Within the space of 50 years mankind had changed almost totally from natural dyes to synthetic ones. Phase 1 of this change was the addition of Chromium. The subsequent, often parallel, phases are discussed below with a bias towards how they apply to fabrics.

The 8 subsequent developments of synthetic dyes from Chromium onwards are:
(1. Chromium)
2. Triphenylmethane dyes
3. Anthraquinone dyes
4. Xanthene and related dyes
5. Azo dyes
6. Reactive dyes
7. Phthalocyanine compounds
8. Quinacridone compounds
9. Fluorescent brighteners

Phase 2: Triphenylmethane dyes

In 1858 Verguin (France) discovers ‘fuchsine’ a rose coloured dye made from aniline and tin chloride. This was the first of a series of dyes later called Triphenylmethanes and this marked the second phase of the growth of the synthetic dye industry. Adding excess aniline made aniline blue. Soon other variations were discovered and the chemistry understood; and before 1900 several hundred colours had been documented.

One problem with these dyes was solubility. That was overcome by the addition of sulphuric acid.

Phase 3: Anthraquinone dyes

In the 1850s and 1860s the understanding of how carbon is structured (tetravalency) led to scientists being able to plan how chemicals might react before doing the experiments. Alizarin and its derivatives (the anthraquinones) gave a huge number of dyes which constitute the second biggest grouping of dyes.

The addition of sulphur created a group of bright, fast dyes for wool. Indrathone blue, a brilliant blue vat dye, was discovered at the turn of the century and related compounds are still today used as pigments spanning colours from blue to yellow.

Phase 4: Xanthene related dyes

Fluorescein was discovered in 1871 but related discoveries were seldom used with fabrics until the late 1880s when some were used for silk. However the dyes had poor lightfastness and usage was stopped – only to be re-used 70 years later when they were found to be particularly good on acrylic fibres. Better products have been found since and now only one chemical dye in this class is used commercially (Safranine T).

Phase 5: Azo dyes

These form more than half of the commercial dyes used today.

The key is the reaction of nitrous acid with arylamine and then with phenols and aryl amines. This chemical reaction forms part of the production process of 50% of dyes in use today having been used since 1875, firstly for wool.

Methyl- related azo dyes were used extensively up until the 1970s but this has now stopped in many countries as they were carcinogenic.

The discovery of the azo dyes led to the development of method called ingrain dyeing. Here the dye is ‘made’ within the fabric. Since the process was carried out at around freezing point, some dyes were called ice colours. In 1912 Naphtol was found to form a water-soluble compound with an affinity for cotton, a major step in the development of the ingrain dyes. Naphtol is able to form a great number of possible end colours although many of these are not adequately colourfast.

Other Azo dyes became the most important commercial colorants because of their wide colour range, good fastness properties, and tinctorial strength (colour density), which is twice that of the anthraquinones, the second most important group of dyes. Azo dyes are easily prepared from many readily available, inexpensive compounds and meet the demands of a wide range of end uses. Cost advantages tend to offset the fact that these are less brilliant and less lightfast than the anthraquinones.

Phase 6: Reactive dyes

Reactive dyes are very adaptable and can create a huge number of colours.

The first reactive dyes utilized monoazo for bright yellow and red shades. Adding aniline gave the azo dye used in the first Procion Red.

Dichlorotriazinyl dyes are now produced by more than 30 dye manufacturers, since the early patents on these dyes have expired.

With the introduction of reactive dyes, cotton could finally be dyed in bright shades with azo dyes for yellows to reds, with anthraquinones for blues, and with copper phthalocyanines for bright turquoise colours.

Phase 7: Phthalocyanine compounds

Phthalocyanines, the most important chromium derivatives developed in the 20th century being introduced in 1934 and marketed as Monastral Fast Blue B and Monastral Fast Blue G.

Copper phthalocyanine is the most important and can be formed directly on cotton. Although not useful for PET and acrylics, some complexes are utilized with nylon. Chemical bleaching alters the shade to bluish-green and green.

Water-soluble versions were developed later by the introduction of sulphur based chemicals also producing a direct dye for cotton (Chlorantine Fast Turquoise Blue Gll), the first commercial phthalocyanine dye.

Such colourants all display strong, bright blue to green shades with remarkable chemical stability. These compounds exhibit excellent lightfastness, and their properties are in striking contrast to those of natural pigments that are destroyed by intense light or heat and mild chemical reagents. The high stability, strength, and brightness of the phthalocyanines render them cost-effective, illustrated by the wide use of blue and green labels on many products.

Phase 8: Quinacridone compounds

A second group of pigments developed in the 20th century were the quinacridone compounds. Quinacridone itself was introduced in 1958. Its seven crystalline forms range in colour from yellowish-red to violet.

