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The intermediate and cold blues

[Translation: Anne Clerget]
French text

The cobalt blue

Typical composition: cobalt oxide + aluminium oxide. It is also called, mistakenly it seems, cobalt aluminate.

Concerning later and current synthesis, many processes are described but they are all associated to calcination (around 1300°C). Indeed, the principle seems always to enclose oxidized cobalt into a mass of alumina.

The oxidized cobalt being one of the most powerful siccative for oil paint, the quantitative ratio between aluminous mass and cobalt is crucial. Overall, we consider this colour as literally really siccative. The contact between cobalt oxide and oil creates "differentials" of siccativation, causing cracks, puckering and other accidents. A possible solution - among others - to this problem consists in applying the paste oil/cobalt between two layers of an isolating glaze while respecting for each layer an especially "cautious" siccativation time.

The cobalt blue is rather strong. It is generally less purple-blue than the ultramarine although its colour, associated to the proportion cobalt/alumina, is not a well grounded reference. François Perego describes however a really specific feature: "it absorbs totally from yellow-green to intermediate red", which can explain this recognizable "strong look".

Some authors describe it as barely hiding, others find it opaque, but the pigment composition could play a part in these differences of appreciation. In fact, the fabrication process requires a know-how that will condition greatly its quality.

Precisely, some cobalts may seem a bit "weak", or dull, at least without any real interest. As Xavier de Langlais writes, not without euphemism,"(...) it gains distinction when blended with white" - it is worth noting that the ceruleum blue is itself a cobalt most often built up with pewter white. According to the same author, it could catch a variant of the "ultramarine disease" in the same conditions as this pigment.

Just like in the case of the ultramarine blue, the synthesis of this colour has been the subject of a prize competition organized by the "Société d'Encouragement pour l'Industrie Nationale" (Encouragement Society for National Industry): what was sought after was a blue allowing more uses than the smalt. This is Thénard who won, in 1802.

The expanded supplying of cobalt blue imitations (typically a combination of sodium aluminosilicate (ultramarine type), zinc oxide and copper phtalocyanine) does not respond to a toxicity matter but in the first place to the necessity of offering a product of quite similar colour with a lower cost.

In any case it is  not very recommended to  get a genuine cobalt blue if it is not one of real superior quality because it will not  bring anything major in comparison to imitations while this blue will cost effectively much more and will be more difficult to apply if the process used is oil paint.

The word cobalt derives maybe from kobold or from similar Germanic terms designating some mining bad genies, accused of substituting this mineral for the one sought by miners (silver according to some, copper according to others). Other explanation: the presence of cobalt would be associated to the presence of arsenic, and the fumes of arsenic occurring at the time of mining calcinations would be the origin of this appellation.

The smalt or smalte

Cobalt and silica-based synthesis (to be distinguished from cobalt blue, see above).

Writers mention a relative success for this colour during the 15th or the 16th century. We may say, in a certain way, that it takes two forms then:

* This is an oxide for glass-work and phosphate or cobalt chloride-based glazes.

* This is a pigment for paint, created from the same base: one produced a glass tinted with this oxide, after what one crushed it. During the 17^th century, the "shimmering" look of the smalt was utilized in decorative painting to embellish metal. Nowadays, this aspect is totally absent from most of the produced varieties because such a fine and perfect grinding confers to it a great banality.

However, this quite new occidental "fashion of the smalt" must not live down fabrications that are much more ancient. Uses in arts of fire are very ancient (one mentions Babylon, 17th century BC; Egypt, 16th BC; etc.) and the reduction into a pigmentary powder seems more recent (from the 11th AC in Asia, apparently).

This pigment was (and remains) very expensive and difficult to use. Its permanence questioned by some writers. In the painting domain, one of the argumentations is the following: the smalt would be made with potassic  and not with sodic glass (the reasons remain mysterious and the information is not confirmed), so the potassium included in the glass could saponify fat such as an oil for painting, a wax or an egg. Regarding the domain of the arts of fire, the same potassium would tend to dissolve in the presence of water or acid. There again, the information is absolutely not confirmed and if you have precise information, do not hesitate to contact us.

The hue of the smalt is not particularly colder than the hue of the lapis lazuli, contrary to what some writers pretend. Others say that its colour is duller than cobalt blue but we will not take this point back because the fabrication processes are diverse, and determinant in one case like in the other. There are beautiful smalt, some cobalt are a bit trivial and vice versa. Moreover, dull colours are not necessarily without interest.

