The author had worked with ancient Asian colours in art, before she came across the ‘mystery’ of purple during a long stay in Lebanon at the end of the 1970s. Moving frequently from one country to another, she was looking for a dry purple pigment, easy to carry along and use when necessary. But here began the problem. Even in Beirut, so close to the sought-after material, close to the hills of shell debris from ancient purple production, no one could give advice. References to purple in literature did not offer a solution. There was only one possibility to obtain it: do it yourself !

 A diver provided the sea snails; they were mostly Hexaplex (Murex) trunculus, a few Bolinus (Murex) brandaris and one Stramonita haemastoma (Thaïs haemastoma). Two scientists from the American University of Beirut, who had dealt with the subject a few years ago, were willing to experiment. The shells were broken; the hypobranchial glands cut out with scissors and the secretion was extracted by dipping them into water. After a short while, a purple-coloured juice was obtained, but no method was found to turn this into a pigment. The purple liquid was centrifuged in a test-tube and has still not changed its colour – a red like fresh dried blood – after more than thirty-seven years.

In the nineties, the author was challenged again and made a new attempt to understand the secrets of purple for use in art. In ancient times, this colorant served to dye clothes for the elite and the pigment had the status of a by-product only. To obtain it, knowledge of the dyeing process was inevitable.

Among all the written testimony on the production of purple in antiquity, the eminent source is Pliny the Elder in the 1^st century AD. In his Natural History, Pliny described the procedure of purple dyeing as it was customary during the Roman Empire, giving posterity a fairly precise account of his observations. He left us a recipe:

The vein already mentioned is then extracted and about a sextarius of salt added to each hundred pounds of material. It should be soaked for three days, for the fresher the extract, the more powerful the dye, then boiled in a leaden vessel. Next, five hundred pounds of dye-stuff, diluted with an amphora of water, are subjected to an even and moderate heat by placing the vessels in a flue communicating with a distant furnace. Meanwhile the flesh which necessarily adheres to the veins is skimmed off, and a test is made about the tenth day by steeping a well-washed fleece in the liquefied contents of one of the vessels. The liquid is then heated till the colour answers to expectations. (Pliny, NH IX, 38)

Until recently, the procedure could not be replicated convincingly. Because translations and annotations were often – not always – erroneous, many trials ended in frustration. Pliny was a reporter, not a dyer; he told us what he saw without really understanding the procedure. He saw that three days were needed to achieve the huge task of crushing thousands of shells before proper dyeing could begin. The glands, containing the precursor of purple, had to be cut out, an easy task which even children could perform. By adding salt, the fleshy mass was kept from decomposing too quickly and protected from an invasion of flies in a warm climate. His suggestion that the glands in the vat, “diluted with an amphora of water, are subjected to an even and moderate heat” is perfectly correct. But the preceding sentence “then boiled in a leaden vessel” is the key to subsequent failures. Any attempt to boil the liquid will render dyeing impossible. A successful purple dye-bath needs the right alkalinity (which Pliny does not mention) and a moderate temperature during the several days required by the dyeing procedure. 

In 2001, the author succeeded in reconstructing a fermentation vat using fresh Hexaplex trunculus from the Mediterranean Sea instead of the dried purple glands mentioned in John Edmonds’ book. John Edmonds was one of the first scholars to prove that a purple dye vat could be prepared following Pliny’s description. He lived in England and had no access to fresh sea snails but used dried ground murex glands, sent to him from Israel, adding the flesh of dog whelks (Nucella lapillus). This brilliant idea gave him the opportunity to bring to life the first purple vat without chemicals. Edmonds’ experiments were preceded by works of Joseph Doumet who, guided by Pliny’s mention of ‘plumbum album’, managed to set up a vat in a tin vessel, the metal helping to induce the reduction of the colorant. The colours he obtained were of light violet hues.

