Gouache often produces a flat-looking, "drawn" result rather than a realistic one.
Gouache, also called opaque watercolor, is a type of water-based paint bound with gum, usually gum arabic. Unlike watercolor, gouache contains an opaque pigment that allows strokes to completely color one another. This medium has been used for centuries and can be found in a number of Renaissance and Medieval paintings and studies. Gouache can take some practice to use successfully, but it produces attractive, painterly results when used by a skilled artist.
Composition
Both watercolor and gouache use powdered pigment bound using vegetable gum, usually gum arabic or gum tragacanth. These gums are mixed with water, combined with pigment and allowed to dry to a paste or hard cake, which can be reconstituted using water. Watercolor paint contains no other ingredients and produces a translucent result on paper. Gouache also contains about 15 percent ground chalk. This relatively inert pigment makes the paint opaque without increasing its value or reducing its chroma, or intensity.
Characteristics
Gouache's high chalk content makes it a fast-drying paint that can be hard to blend. Gouache colors also dry to a different value than the wet paint. Most light colors dry lighter in value, while dark colors tend to dry with a darker value. These characteristics can make gouache difficult for painters trained in other media to handle. Unlike acrylic, egg tempera and many other fast-drying paints, gouache is re-wettable and can be "picked up" with the brush if it dries early on the palette or page.
Technique
Most gouache painters take advantage of this medium's fast-drying characteristics to create painterly works made up of many discrete patches of paint. Few people blend gouache extensively, since this medium makes smooth gradients difficult. Gouache works well on illustration board, watercolor paper and gessoed board, but it does not stick well to very slick surfaces with little or no tooth.
Considerations
While gouache is opaque enough to layer, its tendency to lift up when touched with a wet brush reduces painters' ability to make multi-layered paintings in this medium. Gouache can also be frustrating to work with for painters who are used to mixing on the page. Like watercolor, it tends to stain paper slightly and finished paintings are susceptible to damage from very damp environments.