The term smalt means enamel in franconian language.

The particular case of ceruleum blue

Often considered as a warm blue, its composition like its colour make him yet more similar to an intermediate blue than, for example, the manganese blue.

Nevertheless, considering the (very questionable) custom according to which it is used in the manner of a primary cyan, we classified it among the warm blues. Click here to access the text written about it.

The lapis-lazuli, also known as genuine ultramarine

Etymology : from Medieval Latin, meaning "stone of azure"

Introduction

It is even more expensive than the cobalt blue, and that explains the success of an imitation - or rather of a synthesis - that became famous (the contemporary ultramarine, which has been covered by a specific article). Typically, this is an aluminum, sodium and calcium silicate  thiosulfate: (Na, Ca) Al, Si, O et SO, i.e. a formula  quite noticeably identical - with a few variations, see the ultramarines, pigment family - to the famous synthesis by Jean-Baptiste Guimet, a polysulphured sodium aluminosilicate. What differs is only the presence or absence of calcium (or of other elements) and above all the amount of sulphur. Besides, it seems that it is by varying the sulphur potency that industry had been able to create several types of ultramarine.

Let us go back to our lapis. According to Anne Varichon, "We give to it the name of 'oltramarino (come from overseas) opposed to azurite,  which was once named as azzuro citramarino(the blue that came from this side of the sea)."

This is a semiprecious stone (opposite picture, thanks to Catherine Lisack). It has been extracted since 6000 BC apparently, in Kokcha, a high valley of the Badakhshan, in the Afghan Pamir, at three or four hundred kilometers only from the sources of the Indus. But during different periods, some other mineral deposits have supposedly been worked in Persia and until China and Siberia.

In the past, the cost of a difficult transporting by land and seas was added to the hard extraction work. Lapis lazuli was sumptuous, more expensive than gold, which explains then its presence on the jewels and mortuary masks of the royal Egyptian families.

The antique Afghan mine

It is still in use. The stock, as much colossal as difficult to work, is more or less inexhaustible.
The quarrying conditions are still extreme to date for two reasons:

* the area remains one of the most dangerous and inaccessible in the world, so much because it is a remote high mountain zone as because of a chronic political insecurity and instability,

* snow keeps from any access, except during a part of the summer.

A documentary film shot during a high risk expedition with Gary Bowersox (see below) shows few images of a venerable gallery which, on hundreds meters, bears the stigmatas of a rudimentary work. Walls are covered with soot because the spectacular ore veins are revealed by projection of cold water on the wall that was previously heated with a torch, causing the burst of the stone.

References G. Bowersox :

* The Gem Hunter (video),
Gary Bowersox, dir. W. Knöpfler, Austria 2001,
Media Program of the European Union

* G. Bowersox's Site, gems-afghan.com (English)

Rarity of lapis lazuli on our planet is not the only reason for its cost, whatever the mine location and the transport problems: its extraction is hardly economic. It seems that one needs to process 100 kg of rocks to obtain 3 kg of pigment (information not confirmed). Moreover, this must not make us forget the required works of purification, formerly carried out with the aid of ammoniac, notably.

In western Europe, lapis-lazuli appears only during the 12^th century AC via Byzantium. In Antiquity, Pliny does not even mention it. However, the Syro-Anatolian Orient has been using it since the 9^th century BC for glasswork. Then, during the 5th century AC, artists grind it and use it to paint, for the first time, in Afghanistan (information: Anne Varichon).

Ancient Egypt knows lapis-lazuli too, but uses it in the form of stone while as a pigment, an imitation is made (cf. Egyptian blue). It is true that lapis-lazuli is first and foremost a superb mineral that we would not necessarily like to use as a pigment. We can consider the grinding of this crystal as an original and daring idea which does not come easily to mind. It is therefore a true and major methodological discovery by the Afghan painters. De facto, the same procedure will be applied to malachite, to azurite and to turquoise notably in central and western Asia.

Lapis-lazuli is supposed- and "only" supposed - stable in all blends. It contains sulphur and purifying it is not an easy operation... Many facts  prompting us to be cautious about this point.

To date, none of us tested it, and we do not have any really interesting account. To write to us, click here.

Grinding

Until 12th century AC, the grinding process allowed only a rather irregular and rough granularity. The invention of a new grinding process has been a maybe not so good idea; at least this is what it seems to us who have today other blue pigments of the same category, therefore the choice. Lapis could have been much more beautiful when it emitted less homogenous, more crystalline rays. This question is essential today for those who would like to get this rare colour. It is posed in the same way concerning malachite, azurite and turquoise, but also smalt.

