From indigo to pastel: the search for the most beautiful blue

When Pliny the Elder wrote Naturalis historia , a kind of encyclopedic work, he mentioned pigments used by the Egyptians, including indigo. Some remember that indigo was the name of the paper used to copy drawings and writings, and if you wanted your typewriter to make several copies of your material, before copiers became more widely available. This paper actually did not contain indigo pigment, but carbon black.

However, indigo has been used as a dye for a long time, especially for fabrics. The Tuaregs, a nomadic tribe from the north of Africa, who are close to the Berber peoples (or as they call themselves, the Amazigh, free people ), dye the cloth they wear with indigo, and that color leaves a mark on the skin, which is why they are called “Blue people”. This ancient knowledge of making a blue pigment that varies from cobalt to dark, almost purple, has been passed down since Ancient Egypt.

Indigo is truly one of the oldest colors that man has learned to extract from nature. Traces of its use go back at least to the third millennium BC, and perhaps much further. Blue stripes on the edges of Egyptian linen burial cloths from around 2400 BC are often cited as early evidence of the use of indigo. But this is where things get more interesting: in fact, it's almost impossible to say with certainty whether the ancient blue came from indigo or from a dyer's recruit. One record from Mesopotamia, a cuneiform tile about 2,600 years old, describes how wool is dyed blue – by dipping it in dye, then exposing it to air, then repeating the process.

In Morocco, women also use this plant pigment for skin care, adding it to a special mixture of clay, which takes on that beautiful blue color, and they call it “nila blue”. It cleans the skin and makes it radiant. The Moroccan Blue Nile is part of the hammam tradition of the Maghreb region.

Indigo, that dark blue, purple-blue shade, Newton simply added to the colors of the rainbow, so that there would be seven of them. However, the indigo blue of the Tuareg and Egyptians, the blue of the Nile, is actually a color obtained by fermentation of the leaves of the indigo plant, scientific name Indigofera tinctoria (literally translated “indigo color bearer”) which belongs to the vetch family, just like beans, peas, and soybeans, and precisely because of the color, it has industrial significance. Otherwise, indigo pigment is obtained from several other plants, such as Japanese indigo ( Polygonum tinctoria ), indigo species Indigofera arrecta , Guatemalan indigo ( Indigofera suffruticosa ), Chinese indigo ( Persicaria tinctoria ) and dyer's willow ( Isatis tinctoria ), which comes from the cabbage family.

 

The word indigo itself comes from the Greek Ἰνδικὸν φάρμακον  ( indikon pharmakon ) or Indian blue. Another, older, Latin name for this plant is Anila tinctoria , which is related to the name nila for this blue color.

 

True indigo, from the plant Indigofera tinctoria , is obtained by first soaking the leaves in water and then allowing them to ferment, in a process in which the glycoside indican is converted into a blue substance called indigotin. The precipitate containing the leaves and pigment is mixed with a strong base such as caustic soda.

 

In an alkaline environment, indoxyl is more stable and its oxidation can be more easily controlled. When air is introduced into the previously obtained extraction liquid, indoxyl is oxidized and two of its molecules combine to form indigotine , a blue, insoluble pigment. It is blue at pH 11, but yellow at higher pH values ​​of 13.

Caustic soda is not necessary for the very basic transformation of indoxyl into indigo. Traditional methods of obtaining indigo from Indigofera tinctoria mainly rely on fermentation, pH change and air oxidation, while strong bases can be used in certain industrial or laboratory variants of the process.

This plant grows in the area of ​​West and South Africa, Tanzania and India, which means that the ancient Egyptians had to get the pigment from far away or that they used some other plant like pastel, i.e. dyer's recruit for the production of indigo. Namely, we should not forget that Indigofera tinctoria is only one of the raw materials for obtaining indigo. As for the Tuareg, they trade in finished Nile, which often comes from different parts of Africa.

This and other plant-based blue dyes were and are used more for dyeing fabrics than in painting because this pigment changes color and fades over time.

And the pastel, blue indigo pigment from the Isatis tinctoria plant has an interesting history. Although the pigment is slightly weaker, lighter than the pigment from true indigo, it was a pigment that was used as far back as the Neolithic period. Its use has been recorded on Iron Age pottery, but perhaps the Celtic tribes immortalized this color.

Namely, the Gaelic, or Celtic, tribes of the British Isles used to apply blue paint to their faces and bodies before going into battle to intimidate their enemies. Julius Caesar also writes about this in his Commentarii de Bello Gallico. This is why the Romans called these tribes the Colored People, or Picts .

This dye from the disreputable cabbage plant Isatis tinctoria will be used a couple of centuries later to dye threads for a series of tapestries about hunting unicorns. This same plant, for which you would not look back, will enrich the French province of Languedoc and especially the city of Toulouse, as well as Albi, a region that many centuries later will produce the famous impressionist painter Henri de Toulouse-Lautrec.

The dyers’ pigment merchants would have their own guild, a guild as we would call it, and they were very wealthy people. Toulouse owes part of its wealth and beautiful buildings to the dyers’ pigment merchants. Ironically, Toulouse is known as the “city in pink” because the entire city has a pink tone, due to the color of the pavements and the reddish-pink buildings. In French, this pigment is called l e pastel des teinturiers , which is how pastel colors would get their name. And the origin of the word, pastel, is very ordinary – it means paste or pate.

Pastel des teinturiers is obtained by a similar process as indigo from Indigofera tinctoria . The leaves are harvested, chopped and left to ferment. They contain colorless precursors, primarily indican, which turns into indoxyl during decomposition. After that, the mass or extract is exposed to oxygen by vigorous mixing. Indoxyl is oxidized and combined into molecules of indigotine, an insoluble blue pigment. The pigment is deposited, separated from the liquid, washed and dried. Traditionally, the resulting mass was shaped into balls, known as cocagnes , which were further dried and fermented. Later, they were ground and used to prepare dye for textiles. The difference is primarily in the plant source: chemically speaking, the key blue pigment, indigotine , remains the same.

Indigo as a pigment, whether obtained from real indigo or dyer's recruit, is not exactly the best color for painting, but there are works of art where it is confirmed that artists used this pigment. In addition to the aforementioned tapestry Hunting for Unicorns, there are also the Lindisfarne Gospels, from around 700. This is perhaps the most beautiful example of the use of recruits.

We don't know, and probably never will, how ancient peoples like the Celts discovered that a single green-leafed plant could be the raw material for blue pigment. It was probably a coincidence and it is possible that the process of changing the color from the green plant, through the brown and greenish solution to blue looked like magic to them. Later, this knowledge was perfected and became a lucrative trade. It is interesting that in the dyeing trade, in order to achieve the alkaline environment necessary for the dye to appear on the fabric and fix it, often fermented urine, even human, was used. One of the regions of the Netherlands , Brabant, is precisely known for this, given that it is the region where textiles were produced and dyed, and that workers brought their urine as raw material.

At the end of the 19th century, the German chemist Adolf von Baeyer succeeded in synthesizing indigo. In 1897, synthetic indigo began to be produced, which changed the history of this dye and the textile industry. For his work on organic dyes, including indigo, Baeyer received the Nobel Prize in Chemistry in 1905.

French artist Yves Klein is known for a specific shade of indigo, known as International Klein Blue (IKB). He painted monochrome paintings in a special binder that highlights the intense, almost luminous quality of indigo. However, Yves Klein did not use indigo or other plant pigments, which are too rare, but rather an ultramarine pigment.