Dye Materials - From Plant to Fiber

 

Color, whether animal, vegetable or mineral, fascinates us all. Our obsession with color and efforts to manipulate it must be amongst the oldest of social pursuits. Aboriginal cave paintings using colored clays have been dated to 40,000 years ago, and the practice of daubing the skin with colored patterns may be even older.

Although weaving and dyeing predate all written records, it is reasonable to suppose that early weavers tried to stain their cloth with a pleasing color. The dyeing of animal skins may even have predated weaving although leathers are less amenable to color variation.

Natural dyes

For early societies the most obvious source of stains, which is what dyes really are, were the local plants. Until the middle of the 19th century, the only way to obtain dyestuffs was to laboriously squeeze, crush or boil them out of plants and to a lesser extent insects.

However, transferring the brilliant colors and multitude of shades from plants to fabrics is not simple. Though it is likely an enormous range of plants was examined over time, very few provided dyes which did not fade rapidly or wash out easily. This scarcity of reliable dyes made the trade in them very lucrative. When the first synthetic dye, mauveine, was manufactured in the mid 19th century, it sold readily at a higher price than platinum.

New sources of natural dyes were much sought after. Brazil derived its name from the dyestuff brazilin, a deep red dye obtained from the heartwood of Caesalpinia brasiliensis trees. The abundance of these trees so impressed the Portuguese when they first landed (accidentally) in that country that they gave it the name Terra de Brazil, after the dyestuff. Brazilin was already well known in Europe, where it was imported mainly from India.

Few accurate representations of flowers are found in rugs, owing partly to the difficulty of convincingly duplicating flower color as well as flower form. Only in the sophisticated court or town carpets, which use a much greater range of shades than tribal pieces, are plants recognisable by their colors. Tribal weavers had to make do with relatively few basic dyestuffs for a long time, and the natural dyes still in widespread use at the end of the 19th century were the same as those found in the oldest known rugs and rug fragments.

It should theoretically be possible to make any color by mixing the three primary ones - red, blue and yellow - plus black, in the right proportions. But in practice it does not work out this way. Dyes are not inert substances whose only property is color but chemical compounds interacting with one another to affect color, light-fastness and other properties. An ideal dye is one which does not fade, wash out, chemically degrade the fabric, or interact with any other dyestuff during or after application or on exposure to light. Only two major natural dyes, indigo (blue) and madder (red), come close to this ideal.

Indigo This beautiful coloring substance is the major blue dye which was used not only in all carpet-weaving countries, but as a pigment in painting and cosmetics and for dyeing cloth in virtually every society we know of, Its name comes from the Latin, indicum, a word derived from 'India', the source of the great bulk of indigo imports. Fragments of cloth dyed with indigo have been found in 4000-year-old Egyptian tombs.

In 1900 the economic value of indigo produced from natural sources equalled that of all other dyestuffs made at the time. However the first synthesis of indigo had already been made by the great German organic chemist, Adolf von Bayer. He was so enraptured with the dye that when he was given money for his thirteenth birthday he spent nearly all of it on a block of indigo. By 1910 production of natural indigo had fallen by 90 per cent, for the synthetic indigo had become freely available from the German chemical firm, Badische Anilin (now BASF). Apart from wartime, agricultural production of indigo never reached substantial levels again.

Indigo does not occur naturally. It arises by a series of chemical reactions on a substance called indoxyl, found in a variety of closely related chemical forms in a wide range of plants. The best known are those of the genus Indigofera, such as I. tinctoria, the main plant used for the dyestuff in India. At various times I. arrecta and L sumatrana were also used, as well as others of the Indigofera group. Indigofera species are found in most temperate and tropical areas, including Australia. The abundance of indoxyl derivatives in a wide range of plants simply reflects the widespread distribution of indole (the parent chemical structure of indoxyl) and its derivatives in plants and animals.

