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History of Natural Dyes in the Ancient Mediterranean Civilization

Maria J. Melo

LAQV/REQUIMTE, Department of Conservation and Restoration; Faculty of Sciences and Technology, Universidade NOVA de Lisboa, Monte de Caparica, Portugal

The colors used on textiles and artifacts, their social significance and the scope of their trade are part and parcel of a people's overall history.

Jenny Balfour-Paul, in Indigo

1.1 Introduction

1.1.1 Ancient Mediterranean World

The buildup of Mare Nostrum probably began much earlier than the 6th-5th millennium BC, and there is material evidence pointing to such activity as early as the 12th-11th millennium BC [1]. Mare Nostrum, the Roman name for the Mediterranean Sea, was to become the home of a global market that expanded beyond its natural borders in the first millennium BC. The Phoenicians, the Etruscans, the Greeks and finally the Romans shaped Mare Nostrum; a geographic as well as a cultural domain. It was also home to the first global dye, Tyrian purple, traded by the ingenious and industrious Phoenicians. The purple of Tyre was famous, as were the luxury textiles dyed and produced by the Phoenicians [2, 3]. It is said that the Greeks named the Phoenicians after phoinikes, the ancient Greek word for "red color," probably as a result of their famous purple trade.

By the time of the founding of the Mediterranean civilizations, what we would consider the classical palette for natural dyes had already been established, and the most valued colors were indigo for the blues, anthraquinone-based chromophores for the reds and 6,6'-dibromoindigo for purple. These colors were traded all over the Mediterranean, regardless of the distance to be traveled or the price to be paid. The natural sources for yellows were much more diverse, so yellows could generally be obtained locally. For dyeing, with the exception of some browns, all other colors and shades, including green and orange, could be obtained with these blue, red, purple and yellow dyes. This classical palette was preserved over centuries, if not millennia. The first adjustment resulted from the loss of Tyrian purple after the fall of Constantinople and the subsequent collapse of the Roman social and commercial web. This was followed by a new entry, cochineal red, brought by the Spanish from the New World [4]. However, even with the introduction of cochineal the chemical nature of the classical palette was maintained, as carminic acid is still a substituted 1,2-dihydroxy anthraquinone. This classical palette was only challenged by the audacity of chemists, who created new molecules, and colors never seen before, from the mid-19th century on [5].

1.1.2 Dyes from Antiquity

Natural dyes, discovered through the ingenuity and persistence of our ancestors, can resist brightly for centuries or millennia and may be found hidden in such diverse places as the roots of a plant, a parasitic insect and the secretions of a sea snail. By contrast, the bright colors that we see in the green of a valley, the red of a poppy, the purple of mauve or the blue of cornflower are less stable. Natural dyes were used to color a fiber or to paint. It is useful to distinguish between dyes and pigments based on their solubility in the media used to apply the color; dyes are generally organic compounds that are soluble in a solvent, whereas pigments, used in painting, are usually inorganic compounds or minerals that are insoluble in the paint medium (oil, water, etc.), and are dispersed in the matrix. A pigment lake is formed by precipitating a dye onto the surface of an inorganic substance, namely by complexation with a metal ion.

Dyeing, in red, blue, purple or yellow, is a complex task that requires skill and knowledge [6]; this is true now and has been for several millennia. Color is obtained by a chemical compound called a chromophore or chromogen-that which brings or creates color. To be used as a textile dye, the chromophore must also be captured as strongly as possible into the fibers, that is, it must be resistant to washing. Dyes can bind to the surface of the fiber or be trapped within. They are often bound to textiles with the aid of metal ions known as mordants, which can also play a significant role in the final color obtained (Box 1.1). As a source of aluminum ion, alum is an important historical mordant and was widely used in the past. Other important mordants used in the past were iron, copper and tin ions [6, 7]. Dyes, such as indigo, which are trapped in the fibers due to an oxidation-reduction reaction, without the aid of a mordant, are known as vat dyes.

