This is raw plaster of Paris used alone or made by grinding plaster of Paris in glue size. A single coat is applied with a spatula. It was often used as a rough undercoat. A number of coats of this finishing gesso are applied at right angles to each other over an underlayer of gesso grosso. Precipitated chalk or a fine grade of purified whiting are considered improvements over Plaster of Paris.
Whiting is natural chalk that has been refined.
Gesso is not a flexible ground and should not be used on a flexible support such as stretched canvas. It was used by the Flemish on wood panels, for which it is still very much useful, especially for creating a pure white ground. For a very smooth finish surface, clay can be added. Marble dust is commonly available from art supply dealers for this purpose and some premixed gesso powders have marble in the ingredients. Gesso Recipe Soak 3 ounces of rabbitskin glue in 28 fluid ounces of cold water overnight, or until it has swollen and absorbed its full capacity of water.