MEDIUMS AND ADHESIVES 65
and on this account Venice turpentine has almost completely replaced it in com-
merce. Neither of these oleo-resins is easily obtainable, particularly in an unadul-
terated state.
Strasbourg turpentine was largely used in the XVI century. According to
Church (p. 65), it is undoubtedly the olio d'Abezzo mentioned by early Italian
writers. The best quality is said to come from Abies pectinata DC., a silver fir grow-
ing on the Italian side of the Tyrolese Alps. Dissolved in a terpene, this oleo-resin
was used as a varnish for pictures in tempera and oil, affording special protection
to verdigris and to some other easily decomposed pigments. Strasbourg turpentine
contains about 57 per cent of resinous acids (different from those in Venice
turpentine), 28 per cent of terpenes, and 13 per cent of resins.
Styrene Resins (see also Synthetic Resins and Polymerized Resins). A clear,
colorless resin may be made by polymerizing styrene or phenyl ethylene (CeHsCH
= CH2). Ellis C The Newer Chemistry of Coatings,' p. 129) says that it is
one of the most promising of the plastic materials suitable for coatings. The solid
resin is colorless, like glass, and it is very tough. It is insoluble in alcohol, acetone,
petroleum hydrocarbons, and the glycol ethers, but it is soluble in coal-tar hydro-
carbons, chlorinated hydrocarbons, esters, and turpentine. It is also reported
that polystyrene lacquers are quick-drying and that, even when baked for a long
time, the films do not become insoluble. The resin is thermoplastic and softens
above 150° C. It may be plasticized with dibutyl phthalate or with triphenyl
phosphate. The refractive index is 1.50 to 1.75 (Ellis, Chemistry of Synthetic
Resins, p. 236).
Sunflower Oil is obtained from the common sunflower (Helianthus annuui),
cultivated in Russia, China, Hungary, and India for food. The seeds contain about
50 per cent of a pale yellow, very fluid, slow-drying oil with a pleasant flavor,
It is used in the preparation of oil varnish. It is said to be used in Russia for grind-
ing colors (Doerner, p. 113). It has an iodine number of about 130 and pronounced
drying properties, but does not yield such a satisfactory film as linseed oil; there
is a tendency to produce softer, gummier products. (See also Oils and Fats*)
Sun-Thickened Oil (see also Blown Oil, Boiled Oil, Oils and Fats, and Poly-
merized Oil) is oil which has been exposed to the action of light and air in shallow
containers. During exposure it is stirred from time to time to prevent the forma-
tion of a skin. It thickens in a few days and, when it has acquired the consistency
of honey, it is ready for use. It dries with a gloss and has been used for centuries
as a painting medium and with resin as a varnish. It has already absorbed some
oxygen and dries more quickly than ordinary linseed oil. The old masters further
accelerated drying by putting the oil in leaden vessels. It is probable that no
polymerization takes place on such exposure (Laurie, * Notes on the Medium of
Flemish Painters,' p. 125), and sun-thickened oil is more like a blown oil than a
stand oil.