MEDIUMS AND ADHESIVES 47
Oleo-Resin is a natural combination of resinous substances and essential oils
occurring in or exuding from plants. It is usually a soft semi-liquid in which the
resin is in solution in the essential oil. Deitrich (p. 9) classifies oleo-resins into
four sub-groups: (a) the varnish group, derived mostly from plants of the Anacar-
dlaceae family; (b) the copaiba group, sweet-smelling resins similar to the balsams;
(c) the turpentine group, derived from Coniferae which comprise soft resins; and
(d) the elemi group, which are soft resins containing above 10 per cent of ethereal
oil. Among the oleo-resins most common in pictorial painting are Venice turpen-
tine and copaiba, both used in the older practice of picture restoration and in the
compounding of some surface films. The term, oleo-resin, has been used occa-
sionally to define mixtures of drying oil and resin, but the definition given here
is the one common in standard works on the technology of resins. (See Balsam.)
Ozokerite (see also Waxes). The origin of ozokerite is still, like that of petro-
leum, a matter of controversy. It is regarded by some as an intermediate product
between natural fat and petroleum, but the more common view is that it is an
oxidation and condensation product of petroleum. It is a variable substance. It
may be quite soft or as hard as gypsum. In color, it varies from a light yellow to
a dark, greenish brown. The refined product is known as ceresin.
Paraffin Wax (see also Waxes and Mineral Waxes) is obtained chiefly in the
distillation of shale oil, lignite, and American and East Indian petroleum. It is
a mixture of saturated hydrocarbons of the CnH2n-f2 series. Its melting point has
the wide range, from 35 to 75° C.; it is available on the market in samples melting
at 48 to 50°, 50 to 52°, 52 to 54°, 54 to 56°, 56 to 58°, 58 to 60°, and 60 to 62° C.
The higher the melting point the harder, the heavier, and the less crystalline is
the material. The softer varieties contain not only lower members of the usual
constituents but also more or less of the liquid members which have not been
removed during the process of manufacture. Commercial paraffin wax is a white
to bluish white, translucent material of lamino-crystalline structure. It is an
extremely indifferent substance, being attacked only slowly by the strongest re-
agents. It is freely soluble in mineral oils, ether, and benzene, but is only sparingly
soluble in hot alcohol. Because of its inert nature, it finds many applications in
the arts and industry.
Parchment Size (see also Glue and Tempera) is a nearly pure gelatin'made
from parchment waste. Its preparation is simple: cuttings are soaked in water
until they are soft, and then are heated for i^ to 2 hours in a double boiler with
enough fresh water to cover them. When the size has been taken into solution, the
waste is strained off and the liquor is allowed to cool to a gel or is used while
warm. The jelly that forms can be sliced and allowed to dry. The finished product
may be kept indefinitely and used as desired.
Paste (see Starch and Flour Paste).
Pastel (see also Gums). This kind of painting material is a chalk or crayon
made from pigments and fillers held together in stick form by a weak gum me-
dium, usually gum tragacanth.