Synthetic colorant
A colorant is any substance that changes the spectral transmittance or reflectance of a material. Synthetic colorants are those created in a laboratory or industrial setting. The production and improvement of colorants was a driver of the early synthetic chemical industry, in fact many of today's largest chemical producers started as dye-works in the late 19th or early 20th centuries, including Bayer AG. Synthetics are extremely attractive for industrial and aesthetic purposes as they have they often achieve higher intensity and color fastness than comparable natural pigments and dyes used since ancient times. Market viable large scale production of dyes occurred nearly simultaneously in the early major producing countries Britain, France, Germany, and Switzerland, and expansion of associated chemical industries followed. The mid-19th century through World War II saw an incredible expansion of the variety and scale of manufacture of synthetic colorants. Synthetic colorants quickly became ubiquitous in everyday life, from clothing to food. This stems from the invention of industrial research and development laboratories in the 1870s, and the new awareness of empirical chemical formulas as targets for synthesis by academic chemists. The dye industry became one of the first instances where directed scientific research lead to new products, and the first where this occurred regularly.
Dyes versus pigments
Colorants can be divided into pigments and dyes. Broadly, dyes are soluble and become fixed to a substrate via impregnation, while pigments are insoluble and require a binding agent to adhere to a substrate. Dyes, therefore, must have an affinity for the substance they are intended to color. Chemically speaking, pigments can be organic or inorganic, while dyes are only organic. Furthermore, organic white pigments do not exist, despite the fact that the majority of purified crystalline organic products are white in appearance. This story is complicated somewhat by lake pigments, or lakes, which are dyes modified with a chemical process to form an insoluble pigment. Typically this involves precipitating the natural extracts as salts in alkaline conditions. The historical importance of both pigments and dyes is closely related, as the markets for both, as well as the types and variety available, have always been closely tied.History
Early colorants date to prehistoric times. Human beings were already relying on natural substances, primarily from vegetables, but also from animals, to color their homes and artifacts. Cave drawings like those in Altamira or Lascaux were made in the Ice Age 15,000 to 30,000 years ago. Using pigments for coloration is among the oldest cultural activities of mankind. The important substrates of pre-industrial societies were generally naturally occurring and therefore share similarities, since they are primarily saccharide or peptide polymers.The 19th and 20th century in particular saw an expansion in colorant use and production, yielding many pigments and dyes in use today. The availability of strong acidic or alkaline environments like sulphuric acid and synthetic sodium carbonate was crucial in this process. These conditions became possible due to price drops in reagents due to new industrial preparations like the LeBlanc process, where potassium carbonate formerly obtained from ashes was replaced by sodium carbonate. However, many early colorants are no longer produced due to economics, or high toxicity, for example Schweinfurt green, Scheele's green, and Naples yellow.
The late 1850s saw the introduction of the first modern synthetic dyes, which brought more color and variety of color to Europe. In addition to being multi-varied and extraordinarily intense, these new dyes were notoriously unstable, rapidly fading and turning when exposed to sunlight, washing, and other chemical or physical agents. This led to new systems of categorization and study of colorants, which in turn lead to the synthesis of more color-fast modern colorants. Synthetic colors found themselves in not only dyes and paints but also inks and foodstuffs, permeating consumer culture.
Natural products
In ancient cave paintings natural manganese oxide and charcoal were used for black shades and iron oxides for yellow, orange, and red color tones. Examples of similar earth pigments that persisted to more modern times are the red pigment vermilion, the yellow orpiment, the green malachite and the blue lapis lazuli. Natural sources of white pigments include chalk and kaolin, while black pigments are often obtained as charcoal and as soot.Early production and syntheses
In ancient times, through the Industrial Revolution, various inorganic pigments like Egyptian Blue were synthesized, many with toxic chemicals like arsenic and antimony. These toxic pigments were used for cosmetics and painting. In ancient Egypt, blue was considered the color of the divine. As a result, the early synthetic compound Egyptian Blue, became an incredibly important pigment. It was used for the depiction of eyes, hair and decoration in the graphic representation of pharaohs. Blue, particularly ultramarine pigment made from ground lapis lazuli remained significant for depictions of the divine through the Renaissance. Pre-industrial revolution painters in Europe used ultramarine almost exclusively for the robes of Mary because of the pigment's great expense, until the work of Jean-Baptiste Guimet and Christian Gmelin made it commercially available in larger, cheaper quantities.At the beginning of the 18th century, the first products of the fledgling color industry were Prussian blue and Naples yellow. The first synthetically produced white pigment was white lead. It was known in Roman times. Around 1800, more inorganic white pigments were developed including zinc white was developed, followed by antimony white and zinc sulfide. The printers and dyers at that time had access to lead acetate, alum, copper acetate, nitric acid, ammonia and ammonium chloride, potassium carbonate, potassium tartrate, gallic acid, gums, bleaching lyes, hydrochloric acid, sulfuric acid, carbonates, sulfates, and acetates. Small scale workshops evolved into ever larger and larger manufactories.
