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 (1863). "Synthetics" achieve intensity, color fastness, and reproducibility that natural materials can rarely match. Market viable large scale production of dyes occurred nearly simultaneously in the early major producing countries United Kingdom of Great Britain and Ireland (1857), Second French Empire (1858), German Confederation (1858), and Switzerland (1859), 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 (cotton, silk, wool, leather, paper) 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 (cupric acetate arsenite), Scheele's green (copper(II) arsenite), and Naples yellow (lead antimonate). 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.
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. Antoine 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 [sic] 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.
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