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Pigments, colours

[professional translation]
French text

Recommended Visit:
Chroma at Pourpre.com

But let’s not lament this state of affairs: binding pigments, manufacturing colours is also a way of discovering a new beauty and new pictorial characteristics of unexpected richness, notably with respect to visual aspects. See a pigment is not a colour.

Anyone looking for more detailed information about colours, which is to say the scientific characteristics of colours and not just pigments, is invited to visit the excellent pourpre.com site. In particular Colour explained, the colour charts and also the astonishing curiosities. The linguistic aspect is also fascinating. This theme is thoroughly explored in the "The colour and language" section.

Pulverisation finesse: an essential factor

Xavier de Langlais rightly confirms that the more finely a pigment is ground the better it covers and colours. A very fine powder causes the light to deploy an infinite number of trajectories that prevent any transition of it in the lower layers, giving the colour a great visual impact that is very enriched, almost saturated.

Almost any pigment’s properties can be transformed by simply grinding it more or less fine.

Today all pigments are ground very fine thanks to the evolution of industrial techniques.

However, did not the Flemish and Venetian masters consider transparency a virtue, widely used to great effect in their incomparable works?
And are not smalt, lapis lazuli, malachite, and turquoise more beautiful when they are “badly ground”?

Does the “grinding perfection" evoked by the great Armorican master really consist in an infinitesimal finesse or rather in the harmonic “pictorial intention” of the artist? 

Let’s take pigment for the perfection... that it is! It is one of the roles of the artist to know how to look, and with respect to pigment, to know how to discover the remarkable reality of a pigment that may seem imperfect according to criteria that are ultimately shaped by the punctual evolution of techniques and tastes.

Covering factor (opposite of: transparency)

The term "covering" probably comes from decorative painting. A recurring question in this domain is knowing which surface a paint or a filler can “cover”.  

A coloured substance “covers” a surface all the better the more opaque it is (notably because there are always ways of diluting it or adding colourless fillers to it). Thus the term “covering” is substituted, in artistic painting as well, by the term “opaque”.

Several factors determine the covering characteristics of a paint: 

* the finesse of the grinding (see above),

* the intrinsic opacity of the pigment, which is determined above all by its nature at a molecular level, but also by the finesse of the grinding. The physical nature, whether organic or inorganic, is essential above all because it varies from one pigment to another. In terms of tendencies, organic pigments (synthetic, vegetable, or animal carbides) are generally rather transparent, whereas inorganic pigments (mineral) are often opaque See Organic vs. inorganic.

the way in which the binding agent coats the pigment. This is directly decisive where mineral binding agents are concerned. The white “covering” of plaster employed as a binding agent is radically different from what one can obtain with lime. As for organic binding agents, which are by far the more common, they can offer diverse thicknesses, which is not without consequence on the opaque or transparent aspect of the final result.

* the condition of the paste is also very decisive and this is something that should not be forgotten. Certain binding agents are white and opaque when they are wet and transparent when they are dry. Others tend to accentuate transparency when they are wet.

There are also a few “tricks” that make it possible to measure how important an issue the “covering factor” is. Certain so called covering paints like gouache were initially nothing other than regular paints to which a white pigment was added to increase refraction. On the other hand, certain oil colours called "lakes" are covering pigments that are added to colourless fillers.

Though the binding agent and the additives can play a role in the covering capacity of a paint, it is the properties of the pigments that clearly dominate, to such a degree that covering pigments and fillers are sometimes employed to modify this essential aspect of the pictorial surface. 

Colouring factor

Covering or transparent, a pigment is susceptible to more or less tinting whatever surrounds it. This capacity becomes all the more interesting when colourless (mediums, fillers) substances are added to the paste

The “colouring factor” is not, a priori, a very sensitive characteristic as it often masks the “covering factor”. It is generally difficult to show in what sense a pigment colours more than another if one is not dealing with very colouring varieties such as certain iron oxides. And there... the surprise is great.

