Standards and Requirements

Artists' paints are produced by combining a coloring agent [dry pigment] with a liquid binding medium. The dry pigment may be of natural origin, such as a native earth, or it may be produced artificially in a factory. Through its particular physical characteristics, it gives the paint its color and has other effects on the durability, brushing properties, and drying speed of paint.

The binding medium is the liquid material that holds the particles of dry pigment to each other and fastens them collectively to the ground or support material.

Various thinners or painting mediums may be added to the paint to bring about desirable brushing qualities.

The characterizing difference between the various techniques, such as oils, watercolor, tempera, and fresco, is that each uses as a binding medium a substance distinct from that employed by the others. On the other hand, with comparatively few exceptions, each category of paint lists the same pigments as coloring agents. [p. 4]

General Requirements
A pigment used in artistic painting should satisfy at least the following requirements:

l. It should not fade or change color when exposed to the prolonged action of normal light.

2. It should not interact chemically in a harmful way with any of the materials such as other pigments, binding mediums, or grounds with which it must come in contact during normal painting procedures.

3. It should not ñbleedî through or ñmigrateî through dried layers of binder. Some colors, which dissolve in their binder instead of simply remaining dispersed in it, continue to stain layers of paint put on top of them, even when the underlayer is thoroughly dry.

4. Its color and character should remain unaffected by the acid or alkaline fumes found in the atmosphere to which the painting will normally be exposed.

5. It should form with its binder a tough, physically stable film, resistant to the sort of wear and handling that may be expected for the picture.

6. It should not have poisonous effects when handled with normal precautions in the artistÍs studio.

[p. 5]

Fresco - [Requirements]
In the buon fresco technique, dry pigments are ground with water only and brushed on wet lime plaster. When the plaster dries, the pigment is permanently bound to it but is not varnished and hence protected from the acid effects of the city atmosphere. Furthermore, the lime in the plaster is a strong alkali which bleaches out many pigments.

Therefore, the list of pigments available to the fresco painter is comparatively limited, containing only those colors that remain unaffected both by acids in the air and the alkaline action of lime.

Oil - [Requirements]
Pigments used in oil paints are bound together and to the canvas by a drying oil, such as linseed oil, and are thinned by such solvents as turpentine or mineral spirits. The different pigments ground in the same oil absorb different amounts of oil, dry at different rates, and form films of varying quality, form the desirable tough flexible films to brittle or crumbly ones, which may crack or peel in a short time. There are even colors [such as asphaltum or bitumen] that never dry in linseed oil.

Pigments used in oil technique should dry in linseed oil to an acceptable strong film.

Pastel - [Requirements]
For pastel painting, pigments are very loosely bound into lumps or sticks, like chalks, using a dilute binder, such as gum tragacanth. Colored drawings or paintings are made on paper or cardboard with these chalks.

Since much pigment dust is normally raised by this technique, it is important that poisonous pigments be excluded from the pastel lists, lest they be inhaled by the artist.

Synthetic Resins - [Requirements]
Industrial research has developed synthetic binders such as vinyl or acrylic polymer resins which are now used extensively in artists' paints. Some of these new binders have pronounced chemical characteristics [for example, emulsions of the acrylic resins are quite alkaline] that may limit the list of pigments used in these techniques.

Watercolor, Gouache, Egg Tempera - [Requirements]
In transparent watercolor [aquarelle], qouache, and unvarnished egg yolk tempera techniques, the pigments are bound in comparatively thin films to paper, cardboard, or a prepared wood panel by solutions of gum arabic, animal glues, or egg yolk. The paints are thinned with water. Because these binders do not encase the pigments so completely in a glassy film as is the case in the oil technique, the pigments used in the water techniques are more vulnerable to the effects of sunlight and the atmosphere.

Thus pigments used in water techniques should be as resistant as possible to the chemical effects of acid- and sulfur-carrying gases, which are present in city atmosphere, and to the bleaching effect of lgiht.