The Use of Flake-White

The third point, concerning which a few remarks seem advisable, is connected with the use of flake-white. There are many old oil-paintings in which the only perfectly-preserved parts of the work are those in which flake-white has been used with considerable freedom. Here the continuity of the layer of pigment is intact, elsewhere there are cracks and roughnesses and scalings-off. To what cause is the preservation of the high lights and of the paler flesh-tints attributable? The association of hardness and cohesiveness which these parts show is traceable to the white lead. This pigment was formerly always prepared in such a way as to contain a considerable quantity of lead hydrate. The particles of this hydrate do not lie, as it were, side by side with those of the chief constituent (the lead carbonate), but are so united with the latter as to form one complex compound. This compound acts upon the linseed or other drying oil with which it is ground, forming a substance of great hardness and durability.

This substance - that is, the entire mass of the white-lead ground in oil which has become solid, tough, and hard - seems to contain a small percentage of a lead-soap, formed probably out of the free fatty acids of the linseed-oil. But whatever the complete explanation of this hardening action may prove to be, there can be no doubt that we must attribute to the simultaneous presence of oil and the hydrato-carbonate of lead the preservation of the continuity of surfaces of the whites, and of the pale tints into which white lead enters, in many an old picture. No other pigment in common use is capable of solidifying the admixed oil to anything like the extent that characterizes white lead. Now there are modern preparations of white lead made chiefly by precipitation or the 'wet way,' which produces a pigment containing little or no lead hydrate. Some writers on pigments advocate the use of these newer products. 'Why,' say they, 'should you carefully exclude from your pictures oils, and varnishes, and siccatives which contain lead in solution, and then introduce the same or a like substance in your white lead ground in oil?' Many years ago I tried to answer such a question as this by means of experiment.

I was actuated by a desire, based on theoretical considerations, of preventing altogether the formation of lead-soaps. I tried comparative experiments with zinc oxide, pure lead carbonate, and the Dutch-made lead hydrato-carbonate, or ordinary flake-white. The two lead pigments (with which alone we are now concerned) were washed thoroughly with distilled water and dried before being ground in linseed oil. The oil-paints thus prepared were spread in duplicate series upon glass, paper, and primed canvas; one set was kept in a dark box. the other was exposed to strong light. So decided was the superiority of the ordinary flake-white over the pure carbonate, when both series of specimens were examined after the lapse of various intervals of time, that I was reluctantly compelled to abandon my recommendation of the latter. Ease in working, solidity of body, and rapidity of drying, were not the only points of superiority; for the films of paint, after having been kept a year, showed differences in hardness and in smoothness of surface which were all in favour of the hydrated carbonate. No discoloration was observed in the specimens exposed to light, except in the case of the pair upon paper; the absorbent ground had withdrawn some of the protecting oil, and both specimens had equally darkened.

In darkness all the specimens had become of a somewhat greyish yellow, the discoloration being about equal in all the pairs, the pair spread on paper having, as in the previous case, become darker than the others. The late Mr. G. W. Wigner tried a somewhat similar series of experiments, and came to the same conclusions. I should add that these deductions were corroborated by the results of other trials, in which numerous permanent coloured pigments mixed in pale tints with these two lead whites were treated in the same way. If, however, we feel bound to recommend the ordinary flake-white instead of pure lead carbonate, that recommendation does not prevent us from excluding lead-containing oils from our pictures, seeing that we possess perfect substitutes for them, and that there is no reason for thus multiplying the causes of possible change.