painting materials
at high magnifications (400 to 500 X) fossiliferous remains in the shape of tiny,
hollow shells can be seen. The material is highly birefracting, the shells being
made of tiny calcite crystals with their vertical axes in nearly radial directions;
its refractive index, however, is low, and this, in part, explains its poor covering
power and its discoloration in oil, although it covers well when used in water paints
and in distempers. Mixed with white lead and linseed oil, it makes glazier's
putty. As a filler and adulterant, it is put into cheap paints and it serves as a
base for lake colors. In northern Europe, particularly in England, France, and
the Low Countries, chalk was the inert commonly mixed with glue in the prepa-
ration of grounds for painting just as gypsum (gesso) was used in Italy and in
the south. De Wild (p. 44) found it in the grounds of thirty-six Dutch and Flem-
ish paintings. Under ordinary circumstances, chalk is stable, but when heated
strongly it changes to calcium oxide (lime), and it is decomposed by acids, with
effervescence of carbon dioxide gas. Made artificially, it is known as * precipitated
chalk,' and this is whiter and even more homogeneous than the natural material.
There are many other natural forms of calcium carbonate, some of which are
useful in painting. One of them, marble, is a familiar crystalline variety of calcium
carbonate or limestone. Marble dust has been mixed with lime for the plaster
ground of fresco painting and of lime-wax painting. Another, oyster shell white,
can be made from the shells of almost any mollusk. It was perhaps usual to burn
the shells before powdering them. This white was a pigment in Chinese and
Japanese painting (see Uyemura, p. 47), and Thompson says that it appeared
also in mediaeval England, chiefly mixed with orpiment. Even calcined egg shells
were used as the source of a fine lime white. Coral, the calcareous remains of
various marine animals, yields, when ground up, a pale pink powder that the
Chinese and Japanese made into paint for certain purposes. Uyemura says it
was used as early as the Tempyo period (VIII century) in Japan. Lime white,
derived from lime putty, Ca(OH)2, or water-slaked lime, went into Italian fresco
painting under the name, bianco sangiovanni (Thompson, The Materials of Medi-
eval Painting, p. 97). On exposure to air, this was slowly reconverted to calcium
carbonate or chalk.
Charcoal Black (see also Carbon Black), the residue from the dry distillation
of woods, is made by heating the wood in closed chambers or kilns. That which is
produced from the willow, bass, beech, maple, or such other even-textured wood
is the best. For pigment purposes, the charcoal is ground and well washed to
remove potash. It may be used in stick form for sketching purposes and for the
preparation of cartoons. Charcoal is light and porous; in part, it retains the fine
structure of the wood from which it was made and, for this reason, it is quite
characteristic in appearance when viewed microscopically. It may be seen as
small, opaque, elongated, and splintery particles. This form of carbon has been
used as a pigment since very earliest times. It is gray-black and is weak tinc-
torially. It is found on the wall paintings at Bamiyan in Afghanistan (see Gettens).