PIGMENTS 169
Van Eyck Green (see Verdigris).
Venetian Red is an iron oxide (see also Iron Oxide Red) with a brick-red color.
Formerly a natural oxide, partially hydrated (see Church, p. 180), today it is
obtained by calcining a mixture of copperas (ferrous sulphate) and whiting
(calcium carbonate). The product, a finely divided mixture of ferric oxide and
calcium sulphate, does not require washing but is simply ground and sieved. Al-
though the best Venetian red may contain as high as 50 per cent ferric oxide, the
greater part has 10 to 30 per cent with the rest a mixture of calcium carbonate
and calcium sulphate.
Verdigris is specifically the normal acetate or one of the basic acetates of
copper but, on occasion, the term is also used to indicate copper carbonate or any
of the other blue or green corrosion products which form on copper, brass, or
bronze. The verdigris (vert de Grece) of commerce is usually the dibasic acetate,
Cu(QH302)2*2Cu(OH)2- It is a greenish blue, crystalline powder with acetic
odor. Its preparation was known in ancient times. Theophrastus (p. 225) and,
later, Pliny (see Bailey, II, 41-43) tell how to prepare it by exposing copper to the
vapors of fermenting grape skins or in closed casks over vinegar. There are
numerous mediaeval recipes for its preparation. Production of this material has
long centered about Montpellier, France (verdet de Montpellier)y and the methpds
used there differ little from those of ancient times (see Beckman, I, 171-175). The
crude verdigris produced by the action of acetic vapors on strips of metallic copper
can be lixiviated and the product recrystallized from acetic acid, after which it can
be used as a pigment. Well crystallized verdigris particles have the shape of
pointed needles. De Wild says (p. 78): * They are often united in bundles, while
the larger pieces show fibrous structure. If, however, the pigment is not recrystal-
lized but ground directly, it is seen in transparent grains.' Verdigris is strongly
birefracting (a = 1.53; /5 = 1.56 [Merwin]]), and it is pleochroic, changing from
light blue-green to deep green-blue.
This green is the most reactive and unstable of the copper pigments. It is
slightly soluble in water and readily soluble in acids. When heated, it decomposes
with the escape of acetic acid and water, and leaves a black residue (CuO). Unless
locked up with protective coatings, it is a fugitive color; it blackens readily with
sulphur-bearing pigments. Under very favorable circumstances, as, for example,
where used in the illumination of books and manuscripts which have been kept
closed, it has sometimes endured well. Laurie says (New Light on Old Masters>
p, 45) that Watteau frequently used a mixture of verdigris and ultramarine for his
skies and, in spite of the theoretical incompatibility of these two pigments, there
has been no apparent reaction; his skies are as luminous as ever. De Wild (p. 78)
found it on several paintings of the Flemish School. Thompson says (The Materials
of Medieval Painting^ pp. 163-169) that it was a favorite pigment in the early
days of oil painting in Italy, particularly in landscape painting, but it was a
fugitive color and there are many cases where it has turned dark brown. He says