A good quality designers' gouache contains the following ingredients:
- dry pigments
- distilled water
- wetting agent (oxgall)
- inert pigment (blanc fixe or precipitated chalk)
- binder (gum arabic)
- plasticizer (glycerine)
The dry pigments used are the same quality as those found in transparent watercolors, but they generally are not ground as finely because gouache is not intended for wash form.
Gouache has a greater proportion of binder to pigment than is found in transparent watercolors; for this reason, gouache produces a continuous paint film of considerable thickness and is not as suitable for washes.
The inert pigment makes gouache opaque and increases its brightness.
Only some gouache contains a wetting agent, which reduces surface tension and improves spreadability.
The plasticizer increases solubility, improves spreadability and prevents drying out in the tube.
Gouache does not undergo a chemical change when dry and can, therefore, be rewetted.
Gouache is an opaque paint system with excellent covering power.
If applied too thickly, gouache can be prone to cracking or peeling off.
Gouache dries lighter than it appears when wet; this makes color matching difficult when reworking an area.
Gouache films are less vulnerable than those of transparent watercolor because they are thicker, but paintings should still be protected through proper storage and curation.
The brilliance and luminosity which are characterisitic of gouache come from the surface of the paint film and not, as with transparent watercolors, from the white paper below.
Recommended painting surfaces include hot-pressed watercolor paper, cold-pressed illustration boards, Museum Board and good quality papers with a slight tooth.
Gouache is very effective on colored papers and toned grounds.
Gouache in small tubes (14ml - 15ml) has an average shelf life of 3 to 5 years. Although attempts have been made by several manufacturers to produce gouache in 4 oz. jars, evaporation remains a problem.
In addition to evaporation, a separation problem between the binder and the pigment also occurs with gouache in jars. Subsequent settling of the pigment results from the increased amount of gravity on the larger mass of paints found in jars.
While refrigeration will retard evaporation and reduce separation and settling, it will not prevent these tendencies from occurring.
Several European manufacturers have produced gouache in 4 oz. jars. These have never been available on a consistent basis for the American market because of the above-mentioned problems.