As pigments, natural dyes have been widely applied in painting. For example, anthraquinones have been used as red dyes and pigment lakes from prehistoric times, and we can find written accounts of the use of anthraquinone reds and purples as dyes in ancient Egypt [7, 8]; anthraquinone lake pigments (e.g., madder red) were also very popular with Impressionist painters, including Vincent van Gogh [9b]. Red, purple and yellow lake pigments can be prepared by precipitating the dye extract with aluminum or other inorganic salts, such as alum [9] (Box 1.1). Pure dyes such as indigo have also been used as painting materials, for example, in medieval illuminations (Figure 1.1).

Figure 1.1 Medieval Portuguese illumination, dated from 12-13th-century, Lorvão 15 f. 50 (National Archive of Torre do Tombo, ANTT, Lisbon, Portugal). Dark blues were painted with indigo, whereas the inorganic pigment lapis lazuli was used for the background.

These eternal colors will be described in more detail in the following sections, after a brief account of the analytical techniques used to reveal the secrets of these ancient materials. The natural colorants will be organized according to the color: first, the anthraquinone reds, followed by the blues and purples, where indigo and its bromo derivatives play a major role. Yellows will close this overview. More information about other important ancient dyes is provided in EuroWeb Digital Atlas [10].

Box 1.1 Natural dyes as lake pigments

Figure 1 (Box 1.1) Preparing a lake pigment using lac dye as raw material. Design by Rita Carvalho.

A lake pigment is a metal complex, insoluble in water, prepared by adding a metal ion such as Al3+ to an aqueous solution of a dye extracted from a natural source. The complex will precipitate when a neutral molecule is formed, usually between a pH 6 and 7. In ancient times, the dye was extracted in acidic media (in the presence of Al3+), in neutral (in urine) or basic solutions (prepared with the ashes of plants, in which the oxides of alkaline metals are transformed into the corresponding hydroxides by reaction with water). The main steps for preparing a lac dye pigment are represented in Figure 1 (Box 1.1). Important lake pigments used in medieval manuscript illuminations were based on lac dye and brazilwood extracts, Figure 2 (Box 1.1).

The same kind of metal complex was used to bind the dye to a textile fiber; again aluminum ion was one of the most important metal ions used in the past, as illustrated in the following text. These metal ions, also known as mordants, were used to capture the dye to be resistant to washing. Other important mordants were iron, copper and tin ions. The structures of these dye-metal colors are complex, and there is still much we do not know about them. However, we are getting closer to the answers with computational quantum mechanical modeling (such as DFT, density functional theory).

Figure 2 (Box 1.1) Anthraquinone reds can resist for centuries or millennia, as shown in the precious Andean textiles (embroidered mantle with shaman figures, mfa 31.501, 0- A.N.D. 100; Museum of Fine Arts, Boston, USA). Lac dye, also based on alizarin core, was found on a Persian carpet (17th century, Museum of Fine Arts, MNAA, Lisbon, Portugal) and applied as a lake pigment in manuscript illuminations dating from the 12th century. As for any alizarin-based chromophore, it can take many shades of red, purple and pink, as on the background used to highlight the dragon, symbol of the struggle between light and darkness, but also of renaissance, painted in the monumental Bible Santa Cruz 1 (Municipal Library of Porto, BPMP, Porto, Portugal).

1.1.3 Unveiling the Secrets of Ancient Dyes with Modern Science

Identifying the dyes and dye sources used in the past has only been possible with the development, in the past two decades or so, of sensitive new microanalytical techniques [11-16]. Chromophores are extracted, then separated chromatographically and characterized by UV-VIS spectrophotometry or mass spectrometry; whenever possible, comparison with authentic references is performed. Currently, the use of high-performance liquid chromatography with diode array detection (HPLC-DAD) enables dyestuff characterization from as little as 0.1 mg of thread, by comparison with reference compounds. For the identification of unknown components, analysis by...