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Does a "good" lead white need to have some lead-acetate left in it, or should it all be removed by repeated and thourough washing ? Alvah H. Sabin in "White Lead its use in paint" 1920, questions this and proposes that up to as much as 5% acetate of lead should be left in, or added, to make a better paint. I am aware that he is talking about house paint, but has this merrit ?
In " Mannel des jeunes Artistes et amateurs en peinture" 1831 , M.P.L. Bouvier writes that to use lead-white for watercolour we must take a twig from white wood, peel the bark off, then whip up the lead-white pigment while in water, and only use the froth/foam. After testing the foam/froth from five different batches of lead-white paint I produced, I found no traces of lead acetate while the supernatant had the usual acetate content. Is the foam/froth a different make up then ?
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I am not sure that I can provide the final word on this and
will reach out to others for confirmation. I do have a few thoughts about the
First, residual lead acetate and other lead products left in
poorly washed lead white are responsible for the fast drying reputation of lead
white. Lead oxide, lead tetroxide, and lead acetate all make oil paints that
dry very fast. Lead white is actually quite average in its drying speed.
My guess is that the reactivity of lead acetate makes is a
poor component in lead white paint. I would also bet that it contributes far
more mobile ions that are more likely to contribute to lead soap defects. It
may also be more prone to other changes.
As to whipping lead white in water and using the foam, my
guess is that this was a form of levigation. A watercolorist would want the finest
pigment particles. The stack lead white process produces a large range
of particle sizes unlike modern precipitation processes. The whipped mixture
would allow the finer lead white particles to remain longer in the water than
the coarser particles would quickly fall out of solution.
I am not aware of any research that provides a definitive answer to this question, however, the old Dutch method or stack process of making lead white initially produces lead acetate, which usually exists in excess in the final product and is unstable in the presence of carbon dioxide. Every manual on the manufacture of lead white stresses the importance of washing the lead white to remove limpurities. Although not specifically stating the importance of removing lead acetate, I believe that this is an important aspect of levigating the pigment to remove all traces of lead acetate, which is indicated when the wash water no longer turns blue.I beleive the purpose of Bouvier's instructions of stirring a slurry of lead white in water would cause the finest pigment particles to rise to the top where they can be decanted and used in fine watercolor techniques.