The fresh glands were cut out and soaked in water where they quickly turned into violet colouring the water as well. A glass jar with this mixture was placed into a double-boiler, some potash added to achieve an alkalinity between 8-9 pH and then – with the lid on – heated constantly for a week at about 45° C. Three days later, the liquid turned blue-green. Under correct circumstances, the fleshy parts of the glands provoke a fermentation, which leads to a healthy dye-bath at the end of a week. Wool or silk dipped into the vat and left for a few hours, will come out green and – if the dye bath is kept in the dark- will oxidize into a violet purple. If the dyeing is carried out during daylight, the result will be a blue purple colour.

In the south of France the escargots de mer are available at local markets because the molluscs are considered a delicacy. Two kilograms of Hexaplex trunculus, (Fig.2) bought at a shellfish stand were carried home covered with wet cloth to keep them fresh. As potash is a necessary ingredient for the dye-bath, it is prepared beforehand. Branches of dry oak wood are burnt, the ashes mixed with water and then strained through a paper filter or fine cloth. Wood ash or potash is needed to regulate the pH. 

The murex shells are then cracked open in a stone mortar using a stone pestle (a little hammer will do too) to expose the hypobranchial gland situated diagonally opposite the aperture of the seashell. Two kilograms of medium sized H. trunculus yield about 100 glands. The gland, which contains the precursor, a colourless secretion, so much needed for the purple production, is almost invisible, but can be recognized next to a fine black line. It has – if the mollusc is fresh and alive – a texture reminiscent of shrunken fine muslin. They are then placed in a little water until all glands are removed. There is no need for salt or sea water. The debris of the broken molluscs is set aside in a different bowl and also covered with water. This additional liquid helps to fill up the glass jar, used in the vatting procedure.

When the cut out glands come into contact with air, they immediately start a dramatic change: from colourless to yellow, to green, to blue, then blue-violet or red-violet.

This metamorphosis cannot be used to obtain stable shades of purple. Trying to achieve a colour nuance by pressing it down with a flat stone will stop the development for the time being. Even after years, once the stone is taken away, the colour development will continue. 

Thereafter the mixture is placed in a 1.5 litres glass jar. (Fig.4). Some of the waste water is added to fill the container up to ¾ of its volume. About three days later, the fleshy parts of the glands seem to start a bacterial action which leads to the reduction of the purple dye. The reduction process is the clue to successful dyeing. Shellfish purple is a vat colour of the indigo type. Experiments done in Padden’s laboratory in 1999 on a woad vat identified the newly named Clostridium isatidis bacteria as responsible for the process.  Possibly, a similar bacterial process is the reason for the reduction of the purple dye; the bacteria in question are still to be found and named.

From the beginning, the alkalinity is checked and adjusted by adding the prepared potash to reach a pH of about 9. The jar is covered lightly with a lid to let the developing gases escape and then placed into a narrow, tall pot half-filled with water and a lid to avoid light.

Maintaining a temperature of 45-50° C is the second condition necessary for a successful reduction of the dye during the following days. If there is no electric heater with a regulator at hand, a tea candle will do too, but it needs to be replaced regularly, even at night. 

Every day, taking care to avoid direct light, the alkalinity needs to be adjusted, using potash (potassium carbonate), stale urine or washing soda (sodium carbonate). Use of the latter prevents a too strong dilution of the liquid. Once a vat is dyeing properly, heating can be interrupted. The vat – protected from light – reheated any time later, will continue the dyeing procedure.

