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From binders to processes

[Translation: Anne Clerget]
French text

Another way to get onto this topic:
Dialogs at Dotatpea, chap. I, About binders

Besides, the binder enables the adherence only if there is intervention of an external factor: front of oxygen or baking (commonplace in industrial painting and in fire arts). It plays a part of an intermediary not only with the support, but also with the environment, taken in its widest sense ( incorporating human action and the action of sun, air, siccative agents, or bacteria), which incites us to consider it as a mediator.

The most ancient rupestrian paintings (Arnhem, Lascaux, Chauvet, Altamira, etc.) have been  often made without any binder. Most of them would not have stood up to an open-air exposure, even to a very short one. In Australia, people started very early(cf. Anne Varichon) to use wild orchid sap,  egg  yolk,  wax and various other vegetable origin products as binders. The use of animal fat during Palaeolithic in Europe is mentioned.

But, among  the first very effective binders, we will name particularly the gum arabic and probably also the cherry-tree gum (which we do not know well). Oils for oil paint will appear only very late, after the egg yolk (the egg being the symbol of Dotapea.com), and after numerous other products having shown their adaptation to defined contexts of pictorial use.

The pigments agglutinated by binders will determine a painting process

All paints are constituted at least of:

* pigments

* one, or several binders wetting and bonding the pigments together while enabling them to stick to the support.

A tube, a jar of paint or ink, a pen ink-cartridge, a felt-tip recharge, industrial powder paint, are almost always pigment mixed with a binder. It is usually easy to find those two elements separately and to mix them (see the article "to separate or not binders and pigments"). About the distinction between binders and glues, read the article about glues.

Each painting process refers to a specific binder: One says "oil painting process", "egg , casein or glue tempera", synthetic, acrylic, vinylic, methylcellulose paints, fresco painting on mortar, etc... In principle, the binder itself will condition the way to paint, the possible supports, and most of all the kind of result you will likely get.

The only exceptions are  binders which are also dyes  such as graphite or natural chalk, generally used in drawing because, by definition, they make polychromatism impossible other than by playing with a coloured background or by using mixed media. See further binding or mordant dyes.

Mordant is to dyeing what binders are to painting

Read about this topic the Glossary article "Mordant".

Binder's specific characteristics

The base and the thinner

Aqueous, oil-based, encaustic, cetonic, etc. So called, functions of the nature of the binder (ex. : oil paint), or functions of  the nature of the thinner (ex. : water based paints or distempers).

Some paints have a double chemical base (read emulsions), both aqueous and oily. They possibly allow the use of two thinners of different nature.

Viscosity, adherence and the "pulling on" quality

We have said already that a binder is a kind of glue, which takes action both on the pigment and on the support.

The viscosity (about this term, read imperatively the Glossary article) of this "glue" depends on many factors that have been  themselves objects of experience and research since prehistory. Viscosity interacts with other factors such as the siccativity ability of the binder, and of the pigment. Some viscosities are both slow and very powerful; they produce a very solid result, like in the case of linseed oil. Strictly speaking, for the painter or the artist, the concept of viscosity is not the most useful. The concept of  "pulling on" quality is more precise, taking into consideration important connected phenomenon. We can understand this concept intuitively.

The way the binder agglutinates the pigment ("its way to be viscous") will determine other factors. Watercolour seems to coat nearly every single grain of pigment while oil and acrylic entrap them in their own mass. See further Transparence, fineness, precision.

The adherence to the support is of course essential. Besides, we have to make a distinction between the adherence of a pure binder, used for instance as a coating, and the adherence of the paint, which is always lower because of the presence of pigment (which has no viscosity).

The wet adherence is a technical concept in decorative painting. The matter is adherence on a wet support and/or in a damp atmosphere. It is very interesting because it allows some plastic processes out of a modification of the support. The water will override temporarily, locally or globally, the adherence of oil paint. In the meantime, some things are possible to do.

Inversely, we can apply the same principle using fat, which will alter the adherence of a water-based paint (however, the withdrawal of fat will create more important difficulties).

The " pulling on" quality is the viscosity of a painting paste, but, above all, it is for the painter a "mechanical" sensation, very real and sensible. This sensation can change during a session, as the thinner evaporates. It feels like a concrete resistance.

