painted is protected at the back by a composition of gesso, size, and
tow, over which a coat of black oil-paint was passed. This, whether
added when the picture was executed or subsequently, has tended to
preserve the wood (which is not at all worm-eaten), and perhaps to
prevent its warping."--_Ib._ pp. 373, 374.

* * *

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.

* * *

The perfect whiteness of the gro