Colour: A Brief Historical Perspective

We only have to open our eyes and look around to observe how important a part colour plays in our everyday lives. Colour influences our moods and emotions and generally enhances the way in which we enjoy our surroundings. Our experience of colour emanates from a rich diversity of sources, both natural and synthetic. Natural colours are all around us, in the earth, the sky, the sea, animals and birds and in the vegetation, for example in the trees, leaves, grass and flowers. Colour is an important aspect in our enjoyment of the food we eat. In fact, we frequently judge the quality of meat products, fruit and vegetables by the richness of their colour. In addition, there is a myriad of examples of synthetic colours, products of the chemical manufacturing industry, which we tend to take so much for granted these days. These colours commonly serve a purely decorative or aesthetic purpose, but in some cases specific colours may be used to convey vital information, for example in traffic lights and colour-coded electrical cables. Synthetic colours are used in the clothes we wear, in paints, plastic articles, in a wide range of multicoloured printed material such as posters, magazines and newspapers, in photographs, cosmetics, ceramics, and on television and film. Colour is introduced into these materials using substances known as dyes and pigments. The essential difference between these two types of colorants is that dyes are soluble coloured compounds which are applied mainly to textile materials from solution in water, whereas pigments are insoluble compounds incorporated by a dispersion process into products such as paints, printing inks and plastics. The reader is directed to Chapter 2 of this book for a more detailed discussion of the distinction between dyes and pigments as colouring materials.

People have made use of colour since prehistoric times, for example in decorating their bodies, in colouring the furs and skins that they wore and in the paintings which adorned their cave dwellings. Of course, in those days the colours that were used were derived from natural resources. The dyes used to colour clothing were commonly extracted either from vegetable sources, including plants, trees, roots, seeds, nuts, fruit skins, berries and lichens, or from animal sources such as crushed insects and molluscs. The pigments for the paints were obtained from coloured minerals, such as ochre and haematite, which were dug from the earth, ground to a fine powder and mixed into a crude binder.

Synthetic colorants may also be described as having an ancient history, although this statement applies only to a range of pigments produced from basic applications of inorganic chemistry. These very early synthetic inorganic pigments have been manufactured and used in paints for many thousands of years. The ancient Egyptians were probably responsible for the development of the earliest synthetic pigments. The most notable products were Alexandra blue, a ground glass coloured with a copper ore, and Egyptian Blue, a mixed silicate of copper and calcium which has been identified in murals dating from around 1000 BC. Perhaps the oldest synthetic colorant still used extensively today is Prussian Blue, the structure of which has been established as iron(III) hexacyanoferrate(II). The manufacture of this blue inorganic pigment is much less ancient, dating originally from the middle of the 17th century, although this product pre-dates the origin of synthetic organic dyes and pigments by more than a century.

Synthetic textile dyes are exclusively organic compounds and, in relative historical terms, their origin is much more recent. Textile materials were coloured exclusively with the use of natural dyes until the mid-19th century. Since most of nature's dyes are rather unstable, the dyeings produced in the very early days tended to be quite fugitive, for example to washing and light. Over the centuries, however, complex dyeing procedures using a selected range of natural dyes were developed which were capable of giving reasonably quality dyeing on textile fabrics. Since natural dyes generally have little direct affinity for textile materials, they were usually applied together with compounds known as mordants, which were effectively 'fixing-agents'. Metal salts, for example of iron, tin, chromium, copper or aluminium, were the most commonly used mordants, and these functioned by forming complexes of the dyes within the fibre. These complexes were insoluble and hence more resistant to washing processes. As a result, these agents not only improved the fastness properties of the dyeing, but also in many instances were essential to develop the intensity and brightness of the colours produced by the natural dyes. Some natural organic materials such as tannic and tartaric acids could also be used as mordants. Among the most important of the natural dyes the use of which has been sustained over the centuries, is indigo 1a, a blue dye obtained from certain plants, for example from Indigofera tinctoria found in India, and from woad, a plant extract. A related product is Tyrian purple, whose principal constituent was 6,6'-dibromoindigo 1b. This was for many years a fashionable purple dye which was extracted from the glands of Murex brandaris, a shellfish found on the Mediterranean and Atlantic coasts. The most important of the natural red dyes was madder, a wood extract, the main constituent of which was alizarin, 1,2-dihydroxyanthraquinone (2). Alizarin provides a good example of the use of the mordanting process, since it readily forms metal complexes within fibres, notably with aluminium, which show more intense colours and an enhanced set of fastness properties.

It may be argued that the first synthetic dye was picric acid 3, which was first prepared in the laboratory in 1771 by treating indigo with nitric acid. Much later, a more efficient synthetic route to picric acid from phenol as the starting material was developed. Picric acid was found to dye silk a bright greenish-yellow colour but it did not attain any real significance as a practical dye mainly because the dyeings obtained were of poor quality, especially in terms of lightfastness. However, it did find limited use at the time to shade indigo dyeings to give bright greens.

The foundation of the synthetic dye industry is universally attributed to William Henry Perkin on account of his discovery in 1856 of a purple dye which he originally gave the name Aniline Purple, but which was later to become known as Mauveine. Perkin was a young enthusiastic British organic chemist who was carrying out research aimed not initially at synthetic dyes but rather at developing a synthetic route to quinine, the antimalarial...