About Gouache
Gouache is a French term used to describe a type of watercolour paint. The word is derived from the Italian guazzo which literally means a watering place. Before the 19th century the term guazzo was applied to glue-size painting.
Gouache is opaque. The pigments used are ground in water and mixed with gum. The results of using gouache appear quite different from those achieved with washes of watercolour, the look of which is more dependent on the nature of the paper beneath.
The National Gallery Glossary
Gouache has a considerable history going back over 600 years. It is similar to watercolor because it can be rewet and the paint can become infused with its paper support. It can also form a superficial layer like acrylic or oil paint. Also like watercolor, gouache dries to a matte finish.
Gouache paint is similar to watercolor modified to make it opaque. Just as in watercolor, a binding agent is present. This was traditionally gum arabic but since the late nineteenth century cheaper varieties use yellow dextrin. When the paint is sold as a paste, e.g. in tubes, the dextrin has usually been mixed with an equal volume of water.[1] To improve the adhesive and hygroscopic qualities of the paint, as well as the flexibility of the rather brittle paint layer after drying, often propylene glycol is added.[1] Gouache differs from watercolor in that the particles are typically larger, the ratio of pigment to binder is much higher, and an additional white filler such as chalk, a "body", may be part of the paint. This makes gouache heavier and more opaque, with greater reflective qualities.[2]
Gouache generally dries to a different value than it appears when wet (lighter tones generally dry darker and darker tones tend to dry lighter), which can make it difficult to match colors over multiple painting sessions. Its quick coverage and total hiding power mean that gouache lends itself to more direct painting techniques than watercolor.[3] "En plein air" paintings take advantage of this, as do the works of J.M.W. Turner and Victor Lensner.
Gouache is used most consistently by commercial artists for works such as posters, illustrations, comics, and for other design work. Most 20th-century animations used it to create an opaque color on a cel with watercolor paint used for the backgrounds. Using gouache as "poster paint" is desirable for its speed as the paint layer dries completely by the relatively quick evaporation of the water.
The use of gouache is not restricted to the basic opaque painting techniques using a brush and watercolor paper. It is often applied with an airbrush. As with all types of paint, gouache has been used on unusual surfaces like Braille paper.[4] A variation of traditional application is the method used in the gouaches découpées (cut collages) created by Henri Matisse. His Blue Nudes series is a good example of the technique.
Gouache from Wikipedia
The Seven Gouache Hazards and How to Escape Them by James Gurney