The use that was made of it about the end of the Middle-Age may incidentally appear disappointing. Lapis was sold as a homogenous paste, blended with plaster, waxes or other substances which spoiled its hue. It was only a commercial product anymore, and the quantity that was spread on the painted work was specified by contract (source Anne Varichon).

All information about the use of lapis-lazuli is welcome.

Indigo, indigos

This colour whose name became a commercial issue (one only needs to look for "indigo" on a search engine to get convinced...) is rather particular, a bit mysterious. The least we can say is that it has a charged story which, by the way, is not over. It is still used today in the Indian Union like it was  at least  2000 years ago, but more than 1000 years ago, its single name aroused already the  imagination since people  already usurped it in the West  to name another substance (see below).

The term indigo was already used during Roman Antiquity.

This is to be confirmed: this tinctorial substance is supposed to have been reduced to powder and used by the painters from then and those from the Middle Age. This information is absolutely not confirmed. On the contrary, we have some reasons to think that there is confusion:

The Medieval Indigo (actually, probably antique) used for miniatures is not the dye of Indian origin : it was made from isatis tinctoria leaves , called woad  (read the article of the glossary about this word with multiple meanings), a small plant with yellow flowers. People let ferment the balls of leaves (cocagne) that were reduced into pulp with mills or with less advanced processes in more ancient times. A much more colored product is supposedly obtained with fresh leaves, but in commercial convoys the leaves had plenty of time to dry and loose their quality. The tinctorial process (the "vat" of woad) calls upon a biologic agent, a bacteria named clostridium isatid.

Some writers mention also a mysterious purplish blue substance obtained from an as mysterious "folium" ("the leave", of an unspecified tree) which is supposed to be referenced in medieval manuscripts. The medieval mystery and confusions surrounding the purplish blues come probably from the unbecoming status that was reserved to the violet (a very close colour) by the church of that time (read the introduction of the article about Violets and mauves).

But woad has not been used only during the Middle Age and in the West. Probably original from Western Asia, it would have been brought in other regions during Neolithic. It would have been known in a vast area from Northern Europe to Egypt and India since the 2^nd millennium BC (not confirmed).

In Egypt, these are the Romans who are supposed to have substituted to it the « Indian » indigo. If the woad has disappeared gradually from the East, its presence in the West has been durably settled although this colour was denigrated, or even feared by Latin. The Roman legions would have cowered in the confrontation with Celtic or Germanic warriors who coated their skin with woad dye. Curiously, this blue has been supposedly well-known in the Celto-Germanic world, capable of driving away some animals and spirits. It looks like a legend, but when you think of the importance ascribed to the colour in the military battle less than one hundred years ago, it becomes plausible that the Roman troops were scared stiff, all the more so other chromatic frights, in other places, are well attested by historians .

Later, from the 13^th to the 15^th century, isatis tinctoria, which was in vogue because of the appearance of a new dyeing process, made the fortune of the different producing regions in Europe (we can think for instance of the "cocagne country", in the South-West of France). Initially a dull and not colorfast dye despised by the patricians of the Antique Rome, it has been allowed to gain some permanence thanks to technical advancements.

Indigos from America rivaled afterwards and caused a gradual decline of the woad. 

The pigmentary substance of 'isatis tinctoria is actually the same than the one we find in indigofera tinctoria (indigo) : this is the indican (see below).

Recommended readings:
the woad blue on Pourpre.com

See also
The Mayas' turquoise clay

True indigo is in theory a dark and purplish, even reddish, blue, extracted from the leaves starch of a shrub, the indigofera tinctoria (family of the leguminosae, Papillonacea type), original from tropical Asia and acclimatized later on the new continent. Some close varieties have been used in the warm and irrigated areas of the Eurasian continent.

The hue produced from these plants is actually variable considering the number of vegetable varieties used and the diversity of treatments. It is though quite always a purplish blue, which is characteristic of the Indican.

Some sources recommend avoiding radically this colour in oil painting, for ignored reasons, but it calls for enquiry (any information about this subject is welcome). This is a fact: the true indigo, still used as dye, has not been used - or very exceptionnally - in painting, contrary to the woad. The reason is maybe not because of its chemistry or because of inconsistencies, but because of the concentration of researches on the chemical processing that was presenting the most important economical issues at a moment of the history. There are maybe other reasons, rooted in the history of Indo-European people and those who rubbed elbows with them, from Celtics to Balineses, but this is not the point.

