jg PAINTING MATERIALS
purposes, being mentioned in this connection by writers from Dioscorides to
the XVI century. In the XVIII century the Italian violin makers used it as an
ingredient in their varnishes. Today it is used to some extent in colored spirit
varnishes and for lacquering metals. Laurie (Materials of the Painter's Craft,
p. 203) quotes an auripetrum recipe, a yellow varnish for coating tin foil, which
calls for an ' oil varnish coloured by saffron, aloes, the inner bark of black plum
or dragon's blood' (St Audemar MS., Merrifield, I, 115). These substances are
all easily dissolved in hot pine balsam, which can then be diluted with boiled oil
and turpentine.
Dragon's blood comes from a species of rattan palm, Calamus draco, which
grows in Further India and in the Eastern Archipelago. The variety from Su-
matra, which appears in commerce in the form of eighth-inch sticks wrapped in
fibre, is considered the best. On the surface, the resin appears brown but it gives
a red, lustrous fracture and a light red powder. It is soluble in alcohol, in ether,
and in fixed and volatile oils, and, if heated, it gives off benzoic acid. An inferior
resin comes in lump form from Socotra and is the product of the tree, Dracaena
dnndbari.
Drier (see Siccative and Oils, drying process).
Drying Oils are oils (see Oils and Fats) which have the property of forming
a solid, elastic substance when exposed to the air in thin layers (see Oils, oxida-
tion). This * drying ' power decreases as the iodine absorption diminishes, i.e., it
is proportional to the total amount of unsaturated fatty .acids present. The
iodine values of these oils (see Linseed Oil, Walnut Oil, Poppy-Seed Oil, Tung
Oil, Soya Bean Oil, Perilla Oil, Sunflower Oil, Hempseed Oil, Candlenut Oil,
and Saffiower Oil) range from about 200 to 120. They find their chief use in
commerce as the vehicles for pigments in paints and in varnishes.
Egg Tempera (see also Egg Yolk and Egg White). The whole egg, the yolk, or
the white may be used as a tempera medium. Doerner (p. 213) gives a recipe for
using a whole egg, which requires with it an equal measure of oil, or stand oil, or
oil varnish, and two measures of water added separately with thorough shaking.
According to him, the freshness of the egg is important for the quality and the
permanence of the emulsion. He says that pigments containing sulphur, such as
cadmium, vermilion, and artificial ultramarine, when used with an egg emulsion,
may decompose by combining with the nitrogen and sulphur compounds in the
egg to form hydrogen sulphide, and he finds that the addition of vinegar or phenol
*is inadvisable because they discolor some pigments, and he prefers a drop of oil of
cloves or small amounts of alcohol.
Among many other present-day recipes for egg tempera is that of Kurt Wehlte
in Ei-Tempera und ikre Anmndungsarten (Dresden: Herrmann Neisch, 1931),
pp. 28-29. He requires: I part of whole egg, % part of linseed oil varnish, % part
of dammar resin in turpentine, and i part of water. For a somewhat different
tempera, he suggests substituting oil for the amount of resin in this one. These are