In his Transactions (1806), S. Grandi describes a method of preparing an absorbent ground for panels, but for which he later wrote works equally well for stretched canvas. He described boiling sheep trotters* in water to remove the greasy parts, calcining them and then grinding them to a powder. Next, prepare a thin paste of wheat flour and add an equal amount, presumably by volume, of the powdered bone ash and grind the whole mass well together.
Apply the first coat of the bone paste to the canvas with a stiff brush, working it well into the weave of the canvas. Apply a second coat of the paste, allow to dry and smooth with sandpaper. A third coat must be applied, allowed to dry and lightly sanded smooth. Finally, apply a thin coat of linseed oil or walnut oil by rubbing it into the surface of the ground.
Pigments can be added to the third coat and a fourth coat of the paste, if a colored ground is desired.
Hundertpfund (1849) described the following method for preparing an absorbent ground:
Spread a thin layer of boiled flour and pipe clay over well-stretched, unbleached, even-threaded canvas. Let it dry and repeat the operation until the canvas shows no more open pores. If the paste is the consistency of liquid honey, the canvas will only require three or four coats. Afterwards, he recommends coating the ground with a thin lead white oil ground spread thinly over the entire canvas, and allowed to dry in the sun before painting.
The recipe for the flour paste is as follows:
Take a certain amount of wheat flour, mix it well with a little cold water in a pot, adding gradually more cold water and continually stirring until it appears like thick milk. Let it boil very slowly with constant stirring. As it thickens, stir it quicker, until it is a thick smooth paste, which must look shining. Now stir in some warm water, and let it boil slowly for half an hour. By continual slow boiling, it becomes smooth and smoother, so that it may afterwards be thinned with water, according to your liking. Now place some pipe clay in water until the clay is completely wetted, stir it afterwards with water to the same consistency as the flour paste, mix these two in equal quantities and pass them through a fine sieve.
When this mass is as thin as is required for watercolor painting, after it has been warmed again, spread it on the canvas or panel. If the first priming is laid on warm, it penetrates better into the support.
Bouvier's absorbent ground in his painting treatise called for the use of glue made from starch or 'belle farine' (flour); this was added to pipe clay** 'of the whitest and purest that can be procured from the chemist (marchand drogist).' The clay was mixed with the glue to the consistency of thick cream. The mixture was then rapidly applied with a large brush to an already sized support, the moisture being readily absorbed and the ground dried instantly.
The question of an even thickness, mentioned by Bouvier, seems to have been crucial. Doerner points out that a good chalk ground should appear uniformly thick when held up to the light, and should not show any gaps or crevices. Presumably, unevenness would leave the ground prone to cracking at points where the thickness varied. Bouvier stressed: "your ground should present you with a beautiful unified surface, without the colour being thicker in one place or another" after the recommended three or four coats of priming.
*Instead of sheep trotters bones, one may substitute bone ash that is already in a form that can be used immediately with the flour paste.
**Calcined kaolin is an excellent substitute for pipe clay.