In a few days, the colour of the vat changes from dark violet to a blue-green and finally yellow-brown. A strong odour indicates that the process has begun. Foam develops at the surface of the jar, but contrary to Pliny’s description, no fleshy parts are swimming on top. Instead, fine sediment settles soon at the bottom of the jar. (Fig.5). To the experienced dyer the change of colour from all purple to a clear light yellow brown in the jar is the sign that the vat is ready for dyeing: the dye has been reduced to its leuco form. Leuco stands for the colourless, soluble form capable of penetrating the fibres. Once the dyeing time is over, they are ready to oxidize into the insoluble final shade. When the vat solution is reduced, the pH fluctuation is over. To test the dye-bath, a washed woollen thread is immersed for a few hours, taking care not to expose the vat to direct sunlight. If the colour achieved is satisfactory, a piece of unspun washed wool is left in the vat a minimum time of five hours or better overnight. It is possible to dip wool or silk several times into the dyeing vat to increase the intensity of the colour, as one would do for an indigo bath. The vat though needs a rest of several hours before attempting another immersion. To achieve a very deep shade, however, it is better to prolong the dyeing time and to use a very concentrated vat.

When taken out, the wool – now green-yellowish in colour – is squeezed in the hand (gloves recommended) to avoid exposure to strong light and then transferred into water in a covered container for about an hour. There it oxidizes to its final deep violet. If the same manoeuvre is carried out without protection from daylight, the result is most likely a bluish specimen. Since the very beginning of purple production some thousand years ago, dyers must have known by experience the different hues they could achieve. If light fell into the reduced vat, the result was likely to turn out sky to dark blue. An exhausted fermentation vat will produce pale or even grey blue. Blue shades on textiles were rare in the Bronze Ages. They must have been a much sought-after commodity. Rolf Haubrichs suggests that dyers may have collected the outcome of different vats by separating the nuances into different heaps until there was enough of each shade for commercial demand. 

A purple vat can be prepared with fresh, frozen, even salt dried murex glands. For the latter a cupful of an old vat is vital to start the reduction of the dye. Fresh murex glands are laid on rough salt and dried for storing. (Fig.6). The ancients certainly knew this type of preserving food. Salt dried glands could be transported far away from the coastline. This would explain the purple production in Upper Egypt for instance. Dyeing with salt dried murex glands always ended in an intense deep blue violet colour.

Knowing that Vitruvius  mentioned honey as a preserving medium, the author kept fresh glands in honey for several months. Used in a fermentation vat, the result was too weak to be appreciated. Employing chemicals such as sodium dithionite (Na2S2O4) though, fleece and woollen yarn showed a warm plum like colour. This synthetic reducing agent helped the author to easy dyeing with purpurissum, the dry powder, as well. Instead of a week’s time needed for a fermentation bath, the ‘modern’ vat functions within 30 minutes. The brightness of all shades was amazing.

Experience has it that the colour outcome of a purple dyeing bath cannot be predicted; depending on the amount of glands used, the freshness of the molluscs, the geographical location of the specimen, their gender, the shades obtained vary. The first immersion brings the deepest hue, every following dip looses in brightness, turns out more subtle. When the dye bath (using sodium dithionite) is exhausted, a sample may turn out salmon pink, even turquoise green. It seems evident that dyers of antiquity had their own well kept recipes to achieve certain results. 

The different shades of purple have been widely discussed by scholars. They have been divided into categories, explaining the possible techniques linked to the murex type used. They were given poetic names. The author restricts herself to the solid base of experiences. Her goal was the pigment for painting. Nonetheless, the results obtained may elucidate questions about the colour residues excavated in the Qatna royal tombs.

The choice of wool and silk of different origins must have influenced the hue of purple as well. Modern experiments are surely ‘burdened’ by modern products which may contain pesticides and other ingredients nonexistent in antiquity. Dyeing linen in a purple bath was unexpectedly satisfactory. Adding honey to the vat in order to improve the quality of the colour is of no use at all. For a long time, silk dyeing seemed uninteresting; in a normally heated vat it kept turning blue. Until one day chance had it that the vat got cold: the silken handkerchief reappeared in purple pink!