Phase 9: Fluorescent brighteners

Raw natural fibres, paper, and plastics tend to appear yellowish because of weak light absorption. Bleaching can reduce this but the bleach must be mild to avoid damaging the material. Alternatively a bluing agent can mask the yellowish tint to make the material ‘appear’ whiter (hence the phrase a ‘bluey whiteness’), or the material can be treated with a fluorescent compound that weakly emits blue visible light. These compounds, also called “optical brighteners,” they are not dyes in the usual sense. The major industrial applications are as textile finishers, pulp and paper brighteners.

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A dye is a substance that gives colour to fabric. Usually in a way such that washing, heating or lighting does not change the colour greatly.

Dyes tend to be carbon based (ie organic in a chemical sense) whereas pigments are very fine powders ‘disolved’ in a liquid. Pigments generally give brighter colours and are man-made.

Dyes have existed for at least 4000 years and, before 1850, were almost entirely from natural sources such as plants, trees and lichens but also sometimes from insects. Here are some natural dyes, rarely used today, and their sources:

1. Yellow
Seeds, stems and leaves of the weld plant
The inner bark of the North American oak ‘quercetin’
Dried petals of false saffron (safflower)

2. Red
Crushed insect bodies from Coccus (cochineal) or it’s distant relation Kermes.

3. Blue
From indigo or woad

4. Purple
From the medium sized predatory sea snail ‘commonly’ known as Murex.

5. Black
From the middle wood of the Logwood tree. This is still used today to dye silk and leather and is combined with Chromium. I have written other articles about how this ‘natural’ dye is one of the most damaging to the environment because of the use of chromium.

The art of the dye was historically a closely guarded secret with practitioners having their formulae to produce the colours and to retain them by the addition of various metal salts.

Cotton could not be directly dyed whereas wool and silk could. To add a dye to cotton the cotton had to be first treated with salts made from aluminium (red), magnesium (violet), tin, calcium (purple-red), copper, barium (blue) and iron (black-violet) and then dyed. These salts are called mordants.

The Start Of Synthetic Dyes

In the 1850s Chromium was found to give superior dye retention and so started the decline of the natural dye. Chromium mordants are still widely used for wool and less so for silk and nylon.

More precisely, the first commercially successful dye was ‘mauve’ discovered in England in 1856 and taken to market the following year. It was only sold for about 7 years but that was sufficient to start the dramatic decline of natural dyes and the investment in the science for newer and better dyes.

The Chromium discovery meshed well with the Industrial Revolution. The massively growing textile industry in Europe required a cheap and predictable manufacturing process. Natural dyes and mordants could require up to 20 steps in production, the colour could be variable and the dyes had to be transported unreliably from around the world. Because of these factors and the development of chemical science it is easy to see how by-products of coal tar extraction & coke production, abundant in Europe, became the foundation of the modern dye industry.

By 1900 nearly 90 percent of industrial dyes were synthetic.

Pre-war (WWI) Germany dominated the commercial dye market accounting for 90% of all output. Many German scientists worked with distilled cemicals from coal tar, an abundant by-product of the industrial revolution at the time. The German succes was probably due to their investment in the scientific method and in training scientists themselves. Some further ‘by-products’ of the research include aspirin and saccharin.

After WWI the industry gravitated to Britain (ICI), the USA and Switzerland, also moving away from coal tar to petroleum based research.

Perhaps only now with the ‘green’ movement are we seeing a resurgence of interest in natural dyes. KOTHEA cautions the environmentally conscious reader to look carefully at claims of dyes to be natural. Whilst they may well be made from natural materials the processes used along the way can be VERY damaging to the environment.

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An alternative to a chlorine based bleach is Hydrogen Peroxide (H2O2). This has medical uses and domestic uses such as for bleaching hair.

Hydrogen peroxide occurs naturally by the action of sunlight on water and is simply water plus an extra oxygen molecule (2 lots of H20 plus one lot of 02 equals 2 lots of H2o2 for all you chemists). Hydrogen peroxide is quite reactive and so easily gives up some of its oxygen to revert back to water. This act of giving up oxygen to something else, like fabric, causes the fabric or impurities in it to be oxidised. The oxidised parts of the fabric are chemically changed and lose their colour. They remain there but their colour is changed. That’s what makes it a bleach and so the end products are just the oxidised fabric and water.

Natural linen has a light brown or beige colour. To go lighter than this it has to be either bleached, or bleached and dyed.

If your clients are environmentally conscious and concerned about the environmental impact of the products they buy from you, it would be prudent to ensure that your linen is hydrogen peroxide bleached rather than chlorine bleached.

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