In Europe and the rug weaving areas of Persia and Turkey the best-known indigo-producing plant was dyer's woad, Isatis tinctoria. This is a much poorer source of indigo than Indigofera tinctoria but was used so extensively in Europe that the importation of the Indian productwas bitterly opposed. In other parts of the world the dye was obtained by cultivating Polygonum tinctorium and Lonchocarpus cyanescens. Tyrian or 'imperial' purple, held in the greatest esteem as a luxury in the ancient world, wasa compound of indigo containing bromine and was obtained from a marine mollusc, Murex brandaris. Such is the march of technology that Tyrian purple, once reserved for Roman emperors, is now of no commercial significance. Indigo, almost alone of the traditional dyes, is still used - for dyeing jeans - but almost entirely in its synthetic version which is chemically and visually identical to natural indigo.

Indigo is a good dye for cotton and even better for wool. It is not chemically bound to the fibre and so is never entirely fast to rubbing, particularly on cotton; hence the faded, creased effect so popular in jeans. It is very insoluble, and to be transferred to the fabric it must be chemically transformed to colorless, soluble indigo white which penetrates the fibre. When removed from the dye bath it is reoxidised by air to the original blue pigment.

Madder What indigo is to blue, madder and related dyes are to red. Madder is extracted from the dried crushed roots of species of Rubia, particularly R. tinctorum. Like indigo, it has been used for thousands of years. But unlike indigo, it is a complex mixture of substances, the principal one being alizarin, from the Arabic word for root. The proportions of its various components are influenced by agricultural variables and particularly by the age of the plant. Madder from old plants may contain as many as nineteen components, and a considerable variation in color is possible using plants of different ages from the same area.

The plant has been grown in most parts of the world for its dyestuff. The chemical components of madder are anthraquinones, a class of chemically very stable substances widely distributed in living organisms.

Dyestuffs closely related to the madder components have been found in Australian plants, such as Coprosma australis, and as pigments in marine creatures, particularly the Echinoderms. The insect-derived dyestuffs, cochineal (from the scale insect Dactylopius coccus), lac (from the the insect Coccus laccae which infests a number of Ficus species including the Banyan tree of southern Asia) and kermes (from the insect Coccus illicis found on the oriental oak Quercus i1ex and Q. coccifera) are chemically very similar and have all been used for reds in textiles, although it is not certain that kermes was used in oriental rugs as well.

Neither madder nor the substances just mentioned are particularly useful dyes in themselves, for all must be applied to fabrics with the aid of a mordant. This is a metal salt which is applied to the textile fibres before the dye. The salt combines with the dye to form a new salt, called a lake, which binds well to the fabric and helps fix the color to it.

Not only does the mordant ensure fastness to washing, but varying the particular metal produces different shades of madder. For example, aluminium provides rose-red; tin, redviolet; chromium, brown-violet; iron, violet-black.

The variety of shades from different mordants, plus variations in the madder itself and the possibility of combining madder with other anthraquinone dyes (kermes, lac and cochineal), all gave the dyer a wide range of colors from pale orange to nearly black.

Particularly favoured was Turkey red, a clear brilliant red formed by using both calcium and aluminium in the mordanting process. It was widely used in rug-producing countries and in Europe for dyeing cloth, particularly military uniforms in France and Britain.

Alizarin, the principal coloring substance in madder, was obtained synthetically in 1868 and quickly supplanted the natural products entirely. These are now of no commercial significance, though cochineal is still used as a food dye.

Yellow In marked contrast to indigo and madder, natural yellow dyes are generally not of the highest quality. They are often fugitive in light and many do not give good depth of color. Consequently, old rugs with good yellows are much rarer than those with good reds or blues. The reason lies in the chemical nature of the yellow dyestuffs. They are most commonly related to flavones, a class of substance to which many flower pigments belong and which are chemically reactive towards oxygen in the air.

Yellow dyes can be obtained from a multitude of plants. Possibly the best yellow in terms of lightfastness is luteolin. It was widely used, particularly in old Turkish rugs, many of which still exhibit abundant and beautiful yellows. Luteolin is obtained from a range of plants of which the best known is Reseda luteola, the dyer's weed or weld, but it is also extracted from Verbascum, Hieracium and Digitalis species. Luteolin is more resistant to oxidation than other natural yellow dyestuffs.