Western Africa has also a particular relationship with the indigo, in its mythology and traditions, as confirmed notably by the famous example of the blue men, the Tuaregs.

Besides, it is true that blue men, from Celtic Europe, Sahara or Indus, appear to have inspired a great fear to their enemies, whoever they were. By the way in which colour are dressed our policemen?

Recommended reading: The indigo on Pourpre.com

Indican is the main chromatic element of the indigo; it is a typical glycoside, a starch extract (read absolutely the glossary article about Indican). It has been isolated by calcination in 1826 (Unverborden). This discovery would have allowed the synthesis of mauveine (see the disappearing of archil, anilines) and of different other tinting substances. The first synthetic Indican has been made in 1880 (Adolph Von Baeyer). Today, there are dozens of synthesis processes.

The economical implications for the synthesis of indigo are still very important. You can intuitively estimate them by the number of blue jeans worn by our contemporaries.

Indican needs a particular processing, some adjuvant, in order to be used as a dye because it is indissoluble in water. The complexity of the traditional processes of indigo dyeing, and above all the differences of hue obtained, can be thus partly explained.

Indican is found not only in hundreds of varieties of indigo shrubs (among which indigofera tinctoria is only one kind), but also in woad. But it is found in a proportion twenty times less in the latter. This could explain the reputed dull look of woad dyes and the beautiful look of indigo dyes, not considering inopportune drying problems during transport. If technical advancements in woad processing allowed multiplying its tinctorial power by ten, it nevertheless did not stand the comparison...

In some regions, Indican is still extracted by fermentation of dampened leaves., Considering the number of cultures making products with Indican and the number of plants concerned, adjuvant are innumerable and of amazingly different natures. Since the Celtic period, the European woad used an adjuvant which, by the way, would have given to the dye a bad reputation in Rome: human urine, apparently still used until the XX^th century AC.

The tinctorial substance changes at the time of the dyeing. It goes from a warm blue fairly close to Prussian blue then to a more purplish colour.

We can note the great similitude of the molecule that constitutes the colouring principle of purple. Click here.

The indanthrene blue(C₂₈H₁₄N₂O₄, see The anthraquinones (family)), This nitrogenous pigment, which is very coloring and semi-opaque, has in fact supplanted indigo and woad in the domain of painting but at least without really resembling them because it is not as purplish: several versions are a bit green. But like we said, indigo can be of varying hues function of the substance used as a base of the treatment applied.

The matter of the chemical compatibility with oil is not a problem anymore with this substance, found today in some ranges of oil paint. Indanthrene blue seems however to be advised against fresco painting (manufacturer information).

Thio-indigo is another synthesis, or rather a group, a family of synthetic molecules. It has apparently no point in common with indigos except a structural molecular similarity with Indican (we can compare the opposite scheme with the descriptive of the Indican above). Each N-H group of Indican is replaced with a sulphur atom.

The similarity, apparently, stops here. The thio-indigos we observed or identified are cold red, but we have certainly not finished with the exploration of the members of this family.

Blends containing indigo

Some tints, like the Bayeux violet and the argentine grey are known for having contained an indigo blue. It was presumably, some western indigo, woad.

We will mention, at last, an imitation of indigo possibly ephemeral because aniline blue-based.

See also indophenols.

The Indian blue

Its origin is unknown (thank you for giving us any information about this). We may assume that this is the indigo, but we must not forget that India has been knowing woad and azurite for a long time too.

In the ranges of paint, it is made with indanthrene blue.

Other cold blues

We will mention a vegetable blue used during the medieval period and made from sunflower. It is supposed to have produced different blue and violet. This is very possible because, if our information is correct, it is the same substance coloring the famous litmus paper, a test paper that changes colour functions of the acidity the tested substances.

The France blue, also said royal blue or rex blue is a blend and its typical composition is: 3 pb28 pw4 pw6 (see nomenclatures for pigments). It is a variety of cobalt blue  with white added.

Recommended reading : The France blue on Pourpre.com

The Klein blue(IKB ®) looks like a kind of ultramarine. Its composition is not public and is under an international copyright registration. According to the legend (and presumably), the store manager Edouard Adam, retired today but still well-known by numerous artists in Paris, is supposed to have contributed to this creation.

Recommended reading : The Klein blue on Pourpre.com

The Majorelle blue is mentioned in the article dedicated to the ultramarine blues. Click here

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