PIGMENTS 177
tance of white lead as an artist's pigment. Neither zinc oxide nor metallic zinc
seem to have been known as individual substances in the ancient world, though
certain zinc ores were used in making brass. Zinc was first described as an element
by Margraaf, a German chemist, in 1746. Although the use of the oxide, as a
substitute for white lead, was first suggested by Courtois of Dijon in 1782 (see
Rose, p. 84), more than 50 years passed before it became commercially available.
According to Church (p. 134), as early as 1834 a peculiarly dense form of zinc
oxide was introduced as a water color pigment by Messrs Winsor and Newton,
Ltd, of London, under the name, * Chinese white * (see also Winsor and Newton's
catalogue, 1930 ed., p. 15). The chief difficulty in the way of its commercial use
at that time was its poor drying qualities in linseed oil. In the years 1835-1844,
Leclaire in France showed that this difficulty could be overcome by using with the
zinc oxide an oil that had been rendered siccative by boiling with pyrolusite
(MnO2), and in 1845 he began, near Paris, to produce zinc oxide on an industrial
scale. By 1850, it was regularly made as an oil paint. De Wild says (p. 40) that
the first trial orders of such paint, from the firm of Hafkenscheid in Amsterdam,
were in 1854.
In the French process of manufacture zinc vapor, from molten metallic zinc,
is burned in an oxidizing atmosphere at a temperature of about 950° C., and the
fumes of white oxide are collected in a series of chambers. In the American or
direct process, zinc ores, principally sphalerite (zinc blend, ZnS), are mixed with
coal coke and burned, and the white smoke of zinc oxide is collected in suitable
chambers. In either process the occurrence of such impurities as metallic zinc,
soot, and other metallic oxides can seriously impair the general quality and
whiteness of the product. So-called e leaded zinc oxides,* which are made by the
direct oxidation of lead-bearing zinc ores, contain several per cent of lead sulphate.
Dry zinc white comes on the market in various qualities and degrees of whiteness.
* White seal * and * green seal * zinc white contain over 99 per cent zinc oxide; the
latter has the better hiding power. * Red seal * and * gold seal * are understood
to be slightly less pure, and ' gray seal * zinc white contains metallic zinc.
Zinc oxide is a pure, cold white. In the dry state it is lighter and more bulky
than white lead. It is non-poisonous but is a mild antiseptic. It requires more oil
(18 to 20 per cent) to form a paste than white lead. It has a tendency eventually
to dry brittle and to crack. Mixtures of zinc oxide and white lead combine the ad-
vantages of both pigments.
As would be expected, since zinc oxide originates as a smoke, it is very finely
divided and separate grains are difficult to observe except at high magnifications.
Merwin says (p. 506) that the pigment from zinc vapor (French process) has a
grain size much less than i^ in diameter. The refractive index (o> = 2.00 £Mer-
win]) is about the same as white lead but, unlike the latter, is little birefracting,
In ultra-violet light, the oxide appears bright yellow. It is not affected by strong
sunlight. It is readily soluble in dilute alkalis. Although it can react with hydro-