^2 PAINTING MATERIALS
called ' white tung oil' and the latter,c black tung oil.' An experimental cultiva-
tion of Aleurites fordii in Florida has succeeded, and oil is now being produced
there on a commercial scale. The Florida kernels yield a high quality, very pale oil,
of low acid value (0.3 to 1.4) which is free from the unpleasant odor associated
with the Chinese oil. Tung oil contains a large proportion (75 to^5 per cent)^of
eleostearic acid (CiJHaA), a stereo-isomeride of linoleic acid. This acid contains
two unsaturated double bonds and gives the oil its drying property. The rest is
largely olein with some saturated acid. Its average iodine value is 165. Its specific
gravity (about 0.941) is high, and it has, also, a high refractive index (1.522 at
15° C.). Tung oil appears to dry in about two days in moist air, but the resulting
film is always wrinkled or cracked and uneven. In dry air about fourteen to
twenty-one days are required and a smooth, coherent film is obtained. In either
case it takes twenty-one to thirty days for the full gain in weight (12.9 to 13.3
per cent). From this it appears that tung oil is really a slow-drying oil, and that
the rapid rate of drying in moist air is not * drying * in the usual sense (/.<?.,
oxidation and polymerization), but is a colloidal change in which moisture acts
as the coagulant (W, Schmidt,' Zum Trockenvergang des Chinesischen HolzoFs/
Farben-Zeitung, XXIX [1924], pp. 1261-1262). A characteristic property of tung
oil is that it forms a jelly on being heated. If it is ' bodied * or polymerized (see
Polymerized Oils) by heating at 270° to 290° C. until it thickens, it will dry to a
smooth, glossy film which is highly resistant to the action of water. It is not much
used by artists, although it has various favorable qualities. It is as unsaturated
as linseed oil, but has considerably more tendency to gelatinize or separate in the
heterogeneous phase, so that the films produced are frequently dull or mat. It
also yellows badly and may cause skin diseases. (See also Oils and Fats.)
Turpentine (see also Balsam and Oleo-Resin) is a semi-liquid, natural exudate,
containing terpenes associated with bodies of resinous character, from pine trees
widely distributed throughout the world. The raw or crude product from the tree
is also called balsam, or oleo-resin, and the word, * turpentine/ is commonly used
for the spirits of turpentine, the distillate of the crude balsam. The residue, after
distillation, is colophony or rosin.
Turpentine is commonly used as a thinner or diluent for oil paints and var-
nishes. There is no indication that it was employed in this way by the ancients,
although its solvent power was known and referred to by Pliny. Turpentine and
alcohol are now the usual solvents for spirit-varnish resins. It appears that spirit
varnishes were introduced into Italy in the XVI century and were widely used
by the Flemish painters in the XVII century. At that time, also, turpentine was
used to dissolve verdigris and to prepare lake color (see Resins, history in painting) *
Vaxnish (see also Resins, Oils and Fats, and Waxes). A picture varnish is the
protective coating, usually with a resin content, laid over a paint film. The use of
such a coating was probably introduced at a remote time in the history of the art
for the purpose of protecting water-soluble mediums and fugitive pigments and,