White Lead is used in very large quantities by painters, not only as
white paint, but to mix with and qualify every shade of color, and to
give body to them. It is prepared by different processes, and several
patents have been taken out for improvements in its manufacture; but
the most usual, and, perhaps, the best process, is as follows:--A
layer of spent tan (from the tanner’s pits) is spread out three or
four feet thick, and in it a number of earthen pots are arranged
in rows, each partly filled with a mixture of vinegar, water, and
treacle, or some other acid fermenting liquid, each of these pots is
covered with a piece of lead made in the form of a grating (fig. 1),
and over all these a flooring of boards, and then again a layer of
tan, pots covered with lead, and boards, two or three times repeated;
this is called a white lead bed, and is left for several weeks; the
tan ferments and gets warm, causing the acid vapours to rise and
corrode the lead, at the same time giving off carbonic acid. At the
end of about three months the leaden covers are found to have become
completely changed into a white shining substance (carbonate and
oxide of lead), this is washed from impurities and ground in a mill,
under water, to a fine powder. It is the white lead of commerce; but
what is familiarly called white lead, is this substance ground up
with linseed oil into a thick paste so as to be ready for the use
of painters.
White lead is a very poisonous substance, and produces the disease
called painters’ colic, when taken into the system in minute
quantities and for a long time, so that all who have much to do
with this dangerous substance, as house-painters and artists, should
be extremely careful that their hands are well washed frequently,
and especially before going to meals. Cisterns of lead, used for
containing water, very soon become coated inside with a thin film
of sulphate of lead, this prevents the water from acting further
on the lead, and the water from such cisterns is never found to be
poisonous; but, if distilled water were used, it would act rapidly on
the lead, corroding it, and causing a deposit in the water of white
lead, which would render such water dangerous in the extreme.