174 PAINTING MATERIALS
product by a secret method for years. Finally, in 1859 Guignet patented a method
for making the hydrous oxide by a process which he described (see Guignet, pp.
149-153) as new and unique and which is the reduction of potassium bichromate
with boric acid, outlined above. This new green immediately replaced Schweinfurt
green (see Emerald Green) for printing and other industrial coloring purposes.
It must have been shortly after that time that the transparent oxide of chromium
was introduced as an artist's pigment. Laurie says (New Light on Old Masters,
p. 44) that the date is 1862. Messrs Winsor and Newton, Ltd, state (in their
catalogue, 1930, p. 20) that it was originally introduced by their house and they
say (in a private communication) that this pigment, as well as aureolin, was
popularized by Aaron Penley, a water color painter. Church says (p. 195) that the
pigment came to be best known in England by the name, viridian, and that it was
unfortunate that it came to be called Vert emeraude ' in France and, hence,
confused with the poisonous copper aceto-arsenite or Schweinfurt green, known in
England as * emerald green.'
Weld (arzica) is a natural yellow dyestuff, obtained as a liquid or as a dry
extract of the herbaceous plant, Dyer's Rocket, Reseda luteola, formerly culti-
vated in central Europe. The coloring matter is luteolin or tetrahydroxyflavone,
(CisHioOe). It is extracted with aqueous solutions and may be made into lakes of
various shades of yellow with different mordants. Although weld extract has lower
tinctorial power than some other natural yellow dyes like quercitron (see Querci-
tron Lake), it yields the purest and the fastest shades of all ( Colour Index> p. 294).
Sparingly soluble in hot water and moderately soluble in alcohol, it gives a deep
yellow solution with alkalis. Weld has had a long history as a dye and lake pigment,
and Thompson says (The Materials of Medieval Painting, p. 187) that it is still
cultivated in small quantities in Normandy for dyeing silk.
White Bole (see Bole and China Clay).
White Lead (flake white, Cremnitz white) is the most important of all the lead
pigments; it is the basic carbonate of lead, 2PbCO3'Pb(OH)2, and ordinarily
contains about 70 per cent of lead carbonate and 30 per cent of lead hydrate.
Although normal lead carbonate occurs in nature as the mineral, cerussite, it has
never been important as a source of white pigment. White lead was known in
early times and was one of the first artificially prepared pigments. Theophrastus
(pp. 223-225), Pliny (see Bailey, II, 75), and Vitruvius (VII, 12) all describe its
preparation from metallic lead and vinegar. There are also numerous mediaeval
recipes for making it, A large part of the white lead used today is made by the
* Dutch * or * stack * process, which differs little in principle from the method used
in classical and mediaeval periods. Metallic lead in the form of strips, * buckles,*
or other shapes is exposed for about three months in clay pots which have a sepa-
rate compartment in the bottom containing a weak solution of acetic acid. The pots
are placed in tiers in a shed with spent tan bark or manure separating them. When
the building is closed, the combined action of the acetic vapors, heat, and carbon