It is not only the varnish, or rather the gums which compose the varnishes, that should be considered with great attention, in reviewing this subject,--but the great stress which seems to have been universally laid upon the necessity of purifying the oils. And this necessity is insisted upon from the earliest times. Even after all the precaution and pains taken to purify oils, there will be a tendency to turn yellow upon the surface. Rubens, in a letter, speaks of this, and gives orders for his pictures, which were packed freshly painted, to be exposed to the sun. And this practice of exposure to the sun seems to have been adopted generally in Italy, as well as elsewhere, not only for the purpose of drying the paint more readily, but for the freeing the surface from the yellowing of the oil, the deleterious portion of which is thus taken up by the atmosphere and the heat of the sun. We have unhesitatingly exposed the surfaces of freshly painted pictures not only to the sun, but to all weathers,--and that not for a few hours but for weeks--and always with advantage. There is another method also which will be found equally beneficial. When the surface is greasy, and will not take water from the sponge, it may be truly conjectured that this deleterious quality of the oil has exuded. We always remove it by sand and water--the coarser the sand the better; the finer, being more silicious, is more likely to cut. But we must observe that even though the picture be not fairly dry, excepting under very rough usage, the paint will not be at all removed. Even after this cleansing, the oil will still, for a considerable time, throw up this greasy product. We remove it, therefore, again and again until, after a week or ten days' trial, we find the surface free from grease; and we are strongly inclined to think the colours undergo no change when this clearance has been once well effected. In a letter from Mrs Merrifield, she strongly recommends this exposure of pictures to the sun and atmosphere; and says it was universally practised. This should not, however, prevent the previous purification of the oils; for there is no writer upon the subject that does not insist upon this. Mr Eastlake's book furnishes recipes of all ages. Frequent washings with water, to which a little salt is added, and fine sand to take down the impurities of the oil, may be safely recommended. In describing the process taught by the Gesuate, friends of Perugino, the Padre Gesuato adds, "Observe, that wherever you find oil mentioned, this purified oil is meant."