The most widely used yellow is quercetin, widespread in plants and often extracted from species of Euphorbia growing in Turkey. It is a less satisfactory dye than luteolin, however, and is commonly mixed with madder to give a pleasing color.

Other common sources of yellow, mainly quercetin dyes, are vine leaves, Vit is vinifera, onion skin, Allium cepa, St John's wort, Hypericum empetrifolium, and the pomegranate, Punica granatum. Even straw has been used to produce yellow.

A very expensive yellow dye, crocin, is obtained from the styles of the saffron plant, Crocus sativus. It is chemically unrelated to the yellow dyes already mentioned, belonging to another class of plant pigment similar to that found in carrots. It is expensive because of the huge numbers of flowers necessary to extract a reasonable amount of dye.

All the yellow dyes are applied with mordants, usually aluminium.

Black Wool from black sheep is best for this but is rarely available in the necessary quality or quantity.

Repeated dyeing with indigo gives a very dark blue that is difficult to distinguish from black unless viewed at an angle. This effect is expensive to obtain but is highly regarded, particularly in Iran.

The simplest way to obtain black is to use an iron mordant with tannic acid, a complex mixture of organic acids obtainable from a wide variety of botanical sources. But the acid/ iron combination degrades the wool, and areas of rug pile dyed this way wear more quickly than the rest of the rug. This gives the pile an embossed appearance which is sometimes attractive, but the black wool eventually erodes away almost entirely and exposes the rug's foundation, the warp and weft.

Brown This can also be obtained with limited amounts of tannin and an iron mordant, but again, this combination erodes the pile. A good brown which does not damage the wool is obtained with juglone, a simple naphthoquinone found in the fruits and leaves of the walnut tree, Juglans regia. It is directly applied to the wool without mordant or other treatment.

Green Many rug books will tell you that as green is sacred to the Prophet Mohammed its use is forbidden in carpets other than prayer rugs. This sounds a little like a rationalization of the inevitable. A good, lightfast green is possibly the most difficult color to obtain, which probably explains why large areas of green are rarely found in old handwoven rugs. Although green may be readily formed from double-dyeing with blue and yellow, the two colors do not fade at the same rate. Some very attractive pale blues in old carpets were in fact originally green, but the yellow has faded completely and left only the indigo.

Copper salts have also served as mordants to give green, but this makes the wool brittle.

A good green can be obtained by dyeing naturally yellow wool with indigo, but the availability and quality of the wool are limiting factors.

These problems were unsolved till the advent of synthetic dyes, and the early weavers and dyers had to content themselves with a very limited use of green. This must have been frustrating, given the superabundance of this color in every conceivable shade in nature.

White White in carpets is usually undyed wool, and is in fact not white but cream. It appears white only in contrast to the surrounding colors.

Cotton is occasionally used for white in small areas, but its poorer wearing qualities preclude its more general use.

Bleaching wool white, only damages the fibre and lessens its wearing qualities, so this is not (or should not be) a common practice. Chemical bleaching was probably unknown in rug weaving areas before this century.

Development of synthetic dyes

It is certain that dyers tried all sorts of tricks to increase the range of shades and tones available from natural dyestuffs. They were largely unsuccessful because they lacked knowledge of chemistry, although they did discover long ago that modifications to madder could reduce the proportions of some of the components and thus alter the color.

In the 18th century it was found that treating indigo with concentrated sulphuric acid gave another dye, indigo disulfonic acid. This was more soluble than indigo, and though less fast to washing, it could be applied directly to wool and was soon in wide use in the Middle East.

The synthetic material picric acid was used as a yellow dyestuff, mainly on silk. It was originally obtained by treating indigo with nitric acid and, in a unique duality of function was used as an explosive in World War 1.

Despite the existence of these substances, virtually all dyeing was still being done with plant or animal products when, in 1856, the British chemist William Henry Perkin prepared a purple dye called mauveine.

Perkin was in fact hoping to obtain quinine, used in the treatment of malaria. The purple crystals he eventually obtained proved to be an excellent dye, particularly for silk, a fibre which was troublesome to dye using the techniques of the day.