Permanence (adherence, lightfastness)

The faculty a pigment has of keeping its properties over a long period: intensity, vivacity, and exactness of colour, covering capacity, etc.  Exposure to light is considered to be the most important alteration factor. Sunlight is rarely advisable but it should be pointed out that mineral pigments (notably earth pigments) resist very well.

The separate case of photo-luminescent materials is treated apart on this site. Click here.

By analogy, the permanence of tinctorial substances is sometimes referred to as the tint.

Ability to flocculate (granulation)

This priority is not important for watercolour, nor is it negligible for paints that are highly diluted, whatever they may be. It is moreover not without importance for other materials used in the visual arts (see barbotine).

The earth pigments in particular are renowned for their tendency to gather together in grains or flakes. Under the action of drying, these agglomerates settle in the cracks, colouring the painted surface in a heterogeneous manner, emphasizing the relief of the support.

Certain extra fine water-colour manufacturers offer an indication in their colour charts making it possible to locate the colours having this power.  

In other domains – including industry – a deflocculant is a product that makes it possible to accentuate the homogeneity of a mixture or an emulsion.

Absorbing power and siccative power

We have tested it for you: there is nothing like speaking with a chemist of the “siccative capacity of a pigment” for producing an amused smile. Certain teachers, certain pedagogical works, certain painters confuse – like everyone, quite naturally – absorbing capacity and the aptitude for drying or siccativation.

... and this mocking chemist is not wrong: a pure pigment is solid. It can neither dry nor siccate. It can combine more or less rapidly with such and such binding agent with respect to its own capacity to enter into a solution and to absorb. This is essential. Following this, it is the binding agent that dries, aided or not by the pigment. This plays a siccative role only if it contains a veritable siccative element for the binding agent used, which is not that common. A siccative pigment for oil is not necessarily absorbing and does not accelerate the drying of another binding agent. The absorbing faculty of a pigment is a different property from siccativity. The confusion principally concerns oil paint as the other procedures make it possible to obtain solidification much more rapidly without there being any question of siccativation.

Concerning siccativity independent of absorbing power, numerous authors speak, perhaps a little too jovially, of the “excellent siccativity" of such and such a pigment or contrarily, of the “execrable siccativity” of another (at times with a surprising degree of animosity – tell-tale of first-hand experience). Whereas, in either case it is neither about qualities or defaults. A pigment containing a not inconsiderable quantity of siccative elements creates the same potential for danger as a neutral pigment; the real menace is too much of a “differential of siccativation” when layers containing pigments that do not have the same properties are put in contact with one another. It is this “differential” that creates accidents in the first place: folds, cracks, irreducible sinkage, etc.
It is however perfectly true that pigments containing a particularly siccative element (manganese or cobalt oxides) can also engender annoying mechanical phenomena when they are employed alone, especially in impasto work. Thus the manganese blue, for example, is practically never used in oil paint. 

Curiously, a large part of lightly covering pigments (lakes, green earth, etc.) have the reputation – for the most part erroneous – for not being very “siccative”. Now there’s a troubling coincidence! Is the problem of siccativity perhaps not linked to an error of binding caused by an intrinsically strong transparency that we have not known how to take into account?

Something very obvious appears when we explore the world of pigments, which marks our editorial line: it is the painter that should adapt to the pigment’s properties and not the contrary because colours are never without defects for certain uses and alternative solutions are not that numerous. Even the term “doubtful pigment or colour” makes no sense.

Stability in mixtures (miscibility): compatibilities and incompatibilities

a) Lead and sulphur

In effect, it isincompatibility between sulphur and lead that is by far the most notorious in paint, particularly in the oil process where lead is still used. Putting these two elements together provokes very significant blackening.

Paint manufacturers of tubes or ranges of pigments generally watch out and avoid this combination. They manage to do so by three methods:

* getting rid of these two components (rare)

* getting rid of one of these components (common)

* by eliminating the excess sulphur that is non-molecular.

Thus, if lead has luckily disappeared from tubes and pots of pigments (except for lead white, which is still available under certain brands, see photo) sulphur is still very present. This latter is above all an “additive” (in fact a component) that is indispensable for cadmium and natural whites, but also for ultramarine and numerous other key colours. It is difficult to get around it!