Some glues seem almost "to pull to their side" because they are so viscous. Therefore, we will say that the "pulling on" quality is foremost the sensation of viscosity before being a plain synonym of it.

Drying and siccativation

The distinction between those terms is clarified in a Glossary article.

To say it briefly and simply, we will refer here to the term "siccativity" only as the ability of a binder to solidify.

The siccativity of a binder can be :

* null (pieces remaining in a liquid state, horizontally, difficult to conserve)

* total, but faster or slower, depending on the chemical nature of the paste, its thickness and dilution

* or reversible (waxes, skin glue, gouache).

After-drying or after-siccativation  alteration of the appearance

A binder  can be pure, like linseed oil, or pre-diluted, like the acrylic binder, which is composed of about half water (in volume; see dry extract). Sometimes, the  amount of thinner necessary to use the product is significant, like in the case of shellac (see picture). Just the opposite, a tube of oil paint does not contain any spirit and a jar of watercolour is dry.

The presence of thinner (incorporated or added) implies of course necessarily a removal during drying; but, functions of the way the binder is polymerized - especially the fastness and the homogeneity of this phenomenon - the paste will keep more or less its initial volume.

The chemical base of the binder will condition the type of thinner, but also the average final pictorial aspect of a painting. Dewaxed shellac and oil-based paints are probably the binders that allow the dry result to be the most similar with the wet piece. Nevertheless, in any case a rule remains: when there is a lot of thinner (wash, glaze, tint), the volume will decrease a lot during drying. A drip looking thick when wet looks often quite thin once dry.

Brightness, shine retention

Brightness is overall the privilege of pure binders. When pigmented, they all become matter.

But, a binder or a paint (binder + pigment + possible adjuvant ) can of course be shiny.

In some cases, notably when a painting is exhibited in the open air, outdoors, this shine can fade. Inversely, the "shine retention" corresponds to a propensity to preserve this characteristic in spite of time and bad weather.

Mechanic resistance, resilience

depend on:

1. the concentration of binder and adjuvants (diluting a paint without any viscous adjuvant will weaken it). Some adjuvants such as glycerine and different synthetic binders increase the resilience of  water-based paints. Some resins used with oil paint make the paint itself more resistant as much in the conservation context (a poorly protected pigment will react to air and ultraviolet rays) as in the reticulation context, at the time of drying, when mechanical problems are also posed. These problems manifest themselves especially with non-homogeneous moves, notably on the macromolecular scale and, consecutively (how dreadful!) on the scale of our visual acuity!

2. the binder's own resistance and its density in the paste. Resilient binders such as acrylic and vinyl tolerate more kinds of supports than casein or lime, which are more rigid

3. the right choice and the right preparation of the support: soft binder for a soft support, soft or hard binder for a hard support. A badly primed support, wrongly chosen paint or primer, are the nightmare of restorers, museum curators, collectors and all art amateurs, as a few visits in several museums will suffice to convince anyone.

Permanence: inopportune dyeings (yellowing, whitening, darkening), general resistance

* the inopportune dyeing of a binder is sometimes reversible (see linseed oil)

* a more lasting dyeing can occur as a result of wrong blends, or of an application on inadequate supports (the support being acid, for example, or sulphurous, when  the paint applied on top is mixed with lead)

* it can be related to weak points of the binder (ex: tendency for the walnut oil to go rancid - or reputation) or to a too thin proportion of binder compared to the pigment: this is a problem for watercolour in particular, which is a paint using to the maximum the possibilities of dilution of gum arabic. One has already seen pictures literally falling apart by plates of too poorly bound pigments.

* the question of resistance is often posed in relation with reversibility. A permanent paint must resist to the action of its solvents (especially when it is a question of water-based paints). In addition, the paint must also successfully confront the ambient air, humidity, polluting substances, temperature, ultraviolet rays, bacteria, etc.

Reversibility

It is, for a dried paint (or another product, like a glue for instance) the ability to move back to a solution in the presence of its usual solvents. Gouache is reversible but acrylic and oil are not. A quality watercolour is not supposed to be really reversible. Casein, however, is initially reversible, and then is fixed forever.

