Gouache to acrylic or oil paints in that it is normally used in an opaque painting style and it can form a superficial layer. Many manufacturers of watercolor paints also produce gouache and the two can easily be used together. Gouache paint is similar to watercolor, however modified to make it opaque. Just as in watercolor, the binding agent has traditionally been 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. To improve the
Gum printing Gum printing Gum printing is a way of making photographic reproductions without the use of silver halides. The process uses salts of dichromate in common with a number of other related processes such as sun printing. When mixtures of mucilaginous, protein-containing materials together with soluble salts of dichromate are exposed to ultraviolet light, the protein content becomes tanned and resistant to solution in water. The untanned material can be washed away in warm water leaving a hardened, tanned protein negative. For gum printing a solution of gum arabic is mixed with either potassium or ammonium dichromate. The higher the proportion