Other inorganic pigments developed in the 19th century were cobalt blue, Scheele's green, and chrome yellow. The availability of sulphuric and sulfurous acids facilitated further experiments, leading to the isolation of alizarin and purpurin in 1826. Madder based pigments such as Brown Madder were developed due to research by British and German chemists into Turkey red, also known as Rouge d’Andrinopole.
First "scientific" syntheses: aniline dyes 1858 – 1870
In the mid 19th century, the coal tar industry, particularly in England, produced the precursors needed for a large amount of organic syntheses, in large quantities. For the first eight years after the first marketable synthetic dye, Mauveine, until the middle of the 1860s, British and French firms were the major dye producers. The second half of the 1860s saw German dye works surpassing their competition in both capacity and market share. During 1870, German firms were responsible for roughly half of the world's production of dyes and pigments. Aniline dyes were produced at scale, in part because of many advances in the synthesis of their precursors. Antione Bechamp described a process for reducing nitrobenzene to aniline in 1854, known as the Bechamp Process, making the production of aniline easy. Widespread isolation of phenol from coal tar, made its nitration more economical, generally the path of the synthesis flowed: coal tar → nitrobenzene → aniline → dyes. According to Henry Perkin himself "This industry holds an unique position in the history of chemical industries, as it was entirely the outcome of scientific research."First scientific synthetic dye: picric acid
The first synthetic dye was picric acid. It was prepared in a laboratory in 1771, and commercially produced by M. Guinon in Lyon in 1845. It dyed silk fabric yellow; however the color fastness properties were not good, thus it had very limited commercial success. It was, however, purchased in limited amounts by French dyers.William Henry Perkin’s mauveine
In 1856, 18 year old William Perkin accidentally discovered a dye he called mauve while trying to make quinine from the oxidation of allyl toluene in his home lab for his academic advisor and boss August Wilhelm von Hoffman. Hoffman reportedly referred to aniline, a major step in the synthesis, as his "first love," and was excited to have Perkin working with it. Perkin communicated with the textile industry, including Pullars of Perth, and John Hyde Christie, the chemist and general manager of John Orr Ewing and Co. about how to best market and produce his dye. He started production of aniline purple near London at the end of 1857 and remained the only producer for at least a few months. Perkin began making the intermediates for his dyes in-house, for example, nitro-benzene, expanding the scale of operations. By the summer of 1859, according to a satirical magazine Punch, London had fallen ill with 'the mauve measles'.Rapid expansion
By the end of 1858 there were already eight firms producing aniline dyes. By 1861 there were twenty-nine British patents on coloring matters from aniline. By 1864 68 firms were producing dyes. This was driven by the textile industry, which employed new designs requiring the colorful aniline dyes. Even Hofmann, who had at first criticized his student for leaving his academic research of quinine, later synthesized his own aniline dye, rosaniline. In 1858 the German chemist Johann Peter Griess obtained a yellow dye by reacting nitrous acid with aniline. It didn't last commercially, but it created even more interest in aniline as precursor for colorful compounds. French chemist François-Emmanuel Verguin reacted aniline with stannic chloride to yield fuchsine, a rose colored dye, the first of the triphenylmethane dyes. Further work by Hoffman along with the discovery of benzene’s structure and carbon’s tetravalency, this science built the groundwork for modern organic chemistry.In the late 1860s many companies began offering a full spectrum of colors, and were already outcompeting many natural dyes for market share. Prices continually fell, and new colors and products regularly entered the market. On January 1, 1868, there were 52 producers of aniline dyes. Members of enlightened scientific societies from all over Europe including
the Manchester Literary & Philosophical Society
competed for expertise and authority with dyers and printers in factories and workshops. Many soluble salts of acid dyes synthesized for textile-related purposes were transformed into insoluble salts or lake pigments by reaction with water-soluble salts of calcium, barium or lead, whereas basic dyes were treated with tannins or antimony potassium tartrate to yield pigments.