_Desiccation or drying._ The well-known additions of the acetate, or sugar of lead, litharge, and sulphate of zinc, either mechanically ground, or in solution, for light colours; and japanner's gold size, or oils boiled upon litharge, for lakes; or, in some cases, manganese and verdigris for dark colours, are resorted to when the pigments or vehicles are not sufficiently good dryers alone. It would be well if lead and copper could be banished from the list of siccatives altogether: assuredly, no artist with any regard for the permanent texture of his work should employ them except in extreme cases, and in the smallest possible quantity. The best of pigments may be ruined by their injudicious use, and obtain a character for fugacity which they in no way deserve. It requires attention that an excess of dryer renders oil saponaceous, is inimical to drying, and is otherwise injurious. Some colours dry badly from not being sufficiently edulcorated or washed. Sulphate of zinc, as a siccative, is less powerful than acetate of lead, but is far preferable in a chemical sense. It is supposed erroneously to set the colours running; which is not positively the case, though it will not retain those disposed to move, because it wants the property the acetate of lead possesses, of gelatinizing the mixture of oil and varnish. These two dryers should not be employed together, since they counteract and decompose each other, forming two new substances--acetate of zinc, which is a bad siccative, and sulphate of lead, which is insoluble and opaque. The inexperienced ought here to be guarded against the highly improper practice of some artists, who strew their pictures while wet with acetate of lead, or use that substance in some other mode, without grinding or solution; which, though it may promote present drying, will ultimately effloresce on the surface of the work, throw off the colour in sandy spots, and expose the paintings to peculiar risk from the damaging influence of impure air.