IX6 PAINTING MATERIALS
depends to a great extent upon purity. When the surface of pure gold leaf is
highly burnished or polished, it becomes a very good reflecting surface, less yellow,
more dark and metallic.
Gold powder can be made in various ways, but not by direct stamping or
rubbing because it is too plastic. One method during the Middle Ages was to
make it into an amalgam with mercury and then to drive off the mercury by
heat, leaving the gold in powder form. Another was to grind gold leaf in a mortar
with honey and then wash away the honey with water. Today there are various
electrolytic and reduction methods that serve the purpose equally well. There is
evidence that river gold or gold dust was used on certain English manuscripts
(Thompson, loc. cit.> p. 198). Powdered gold leaf was used in mediaeval times for
a writing ink; it was applied mixed with egg white or gum and, when dry, was
burnished so that the letters looked as though they were cut from gold leaf.
Unburnished, powdered gold was used quite freely in panel paintings where
brilliance and luminosity were demanded; it was even mixed with transparent
colors; painted hair was sometimes streaked with gold to increase its luminosity.
Chiefly, however, gold was laid as leaf. There were various ways of making
the leaf adhere to the surface, but for large areas a bole or fine earth with size
was usual. For initial letters and illuminations on manuscripts, an aqueous me-
dium (glair, size, honey, plant juices) was brushed onto the part to be gilded,
The film was allowed to dry and was moistened by breathing to make it sticky
just before the leaf was laid on. For panel and wall painting, however, oil mor-
dants were more common. These were really thin coatings of an oil varnish on
which the gold was laid while they were still tacky. (See also Bole and Mordant.)
Golden Ochre (see Ochre).
Grain Lake (see Kermes).
Graphite (see also Carbon Black) is a crystalline form of carbon which is
widely distributed naturally as a mineral in different parts of the world. The
most important modern source is Ceylon; European sources have been Cumber-
land, Bavaria, and Bohemia, where deposits have been worked for centuries. It
has also been made artificially by a furnace process (Acheson process) since about
1891, Graphite has long been used as a writing material and it gets its name
appropriately from the Greek, ypafaw (to write). It was early confused with
lead which was also used for writing purposes, and hence the names 'black lead'
and * plumbago' are also used for it. For use in lead pencils, it is compressed with
very fine clay. Graphite has a greasy texture and is dull gray. Merwin in de-
scribing it says (p. 514): 'Microscopic flakes thin enough to transmit grayish
light have been prepared. Its refractive index is about 3 and its reflective power
high (about 37 per cent)/ Graphite is one of the most stable and refractory of
materials and would be permanent in any technique. It has been used chiefly as
a drawing material, however, and rarely as a pigment (Thompson, The Materials
of Medieval Painting, p. 88).