The ground that covers the canvas ought to be of pure white lead mixed with linseed or poppy oil, and only enough should be applied just to fill up the pores, not to form a thick bed. The prepared canvas should not be used till this white ground is so dry that the nail makes a grating noise when passed over it. Whiting and glue should never be used, for though the mixture lasts well if kept thoroughly dry, damp must ruin it in time., ,,