On the white ground, scraped, when it was perfectly dry, till it was "as white as milk and as smooth as ivory" (Cennini), the outline of the picture was drawn, and its light and shade expressed, usually with the pen, with all possible care; and over this outline a coating of size was applied in order to render the gesso ground _non_-absorbent. The establishment of this fact is of the greatest importance, for the whole question of the true function and use of the gesso ground hangs upon it. That use has been supposed by all previous writers on the technical processes of painting to be, by absorbing the oil, to remove in some degree the cause of yellowness in the colors. Had this been so, the ground itself would have lost its brilliancy, and it would have followed that a dark ground, equally absorbent, would have answered the purpose as well. But the evidence adduced by Mr. Eastlake on this subject is conclusive:-- * * * "Pictures are sometimes transferred from panel to cloth. The front being secured by smooth paper or linen, the picture is laid on its face, and the wood is gradually planed and scraped away. At last the ground appears; first, the 'gesso grosso,' then, next the painted surface, the 'gesso sottile.' On scraping this it is found that it is whitest immediately next the colors; for on the inner side it may sometimes have received slight stains from the wood, if the latter was not first sized. When a picture which happens to be much cracked has been oiled or varnished, the fluid will sometimes penetrate through the cracks into the ground, which in such parts had become accessible. In that case the white ground is stained in lines only, corresponding in their direction with the cracks of the picture. This last circumstance also proves that the ground was not sufficiently hard in itself to prevent the absorption of oil. Accordingly, it required to be rendered non-absorbent by a coating of size; and this was passed _over_ the outline, before the oil-priming was applied."--_Ib._ pp. 383, 384. * * *