We get a hint of what's involved in the use of gouache (pronounced "gwash") by considering the origins of the name: it comes from the Italian aguazzo, for "mud."
Like Italian, gouache is a continental European invention. It was transplanted to England via French and Italian decorative and landscape painters in gouache, working in London in the 18th century. Like mud, gouache is wet and opaque. The topic is also muddied by different definitions of what gouache is or how it should be used. And many artists react to gouache in watercolors as if it really were mud — it's probably the most vehemently controversial technique in watercolor painting.
The technique has a long and complex history. It first appears in the decorative and pictorial embellishments to medieval illuminated manuscripts. The earliest modern examples are nature paintings by the 16th century German artist Albrecht Dürer (the fur on that famous hare, for example), and continuing in a series of paintings by Gaspard Dughet (1615-1675, called "Poussin" in England). It reached a prolonged high point in the 18th century, particularly in France, in decorative works by the French painter François Boucher (1703-1770). Gouache was probably introduced to England by painters with French ancestry, such as Joseph Goupy (1689-1763) who was drawing master to the family of George I, or by painters who became influential in England after a career on the Continent (such as Marco Ricci, 1676-1729, Francesco Zuccarelli, 1702-1788, or Charles Clérisseau, 1722-1820). From around 1740 English topographical artists used the method frequently; Paul Sandby in particular handled bodycolor with considerable skill and variety, and knew Goupy, Zuccarelli and Clérisseau personally. By that time, use of gouache or bodycolor on the Continent had begun to decline into purely decorative painting on fans, screens and theatrical scenery. The Swiss topographical artist Louis Ducros (1748-1810), working in Italy on large format gouache and watercolor paintings, continued almost alone to use the medium in fine art works. In the 1830's the method was revived by several Victorian watercolor artists, who used it until the end of the century.
First, some distinctions. The method of mixing watercolor pigment with an opaque white pigment in a watercolor vehicle (made with gum arabic) is traditionally referred to as gouache. The method of mixing concentrated watercolor pigments with a vehicle that is made with fish gelatin (isinglass jelly) or animal gelatin (size) — without the addition of any white pigment — is traditionally called bodycolor (or distemper in England). However, the two terms are sometimes confused or used interchangeably, both in historical writings and current usage: some "designer's gouache" paints are made with concentrated pure pigment in a watercolor vehicle, without any added white pigment.
The core meaning in all cases is that gouache or bodycolor is an opaque watercolor paint. This arises from six properties that cause gouache technique to be different from transparent watercolor technique:
- Gouache has a much thicker paint layer, and beyond the minimal amount of paint required to completely cover the paper or support, the thickness of the gouache paint layer does not affect the apparent color. Unlike transparent watercolors, gouache can be painted on a white or tinted support, with little or no difference in the finished color appearance.
- Colors must be lightened by adding white pigment, as in oil paints; they are not lightened by dilution to show more of the white paper, as in transparent watercolors.
- Paints are not applied in glazes or tints (unlike oils and watercolors, where glazing one color over another is a common technique).
- The applied paint is not absorbed into the paper but remains on the surface in a thick layer, allowing for limited textural effects created with brushstroke variations in the surface of the paint (although gouache will crack when dry if laid on too thickly in "impasto" layers). Transparent watercolor is sometimes described (somewhat inaccurately) as "a stain on the paper," which only means that textural effects in watercolors are limited to pigment granulation, water based diffusion such as blossoming, brushwork, or the pinhole texture of rough finished papers made visible by unevenly applied paint.
- Gouache creates flawless, flat color areas, which are more difficult to attain in watercolors. Because of the concentration of pigment and filler, gouache is resistant to water induced variations in paint appearance such as blossoming or blooming.
- The paint covers all paint layers below it, so that the method of painting is more direct, especially for complex patterns such as leaves or flowers. In transparent watercolors, a dark background must be carefully painted around the white flower in front of it, and painters normally work by laying darker colors on top of light; but in gouache the background can be painted first, then the flower directly on top of it (just as in oil painting), and lighter colors can often be layered on top of dark if that is a more convenient way to work.
Many of these points have caused gouache to be especially popular with architectural or commercial artists; some types of gouache paints are even labeled "designers' colors." The flat color fields photograph and reproduce very well, which makes gouache ideal for photoreproduction artwork. Gouache does not undergo a chemical change when dry and therefore can be rewetted and reworked, just like watercolors. And the more direct painting method and rapid drying times mean that a project can be completed relatively quickly.
The similarity between oil and gouache painting techniques allows watercolor artists who use gouache to emulate the free, vigorous style and strong contrasts of value that are possible with oils. However, gouache will crack or discolor if applied too thickly, so texture is created by the surface of the paint, not its thickness. Standard watercolor brushes can be used, although the same brush should not be used for both gouache and watercolors if the gouache is made with a white filler. However, because gouache is relatively thicker than transparent watercolor, a stiffer synthetic or acrylic brush may work better than a natural-hair brush.
Gouache paintings are typically done on hot pressed papers or smooth art boards, since the paint imparts most of the texture and these surfaces help to create a perfectly flat paint film. Tinted papers are also more commonly used, since the tint is easily covered wherever desired, but lends a pleasing background hue in unpainted areas.
Gouache paintings should be handled just like finished watercolors: matted and framed under UV shielding glass or plastic.