Certain manufacturers of high-quality products have regrouped their pigments in groups: a group that mixes with lead and a group of colours that can be mixed with each other.

Using a sulphurised pigment poses hardly any problem in itself, but in oil paint it poses a major one: the best siccative is nothing other than lead oxide.

White or brown Courtrai, Flemish, Harlem, "cooked oils", “classic” siccatives and litharges can be incompatible with sulphur pigments when these are not sufficiently “washed” of their non-molecular sulphur. 

For this reason, all the elements containing sulphur on our site are marked with the following sign:

Or in french, ATTENTION SOUFRE !

Substances containing lead are accompanied by the following image:

Or in french, ATTENTION PLOMB !

ATTENTION: it is not because a material contains sulphur that there is necessarily a risk of reaction. Free sulphur, a kind of waste matter from incomplete preparations, is otherwise more likely to produce interactions than molecular sulphur. Without comparison, it is undeniable. But there we are talking about phenomenon that happens at a “probabilistic” level, notably in liaison with the electronegativity of the atoms that are present, which partly determines the solidity of the bonds. It so happens that certain bodies “break”, notably in the presence of heat. But let’s not be alarmist: only complex bodies (for example polysulphides), which are often more fragile, are really concerned, but as pigments, rather as supports – and this is rare.

In other words, our warning is not uniquely but principally valid for pigments that are not sufficiently well prepared, for example, old jars of cadmium. Preparation techniques seem to have evolved in the right direction. However, a certain mistrust endures (even among manufacturers, see above) and we are not in a position to refute their arguments for the time being. In order to reduce this mistrust, we think that manufacturers of quality products would certainly benefit from providing more information on this subject, backed up with relatively detailed scientific and or historic arguments for the consumer, who moreover, always justly appreciates it when their concerns are addressed. 

Some authors assert that chrome poses the same problem as lead: notably, that it is incompatible with sulphur. In reality, orange and yellow chromium pigments – today rare because they have turned out to be toxic and even highly toxic under a certain form – were associated with lead. Lead chromates effectively turned black in contact with sulphur, but not because of chrome (a non-confirmed hypothesis). 

This latter is finally very present in palettes in forms that are without danger: chromium oxide green, viridian green.

However, it seems the authentic emerald green, composed nonetheless of copper arsenate – so neither of lead or sulphur – and happily impossible to find today, is very sulphur reactive, and even so with lead, according to certain sources. This horrible poison has given way to imitations that are less unhealthy and more stable.

b) Pigments that are too alkaline and thick (grease) paints

The binding agent of an oily paint is a fatty acid esther. The use of alkaline pigments like white earths with a binding agent like an oil paint can cause saponification. This generally translates into browning that is accentuated over time.

Brilliance

The pigments themselves are more or less brilliant independent of the aspect that the binding agents and mediums, confer upon them, which are capable of considerably modifying these characteristics. 

On the whole, organic pigments are brilliant and inorganic, matt.

Certain pigments, which are generally synthetic are literally "dedicated to brilliance": interferential or iridescent pigments and metallic pigments.

Toxicity, precautions

Pigments and colours are not all harmless products. The rulings governing the obligation for manufacturers to place a warning pictogram on products fluctuates in France. Certain North American brands carry the health label, which can only be applied when it is paid for, which a priori implies neither a guarantee of impartiality nor the contrary.

We advise our European readers to take into account (neither more nor less) any warnings while waiting for clear and lasting rulings to be established throughout the European Union. In the case of other Francophone countries and the other non European countries, we are unfortunately not in a position to comment at this time, but any information will be welcome.

Certain pigments display pictograms.

Toxic pigments (see pictogram opposite). They are highly dangerous. It is best to buy them in liquid or semi liquid form and to avoid any type of contact. Wearing appropriate masks, gloves and other protective material is necessary in most cases. It is highly recommended that you inform yourself about the measures to be taken by consulting the services specialized in this field (see Art, security, pollution).

