PIGMENTS 175
dioxide from the fermenting tan bark, oxygen of the air, and water vapor slowly
transform the lead to basic lead carbonate. After being washed, dried, and
screened, the product is ground in linseed oil. Various other processes, including the
chamber, electrolytic, and precipitation processes, most of which are more rapid
than the Dutch, are also used for preparing white lead.
Although white lead can be purchased in a dry powder form, the greater part
of it comes on the market ground to a thick paste with 9 to 10 per cent of linseed
oil. Since white lead is a poisonous compound if inhaled as a dust or if ingested,
grinding and manufacture into paint was long regarded as a hazardous industry
and in several countries it was curbed by legislative action. Now, because of
improved factory methods, such dangers are no longer attendant. Painters, how-
ever, still suffer from * painters' colic * or ' plumbism ' if they are careless with it.
White lead is a very finely divided yet a definitely crystalline compound. At
400 X magnification, it can be observed to be highly birefracting. Merwin says (p.
514): 'Individual grains seen in several samples were tabular (perhaps twice as
broad as thick) and hexagonal in outline.' The refractive index is high, a? = 2.09.
It is commonly understood (see Beam, p. 45) that the lead hydroxide, Pb(OH)2,
part of the white lead molecule, is able, partially, to saponify linseed oil and to
form with it a lead soap (lead linoleate). This fact has been used to explain why
white lead in oil forms such a homogeneous, durable, hard, and non-porous paint
film. (White lead films are conspicuously strong, and their strength extends to all
mediums.) It is also given as the reason for the apparent increase in transparency
of old white lead films (see Eibner, Malmaterialienkundey p. 121) with the striking
through of darker underpainting, sometimes called, * pentimento*
The siccative or drying action of white lead upon oils is another reason for its
being so widely used. Pure white lead in oil is favored as an outside white paint
because it chalks on weathering (does not check or crack) and leaves a satisfactory
surface for repainting. On indoor exposure, however, it has a tendency to yellow,
particularly in the dark. It is darkened, even blackened, by contact with sulphide
pigments and hydrogen sulphide in the air because of the formation of black lead
sulphide. When protected by oil or varnish films, this reaction is very slow and the
effect may be negligible. In fact, white lead is commonly seen in paintings where
it has been mixed with vermilion (HgS), ultramarine, and other sulphur-bearing
pigments for centuries without any sign of incompatibility. With impure pigments
that bear free sulphur, however, a darkening effect may be quickly noticed. In
water color films, it is often seriously blackened. When heated at a moderate
temperature, white lead turns bright yellow because of the formation of massicot
(lead monoxide, PbO); higher temperatures melt the massicot and change it to
litharge and even further oxidize it to red lead. White lead is readily soluble in
dilute mineral acids and in acetic acid with effervescence (CO^). There is perhaps
no question that, so far as general use is concerned, white lead is the most im-
portant pigment in the history of Western painting. It is practically the only