In some cases, the  reversibility of a paint is related to its ability to saponify. Since most of the binders are esters, they can actually react to a base, to water or a fortiori to an alkaline water such as many tap waters.

It is said, that a water-based paint would generally be less reversible in a warm atmosphere. We did not check this information.

Opposite to reversibility is the washability (with or without detergent), property of paints that are resistant to washing, even to some abrasive action. Acrylic paint is washable, oil paint a little bit less; but decorative paints such as glycerophthalic, the different alkyds, polyurethane paint, and even more, powdered paints and other industrial processes along with enamels are representative of this ability to resist reversibility.

Thermosensibility

Heat sensibility  delimits a type of "reversibility" which is particularly determinant for the paints containing waxes.

When wax cools, it coagulates. It is very quick, it allows to arrest some unwanted mechanic moves. But once solidified, wax becomes brittle and frail.

When heating up, it liquefies, and causes irreversible accidents without delay.

It is therefore important to wisely use thermo-sensible products. Too close lighting devices especially represent a potential hazard.

Read Waxes as adjuvants, waxes, encaustic.

Impasto ability

Except for very special techniques, impastos are possible only with acrylic and oil, on some conditions. Auxiliaries are however generally essential from a thickness of two or three millimetres.

Is it possible to consider fresco mortar and especially plaster as impasting substances? More or less, though we would be inclined to call them "plastic" binders.

Spreading, smooth flow, surfactant

To also see passage in the dialogues of Dotapea, chap. I, In connection with the binders.

See without fail Tension, superficial tension, tensioactive.

About the concept of surfactant,
read definition in Dialogs at Dotapea, chap. I, About binders

The concept of spreading must not be confused with the concept of dilution. A very "dilutable" paint such as watercolour is absolutely not " spread " in a "smooth" way.

Then, what is the signification of these terms?

A paint, because of its own nature and because of the nature of its thinner, can have or not a tendency to spread on the painting support and to form quickly a homogeneous surface where brush strokes do not leave any marks (as in the typical case of shellac).

Seemingly, in the fields of plastic or decorative arts, the term "smooth" well matches- subject to confirmation- the physicochemical concept of tensioactivity.

Standoil with oil painting and diffusers with acrylic help to increase the sensation of smoothness of the flow.

Plastically speaking, a smooth paint, or made as such, allows very homogeneous and "soft" transitions, but also flat colours, or fuzziness.

Read also passage in Dialogs at Dotapea, chap. I, About binders.

Transparency, fineness, precision

Those properties are proper to rather fluid paints (or made as such) diluted (see Thinners, solvents, remover: physicochemical concepts).

Some binders are rough, almost granular. Others are fine but fuzzy. Precision is not always a given thing. It is inherent not only in the binder, but also in the thinner. For example, alcohol, which is the Coromandel shellac (see picture) thinner, does not permit a precision such as the one you would get with an average binder diluted with water or petrol. On the other hand, the transparency of this shellac has become legendary.

These three characteristics, transparency, fineness and precision, have greatly determined the use of the binders. A binder as fine as the Coromandel shellac  requires a high level of technical skills in order to get a precision that other binders allow a lot more easily. Therefore, it is not an accident if shellac is practically never used in painting whereas it is commonly used in cabinet making. Nevertheless, intrinsic properties of shellac can advisedly be taken advantage of in art.

Permeability, respiration

A permeable paint is not only a paint letting other substances pass through. To deserve the epithet "permeable", the paint must also bear to be gone through.

In other words, a permeable paint does not self-dilute or self-dissolve.

The most exemplary permeable binder is without any doubt lime.

A gouache lets pass the humidity, but is moved by it though:  the paint gets diluted at the same time.

Opposite, binders such as acrylic, vinyl, linseed oil or shellac do not let anything pass through. Those paints are impermeable and do not let the support breathe.

Binding or mordant dyes

Are rather uncommon particular cases.

A few examples :

Graphite is both a binder and a pigment.

Curcuma (see picture) is a substantive dye pigment used in dye works.

Gallnut black is another substantive tinctorial dye pigment, used to make ink.

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