Noxious pigments - see pictogram opposite. They should be handled and stored with a few elementary precautions: work calmly, hold open jars of pigments away from the face and open them only when covered with a transparent plastic or paper that prevents the powder from diffusing in the air during the few seconds following this operation; wash your hands often and avoid any sustained contact with the skin; store them out of reach of children and animals. Do not panic: these products, contrary to toxic pigments, only have pathogenic effects if they are repeatedly handled in unsatisfactory conditions or if they are massively inhaled or ingested.  

A pigment is not a colour

The chemical and physical considerations evoked below remind us of an essential point: a pigment is not a colour. It is a material. Its composition is more or less precisely defined. Very often, in reflecting light, it absorbs some of it and becomes what we call “its colour”. A fact which is also just as decisive; it is altered by the kind of source(s) of light and by its environment: binding agents, additives, the surface and of course lighting and atmosphere. Not to mention the eye of the observer (cf. The Eye and Observing at Pourpre.com), which is the other half of the phenomenon.

So we naturally come to ask ourselves just what exactly is a colour?

Light that reaches us is characterized by what physicists call a spectrum (see photo above – thanks to Alain Klotz). The spectrum is, among all the light rays of all possible wavelengths, that which actually reaches our eyes, divided into rays, which is to say, frequencies. Blue takes up such and such a frequency; yellow such and such another, etc. Certain "rays" can be emitted (emission rays); others represent the absorption of emitted light (absorption rays). Each ray is representative of the presence and state of an element, of an atom, in the field being viewed. 

Certain radiances can either not be emitted, not reflected or be absorbed. Moreover one must take into account the intensity of the radiance of each wavelength (see diagram in black body) and this for each phenomenon: reverberation, absorption, and emission.
The purely physical approach is therefore centred on radiance. It essentially describes colours in terms of emission rays, absorption rays and the intensity of radiance for each ray – a “point of view” that cannot be ignored because it describes a concrete reality.

However, in the context of visual arts, as in daily life, colour takes on more the meaning of a cognitive phenomenon. On the basis of electric impulses provoked by an outburst of photons striking the retina, our neurons can interpret the world: “I guess” that between me and the pigment there is a hazy air, I guess the time it is and the season with respect to the colours I see, but I also guess the emotions of a living being and the passion of a painter, etc.  Sometimes too “I let myself be taken in” by pure illusion. Read Illusions in Pourpre.com.

Colour is certainly, in our human sense, mainly an “imaginary phenomenon".

From this imaginary dimension are born all kinds of taboos, of fears ("peurs bleues"), of connotations and preferences interwoven in cultural dimensions, anchored notably in language (read Colours on the Tongue in Pourpre.com). The same goes for pigments. From this point of view, the status of pigments is often directly associated with that of colours, but exceptions have existed.  For example, purple probably succeeded saffron for religious and political reasons linked to the exceptional cost of these two substances, in addition to the symbolism of their hues. In India, certain colourings must not be touched, while others, like turmeric, continue today to bring good luck. Lapis lazuli survived a certain blue hobia in the West because it had an intrinsic value. There are certainly other examples because human beings have never been able to fully ignore – even in fundamentalism – the material dimension of pigments.

The variability of composition and colour of a pigment

A pigment is therefore not an abstract concept.

In decorative painting, there is a relatively common practice:  mixing jars of paint or pigments of the same colour to obtain maximum homogeneity.  Which is to say to what degree only in a statistical manner a pigment corresponds to a colour.

Painter decorators know how multiple factors intervening in the place where they are extracted, discharged or synthesized can yield products and thus colours that are perceptibly different.

An edifying experience consists of comparing two tubes or pots of pigments that are supposed to be the same colour.

To a certain degree, these divergences are not flaws, especially when they concern mineral pigments. Manufacturers are limited by what nature imposes on them: deposits are what they are and it is up to the painter, artist or decorator to address the task and manage the differences in composition with all that this implies in terms of colour, siccativity, absorbing power, etc. 

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