In 1859 the next synthetic dye appeared, a brilliant red-purple called fuchsine or magenta. It was of an entirely different type from mauveine and was widely used in rug-weaving countries.

The discovery of mauveine and fuchsine, coupled with the rapid development of organic chemistry, led to the introduction of many new dyes, but not all of them were of good quality, particularly the earlier and cheaper ones. Some like fuchsine shawed very poor fastness to light and others to washing, so they faded rapidly or bled into adjacent colors when washed, leaving large areas of a rug drab and discolored. This had a catastrophic effect on the weaving industry, and draconian steps were taken in Persia to prevent the use of these dyes.

These early synthetics are often termed aniline dyes in the rug literature. The word derives, ironically, from the Arabic word for indigo, anil, but its use to describe these early dyes is usually inaccurate. When indigo is strongly heated it chemically decomposes and the colorless liquid aniline distils off. It was when oxidising a crude sample of aniline that Perkin first obtained mauveine, although its formation depended on the presence of an impurity and not on aniline alone. Fuchsine was also obtained by oxidising crude aniline' although under different conditions. Aniline became an important starting material in the young organic chemical industry when other methods for its preparation were developed, but the vast majority of the earliest synthetics were not prepared from aniline.

The first of the somewhat better azo dyes was developed in 1863. Azo is from azote, French for nitrogen, and the dyes were so called because their chemical structure includes a group of two nitrogen atoms. Several a ' zo dyes developed in the mid 1870s were later used in rugs, including Ponceau 2R (a bright red), Orange IV and Roccelline (a purple-red that often ran distastrously during washing). Apart from Roccelline, most azo dyes did not suffer from the problems of the aniline dyes, but the reputation of synthetics generally had already been damaged by the poor quality versions and they retained a bad name for some time.

Nevertheless within 50 years of Perkin's discovery of mauveine the natural dyestuffs industry was almost defunct. Few carpets made since 1900 are entirely free of synthetic dyes, while those made since World War 11 are unlikely to contain any natural dyes at all. It is often difficult, however, to distinguish natural from synthetic colors in rugs without detailed chemical analysis.

Advantages of the synthetics

Why did dyers persist with synthetics after their unfortunate early experiences, especially as they were certainly not cheaper initially than the natural dyestuffs?

To understand this, it should be remembered that dyeing with natural colors was extremely laborious. Recipes for special effects were jealously guarded and handed down from one generation to the next. There was little room to develop new colors since the underlying chemistry of natural dyes, though fairly simple, was unknown, and the only route available was trial and error. In contrast, synthetics offered the dyers vivid colors and shades they did not have and were much easier to apply.

Most important was the reliability of synthetics; each batch of dye gave the same results. This must have been a great boon, for natural dyes were often unpredictable. Natural materials contained only very small amounts of pigment, and their color properties varied according to location, growing conditions and other factors. This explains the variations in color intensity called abrash.

Even if natural dyes were bought in the markets, their source and quality were uncertain and adulteration was commonplace. It took just as much labour to produce a poor result as it did a satisfactory one, so synthetics that gave consistent results must have seemed well worth the sacrifice of some degree of quality.

.... and disadvantages

For all these advantages, synthetics often seem less pleasing aesthetically than natural dyes, though it is probably a mistake to assume that what appeals to European tastes also appeals to the weavers.

The contrast can be seen where synthetic dyes have been used for some areas of a rug and natural colors for other parts. While this gave greater scope to the weaver's imagination, it sometimes produced a garish effect: the good synthetic dyes changed or faded very little, but the natural dyes did change or at least mellowed. This imbalance is particularly evident where vivid shades of synthetic orange and green, some of them almost fluorescent, clash with a rug's more mellow natural dyes.

Hence this tendency of good synthetic dyes to resist fading or changing is not the advantage one might think. Synthetics generally do not provide the softened, mellowed effect that makes older, naturally dyed rugs so attractive. This mellowing is due to slow reaction with the oxygen in the air, coupled with the effect of light on the dyes and presumably to a lesser extent on the fabric itself. The results are largely fortuitous, and in many instances the color harmonies we admire in old rugs are certainly very different from the weavers' intentions.

