corresponding British patents to prevent competition until UK patent law was revised in 1907. With the new colour theory and patent protection, some companies formed their own research laboratories.

US participation in the early synthetic dye industry was inhibited by the Civil War (1861–1865) and its aftermath; the reluctance of coke and gas companies to separate coal tar; and the powerful textile lobby which wanted the cheapest dyes possible, reducing tariffs on imports. The important fraction aniline oil was admitted tariff free, discouraging investment in its production and causing early businesses to shut down.

By 1900 nearly all countries imported German dyes. Cassella, founded in 1870 as “Frankfurter Anilinfarbenfabrik von Gans und Leonhardt” (Frankfurt Aniline Colours Works v Gans & Leonhardt) was the largest single dye producer in the world (in 1995 it merged with Hoechst). Along with Kalle AG (1863, also merged with Hoechst), AGFA Bayer, BASF and Hoechst they dominated the market. In 1914 with the start of World War One, there was a huge upheaval including a much larger sourcing of chemicals from petroleum. The changes this brought to the world, especially the UK, will be dealt with in another series of articles.

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Martin Pitt CEng FIChemE is a regular contributor. Read other articles in his history series: https://www.thechemicalengineer.com/tags/chemicalengineering-history