Aqueous Paints

The bulk of the vehicle in an oil painting is linseed oil or, in some cases, a mixture of oils and resins; whatever volatile material may be used as a thinner [such as turpentine or petroleum solvent] is normally present in a relatively small proportion.

On the other hand, the binding ingredients of all the water mediums are very powerful adhesives and binders; even with the least powerful of these, the greatest bulk of a normal water paint, when it is ready for use, is water.

As has been noted, the oil vehicles do not evaporate, but remain in the dried paint film in approximately the same bulk as that which they occupied in the wet paint. Consequently, the pigment, still surrounded by a substantial excess of dried vehicle, will display the same or nearly the same color effect as did the wet paint. This same surplus vehicle also serves as a protective casing for the pigment. It has such flexibility, and such resistance to harm from moisture, temperature changes, cleaning agents, and wear and tear, that a well-made oil painting may be done on canvas, framed without glass, and, if kept under the normal conditions of preservation which works of art are intended to receive, may be expected to survive for hundreds of years.

Paints made with water vehicles [as has been pointed out in commenting on the reasons for their mat effects as compared with the glossier oil paintings] contain much less of such surplus binder; unlike oil paint, the bulk of the paint--the water--evaporates completely on drying, leaving the pigment particles exposed or partially exposed to the air. This not only causes the mat effect and the brightness of the hues, but also eliminates a good many of the troublesome ills to which improperly painted oil paintings are subject. It makes necessary a different set of precautions as to the durability of the works, because they are inherently less resistant [p. 157] to wear and tear or to atmospheric conditions; yet many of the oldest paintings in our museums were painted in water techniques.

Regarding permanence, they can be said to be more fragile but just as permanent as oils if properly preserved. For example, watercolors are framed under glass and backed with rigid cardboards [or mounted on them], whereas oil paintings are normally framed without glass. Some of the aqueous painting techniques have a considerable degree of material flexibility, but none are so elastic as oils and are therefore not suitable for use on canvas.

The principal difference between the handling or brushing out of water paints and that of oil paints is the more limited special brushwork necessary for using water paintings because of their extremely rapid rate of drying as compared with the slow-drying of more plastic oil paints. This narrows the working or manipulative properties of water mediums, and limits the brushwork either to free, fresh stroking or to more complex methods, such as allowing the paint to dry and then going over it with other manipulations. However, watercolor and gouache can be used in blended, graduated, and wash effects much more successfully than tempera or fresco, where handling is in general confined to pencil-like touches applied in single strokes and left alone. Oil paints are the most versatile of all.

Paints with simple gum or glue binders, including watercolor, gouache, and poster colors, dry by simple evaporation of water, and the dry paint's physical properties and solubility remain unchanged. Casein colors and tempera colors become set by going through a chemical process [denaturing of a protein], and their binders or film formers become new substances, no longer soluble, which puts them in a different class or group of film formers along with oil paint and the acrylic polymer colors, both of which when dry are unaffected by the thinners or solvents that diluted them when they were liquid. [pp. 156-158]

Preservatives and Odorants - The following mold preventives or preservatives are efficient for use in aqueous paints:
Small amounts of a weak solution of phenol [carbolic acid], about a half teaspoon of the 1 percent solution to a pint. Drugstores will generally sell it up to about a 5 percent solution, this low concentration being deemed harmless.

Sodium orthophenyl phenate [trademark "Dowicide A"]--a powder obtainable in chemical supply stores. Small amounts [1/4 teaspoon to a quart of binding solution] will effectively preserve gums, casein, and glues.

Another older but effective preservative for gum solutions is beta-naphthol used in small amounts.

A 4 percent solution of formaldehyde. Formaldehyde is likely to react erratically with some binders and is better suited to industrial than to amateur use.

The odor of water paints is sometimes a little disagreeable; a [p. 163] few drops of any pleasant odorant will correct it. Care must be taken not to perfume the product too strongly, for that will usually be more objectionable than the natural smell of the paint. A "clean-smelling" material such as oil of cloves or sassafras is more appropriate than sweet, flowery odors. Oil of cloves also has a mild preservative action. [pp. 162-163]

[Mayer, Ralph. The Painter's Craft. An Introduction to Artist's Methods and Materials. Revised and updated by Steven Sheehan, Director of the Ralph Mayer Center, Yale University School of Art. New York: Penquin Group. 1948. 1991.]

Water Paints
The history of water-thinned paints, similar to present-day poster paints and watercolors, goes back many centuries before the Christian era. The Egyptian, Greek, and Roman artists used binders made of vegetable gums, hide glues, milk products (casein), and many other sticky materials which could be thinned in water. Pictures made with such colors have ranged in expressive character from the cool decorative balance of the Egyptian tomb paintings to the vibrant intensity of the early Christian manuscript illuminations. One should not assume that there is anything inherent in any of the water techniques that restricts these media to sketchy spontaneity or to a weak visual impact . . . .

For a painter familiar with the characteristics of oil colors, one distinguishing factor about the water techniques is that they dry rapidly. This makes it possible to work one layer of paint over another in rapid succession and to develop a color idea quickly. For this reason water media, such as gouache or transparent watercolors, have often been favored as materials for quick color notes or sketches . . . .

Oil painters also find that they can safely dilute water media to a very free-brushing consistency, and that they can work in a flowing line with an ease impossible with buttery oil colors. This line may become an element of a free, robust style, such as we see in Rouault's watercolors, or it may be used in a restrained precise delineation, as is shown by some of Dürer's transparent watercolors.

Furthermore, it usually becomes apparent that water techniques tend toward brilliant clear tones in the lighter range of values, rather than toward the rich mysterious darks commonly associated with oil techniques. This by no means indicates "candy-box" harmonies, but rather the high brilliance of some of Klee's watercolors . . . , as opposed to his oils, which are deeper in color, or the rather restrained elegance of Bonnard's gouache paintings, as contrasted with the rich color of his oils.

On the other hand, it is worth noting negative characteristics that limit the water techniques. First, their films will usually crack if they are applied as thickly as an oil paint impasto. Therefore their textural range is quite subtle by comparison to the possibilities of high relief in oils or encaustic painting. Second, colors in water technique cannot be fused and blended with the easy precision that the oil technique allows. Thus it is difficult to produce in water technique a kind of explicit reconstruction of weighty volume, such as Rembrandt developed in the oil technique, with its fluent blending and rich textures. Instead the water media usually lend themselves to graphic suggestion of volumes and to broad simplifications of light and color relationships.

It may be useful to consider the water media in two main groups: those forming films that can be easily dissolved again when water is applied to them, and those that when dry are resistant to re-solution in water.

In the first group are such paints as (1) transparent watercolor (aquarelle), gouache, size or distemper colors, and various so-called poster paints. These employ gums, glues, or starch products as binders, which do not undergo any chemical change when they dry or when they dissolve. Rather, they dry only by the evaporation of the water content of the paint and may be dissolved by the addition of some water to the dry material. In the second group are the various (2) tempera paints, which change their chemical composition or are polymerized as they dry and so become relatively water resistant after they harden. The binders of these paints usually consist of a combination of an adhesive material, an oily or resinous material, and water.