Of the remaining colours which we mentioned as supplied by patrons by reason of their costliness, the chief is purpurissum. Silversmiths’ white is dipped at the same time and in the same manner as purple cloths, and absorbs that colour more quickly than the wool. The batch which is the first steeped in the cauldron seething with fresh dyestuff is the best. The next quality is obtained by adding silversmiths’ white to the same liquor, when the first batch has been removed, and, as this operation is repeated, the excellence of the product diminishes with the fall in concentration of the dye.

Mollusc purple and purple dyeing is a vast subject which is extensively described in recent literature. It is most likely that the production of purpurissum was a by-product of the purple dye factories. The “painter’s paint” as Pliny called it was also used as make-up: this was a favourite colour for embellishing the feminine complexion, sharing this distinction with ceruse.

Besides the fossilized purple fragments found in the Royal tomb at Qatna (1400 BC), not many historical samples of pigment or purpurissum have been discovered to date. Excavations at the site of Akrotiri, a Late Bronze Age town on the island of Santorini, Greece, brought to light purple in small amounts on several wall paintings. This is the earliest assessment of purple pigment in painting used in the Aegean world during the period from 1700 to 1600 B.C. More purple was identified at the archaeological site of Trianda on the island of Rhodes, supporting the results of Akrotiri. Samples of clay impregnated with 6, 6´-Dibromoindigo, dating from the 3rd century B.C.E, were discovered on the island of Zembra, Tunisia. It is still unclear if the presence of this coloured earth is associated with the waste waters from a purple production site or accidental in another way. Purple was found in Pompeii as well, not on paintings, but in the form of small cubes ready for use, well described by Selim Augusti and Monica Salvadori. They were first analysed by Humphrey Davy in 1825. He concluded that the material contained silica, alumina, chalk and traces of iron, but he was not able to determine if an animal or a vegetal component was used. Later on Selim Augusti, through microscopic observations of many samples, showed that all of them contained diatomeas (silica algae) and some of them, fossil residues of foraminifera and radiolarian. Chemical analyses confirmed the presence of silica, calcium carbonate and traces of iron. A multi-partner study lead by Mark Clarke in 2005 still left open many questions about this pigment. Few attempts have been made so far to reproduce Pliny’s purpurissum.

Heinz Roosen-Runge is known to have carried out experiments in Naples in the 1960s. He tried to reproduce old recipes for his research on purple manuscripts. He writes: «The method of reproducing purpurissum mentioned by Pliny (H. N. XXXV, 44) as a painter’s pigment, by adding chalk to the juice of one of the murex types, resulted in a rather weak and subdued tone because the chalk breaks the intensity of the purple red». He was not convinced that his result was good enough for manuscript painting. He guessed that the thirty murex glands he used were not sufficient to create a beautiful colour, but he did not repeat the experiment

Preliminary work on purpurissum following Pliny’s description using a vat prepared according to the method described by John Edmonds was not satisfying. The creta argentaria or even crushed, powdered murex shells dipped into the dye-bath sank into the sediment, consisting of disintegrated murex glands, and seemed to have no affinity with the colorant. Christine Macheboeuf suggests a fine sieve to retain the chalk, but the powder was diluted by the liquid and quickly fell to the bottom of the jar. The sediment gathered in a filter showed a hint of violet on the paper, but the chalky mass remained uncoloured. 

 In 2005, a different technique was tried: the direct dyeing method was used instead of a reduced vat. Fresh glands of Hexaplex trunculus were cut out and placed in a Petri dish with water which soon turned violet. At this stage it was helpful to use a mixer or mortar to blend the glands in order to liberate the water soluble precursors. Various substrates like calcium carbonate, aragonite, and ‘gofun’ (crushed oyster shells) were added, but only acidic insoluble white pigments were found suitable to do this. Also, the white of the pigment should not dominate to prevent the purple colour from turning pastel pink. The remains of the fleshy material and other shell debris, which always stick to the separated hypobranchial glands, had to be discarded. Vinegar, or more efficiently, diluted hydrochloric acid, was used for the hydrolysis and separation of the unwanted residues. Continuous heating of the extracts (up to 50°C) helped to accelerate the cleaning process. After that, careful washing with water was needed in order to remove the acids and obtain a neutral product. Obviously this acidic treatment cannot be applied with calcium carbonate based pigments. A good choice seems to be talcum powder (magnesium silicate). The use of this mineral does not alter the strength of the purple hue. After filtering, the wet mass was dried and then ground into a fine purple pigment ready to use as a ‘painter’s paint’. Maybe the purpurissum described by Augusti and recently identified as an organic material on kaolinite (aluminium silicate) was obtained using a similar procedure. 

