94 PAINTING MATERIALS
mercury vermilion. It is soluble in alkalis and in strong acids, and turns black
on heating (Rose, p. 16).
Antimony Yellow (see Naples Yellow).
Antwerp Blue (see Prussian Blue).
Armenian Bole (see Bole).
Artificial Pigments (see Synthetic Pigments).
Arzica (see Weld).
Asphaltum (bitumen) is a brownish black, native mixture of hydrocarbons
with oxygen, sulphur, and nitrogen, and often occurs as an amorphous, solid or
semi-solid liquid in regions of natural oil deposits. It is thought to be formed from
the evaporation of the lighter components of the petroleum and from polymeriza-
tion and partial oxidation of the residue. It is found widely, but that used in
European paintings came, perhaps, from the region of the Caucasus or the borders
of the Dead Sea. In Mesopotamia and Egypt in very early times it was known
and used for various purposes (see Partington, index). Asphaltum has little use
now, but is still listed by artists' supply dealers. Not much is known about its
preparation, but Church says (p. 235) that the crude asphaltum is usually heated
to a fairly high temperature to drive off moisture and volatile materials before it
is ground in oil or other mediums. The pigment is partially soluble in oil, like a
stain, and gives a semi-transparent, reddish brown film. In the film, it may be
occasionally observed microscopically as tiny brown flakes without structure.
Only thin grains are transparent brown. It is soluble in turpentine, naphtha, and
other organic solvents.
Asphaltum and other similar tarry compounds are among the least desirable
pigments known because they never become permanently dry. In thick oil films,
they have a tendency to run and to crawl, but, if they are properly prepared,
such difficulties may be partially overcome (see Church, p. 236). Doerner says
(p. 189) that Rembrandt used asphaltum as a glaze with no harmful effect. It is
unaffected by acids and is unsaponifiable; it requires about 150 per cent of oil to
grind. Under ordinary circumstances, it is unaffected by light but is faded by
strong exposure. It was much favored by the XVIII century English school,
with unfortunate consequences; those paintings which contained it have become
disfigured because of shrinkage of the paint films and 'alligatoring/ Harder paint
films put over it sometimes crack and curl. Neuhaus says (see footnote in his
translation of Doerner's The Materials of the Artist, p. 89): 'Under high summer
temperatures in museums without thermostatic control whole areas of the pic-
ture surface have moved and become permanently dislocated. Thus in several
warm climatic belts of America it has caused the destruction of many paintings
of the Munich school which at one time was passionately fond of asphaltum as
a frottie/
Asphaltum is also sold to the artists' trade under the name 'bitumen/ Both
mummy (see Mummy) and bistre (see Bistre) are similar in color and composition