It is possible that the synthetic dyes will ultimately mellow, but the process will take much longer than with natural dyes. This is especially true of modern chrome dyes, so called because they are mordanted with potassium dichromate. Chromium salts as mordants have only been available and used relatively recently.

Analysis of dyes in rugs or other weavings attracted little attention until very recently. The techniques are not too difficult, but they require removing a sample of the pile - a sacrifice that is understandably not popular with rug owners. Besides sometimes giving an indication of a rug's origin (for example certain combinations of dyestuffs are found only in plants from certain regions), a synthetic dye can occasionally give a maximum age for a rug - it cannot have been woven before the patent date of that particular dye.

Modern methods make it possible to determine exactly all the dyestuffs in any rug and to duplicate them in the laboratory, but the costs would be prohibitive and it is unlikely ever to be done.

The dyeing process

The early dyers went to great pains to keep their techniques secret. It was their livelihood and over the centuries the whole industry became imbued with a self-protective guild mystique. Nevertheless, it is remarkable that so complex a chemical process as dyeing ever developed. To illustrate what was involved, let us briefly examine the use of indigo.

Dyeing wool for oriental rugs was largely a sedentary occupation because it was done in large earthenware vats which were difficult to transport. Thus rug weavers either brought their wool to the dyehouse to be colored, or purchased pre-dyed wool. Some of the nomadic weavers dyed their own, but this was probably not common since a great deal of knowledge and skill was required for good results.

The fundamentals of the process were quite simple. In essence the wool was washed thoroughly, steeped in dye in the vat (called vat dyeing) for variable but often considerable periods of time, removed, rinsed thoroughly in water and dried. Different color intensities were obtained by varying the time in the vat or by repeating the process until the desired shade was achieved. If mordant dyes were used (as they mostly were), the wool was first thoroughly impregnated with a solution of a metal salt, usually alum. The final washing was important, particularly for mordant dyes, and water with a relatively high limestone content was preferred.

With indigo no mordant is needed, but the dyestuff must first be converted to the colorless form, otherwise it is too insoluble to incorporate into the fibre. This process was carried out by fermenting the dye with organic material in a large vat heated over a fire. It was a slow and tedious process which often took many days and necessitated constant attention, especially to adjustments in acidity and temperature. When the conversion was complete, dyeing could be carried out till the dyestuff was exhausted.

Though it sounds simple, the constituents needed for each step had to be discovered by trial and error, so that virtually every dyer had individual and often strange-seeming recipes. For example, green fruit was used to increase acidity and urine to reduce it. Some of these fascinating recipes no doubt enhanced the notion that dyers possessed magical powers.

Dyeing with madder and other mordant dyes was similar but somewhat less complicated, since fermentation was unnecessary. Special effects were more laborious: Turkey red, for example, took nearly three months to achieve and involved many steps.

Most plants and other sources did not contain large amounts of dye, so the vats became exhausted fairly quickly and the whole process had to begin again. This is probably why dyers often used indigo in prepared and partly concentrated form, rather than bothering to extract it from the plant themselves. Indian indigo was commonly imported into Turkey and Persia, even though the plant grew in both countries. Natural indigo, even of the best quality, also came mixed with other material extracted at the same time. In the case of woad Isatis tinctoria, the amount of actual blue pigment in each block was often as little as three per cent.

The return of natural dyes?

In Turkey recently, a program was established in some of the rug-weaving villages to revive production of rugs with handspun wool and natural dyestuffs. It is doubtful if the villagers go through the whole laborious process, however, as this would make the finished article prohibitively expensive.

The production of handwoven carpets at an affordable price depends on cheap manual labour. As living standards rise, so inevitably will the cost of the rugs, unless ways can be found to shorten and simplify the process. Sadly, perhaps, the very labour-intensive dyeing process is an obvious place to economise, and what we might view as the romantic era of natural dyeing is probably almost over.

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