In October 2008, a more time consuming but easier way to produce purpurissum was found. Twenty mashed fresh glands of Hexaplex trunculus were stored in a small bottle with water and the substrate. A few days later the violet coloured sediment had settled, leaving dirty water on top. It was carefully discarded; the decanted solid was mixed with fresh water and kept in a warm place to speed up the cleaning procedure. This treatment was repeated as often as necessary until the bad smell had disappeared and the water remained clear. About three months passed before the sediment appeared clean and odourless. The solids were then filtered, washed one more time, and dried: a very vivid purple was the result.

Once filtered and dried this purple powder is a paint (when used with a binding medium), or a dye (when reduced with sodium dithionite).

Why should the Ancients not have used this very simple way of producing purpurissum? Only time and water are required to clean the pigment from the undesired organic by-products, while purple cloth dyeing – in much greater demand of course – needed all their attention.

The purple pigment, called purpurissum since Pliny, needs the correct binding medium for a good result. Pliny mentions melted wax, known as encaustic, which could be applied on wooden support. He also describes ships and funerary pyres being decorated with this colour and rages against such luxury. 

In his writings appears another binding medium, ‘ovo’ the egg, without further details. Bailey’s translation reads ‘white of egg’, but others just repeat ‘egg’ meaning egg yolk. However, the use of egg yolk will quickly turn the violet coloured purpurissum into a blue, even grey-blue shade. 

While handling thousands of murex snails it appeared regularly that the red violet hue can’t be counted on. Many factors may intervene such as the geographical location where the snails were caught. Personal observation with Hexaplex trunculus confirms that in the warmer part of the Mediterranean, i.e. in Lebanon, a more red violet purple can be obtained than in a cooler region, i.e. southern France, where the blue violet shade is strongly dominant.

 It is known that the gender of the sea snails is supposed to have an influence on the colour too. Sex determination of murex is a rather complex matter. Interesting is the fact that in more than 80% of big Hexaplex trunculus (over 3.5cm large and 7cm long, weighing more than 30g each) one can distinguish a “beige coloured” gland which Chantel Westley calls a prostate gland. Specimens with such a prominent gland were picked out from among the others and processed separately, which led to an amazing result: the purple pigment from Hexaplex trunculus with a “beige coloured” gland yielded a brighter, more vivid pigment compared to the ones without. In one case even, the outcome was violet purple for the bigger type, and blue purple for the second type, in other words, the smaller ones. This colour difference was observed all year round and with all the processed batches.

Of all purple shades between ‘fresh dried blood’ to the colour of jeans, the blue purple gained a high status for the Jewish people. Moses already distinguished red purple (Argaman) from blue purple (Tekhelet) when referring to the ritual objects used in the worship of God. He tells his people to include a thread dyed with Tekhelet in the fringes of their garments in order to be constantly reminded of God and to obey his commandments.

The issue of red and blue purple colour well known in Hebrew literature as Argaman and Tekhelet or the Biblical Blue, has been discussed extensively in the past. The author herself reproduced this famous blue in 1993 in a water colour painting, reducing the violet purple to blue by treating it with sodium dithionite. 

 During the demonstration of a purple vat at conferences, the glass jar is lifted from the double boiler and the lid opened to immerse or withdraw a piece of wool to show to the audience. Light enters the dye-bath. Consequently, the result is blue coloured wool or silk. At home, dyeing purple is always carried out in the dark and the blue tint does not occur. This observation explains that, due to the exposure to light, debromination of the dissolved purple takes place and indigo appears: indigo derived from Hexaplex trunculus. The rediscovery of the long lost Biblical Blue or Tekhelet can be attributed to Otto Elsner when he prepared a purple vat in his laboratory in Ramat Gan. Knowing about the famous stench of vat dyeing, he kept his jar on a window sill, in full sunshine. To his surprise, the yarn turned blue!

To quote John Edmonds: “A prosaic explanation for the use of Tekhelet in the Jewish faith may be that imperial purple produced by Hebrew dyers was at one time a Roman imperial monopoly and not available to be sold to others. At the time the indigenous population could only use the blue degraded pigment from the murex for own purposes, and thus made its use a virtue from necessity.”

When the indigo master dyer is testing the maturity of his dye-bath, he listens to its sound, he smells the vat, and he watches its surface, feels the water and tastes the liquid. The preparation of the purple colour calls on all the senses too: 

• the pounding sound of the shells being cracked open must have been a familiar noise at the imperial factories 
• a purple production site was marked by the unforgettable stench of garlic and asafoetida emanating from the pits
• only those who open the shells and extract the glands can actually watch the exciting metamorphosis from a transparent slimy secretion, to yellow, green, blue, blue-violet and finally violet-red colour 
• the precursor-bearing gland contains a toxic substance, a piece dropped onto the skin leaves a burn-mark similar to that of a cigarette butt (Personal observation) 

All of this can be observed while working with fresh murex for a dye-bath or even in art. The synthetic pigment –if affordable – cannot compete with these sensations. The capacity to invent is stimulated by the substance itself, by its complexity. “To any vision must be brought an eye adapted to what is to be seen”.

Though Pliny the Elder gives interesting hints for reproducing the purple pigment, his recipe did not yield a satisfying result; instead it led to further experimentation. The direct adsorption of the secretions of the hypobranchial glands on a mineral substrate like talcum powder or any other calcareous substrate (if the cleansing procedure avoids aggressive materials) led to a purple pigment. Respecting this procedure, the colorants were well adsorbed, and could be used after drying as a pigment for the painter’s palette. A broader range of colours was achieved by treating the above mentioned snails according to size, gender, geographical locations, or personal recipes.

Purpurissum may have been an easily available by-product in purple dye factories. Purple pigment as well as purple-dyed fleece or silk threads may have been stored according to shades until there was enough quantity for trading.

It was a long way to the shades of purple. The author, an artist wanting to paint with genuine purple, was stubborn enough to step onto ground unknown to her: the dyer’s domain. Stimulation to keep going came from the purple giving snail itself. The opened murex promises royal luxury, evokes ancient history and cultures. Pliny the Elder is one of the best sources if read correctly. His descriptions will guide one, but one needs patience and a passionate interest until one understands the magic of Nature. One can share the enthusiasm felt by people in ancient times and the power of purple that lasted for thousands of years. The secretion of an animal, a modest marine mollusc, once exposed to air and light, reveals the secret: from translucency the colour goes to yellow, then green, from blue to violet. Once the multitude of violet shades is achieved, these resist for an unexpected length of time of which the findings in Qatna are a famous example. 

Inge Boesken Kanold

It is time to thank all those who helped and guided me on my way to this royal colour which became the centre of my artistic quest. Purple makes friends; this happened to all of us. Being part of the ‘purple family’ added to the joy that I felt during years of research, not as a scientist but as an artist. The most loyal friend and adviser since almost twenty years is Rolf Haubrichs. He supported me unfailingly, respected my many trials and failures and encouraged me in my artwork. Without him, I would have never achieved the knowledge necessary to be able to paint with purpurissum.

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