,o	painting materials
40
PeriUa Oil is obtained from the seeds of the Perilla ocimoides, an annual plant,
occurring in China, Japan, and the East Indies; it is of the mint family, and is
closely related to the highly colored Coleus seen in gardens. In appearance and
odor, perilla resembles linseed oil. It is highly unsaturated and is characterized
by its iodine number (190 to 205) which is the highest in any of the known oils.
It dries quickly but gives a dried film which is somewhat marred by irregular
marj
•kings and spots. During the last few years perilla oil has become of industrial
importance*
Phenol-Formaldehyde Resins (see also Synthetic Resins). Although the phenol-
formaldehyde resins (bakelite) are much used commercially, because of their
yellow to deep brown color they have found little application in the treatment of
works of art. Certain of the phenolic resins are soluble in oil and can be added
in oil varnishes. Stamm and Seborg have recently described how the shrinkage and
swelling of wood may be lessened by impregnating it with a phenol-formaldehyde
mixture and condensing the mixture directly in the cell structure.
Pitch is a term which is often improperly applied to the resin or crude tur-
pentine that exudes from pine and fir trees. It may mean the residuum from the
distillation of turpentine (see Turpentine and Colophony), or it may mean asphalt
or bitumen. More broadly, pitch is any tenacious, resinous substance, black or
dark brown in color, which is hard when cold and is a thick, viscid, semi-liquid
substance when heated.
Plasticizers are non-volatile, or little volatile, liquids or solids that are incor-
porated, usually in small amounts, in a lacquer or varnish and are retained in the
film after escape of volatile solvents for the purpose of keeping the film adhesive,
elastic, and flexible. A plasticizer is a necessary component of most cellulosic
coatings (see Cellulose Acetate and Cellulose Nitrate). Although such natural
products as camphor and castor oil have been used for this purpose, the modern
trend is toward * chemical * plasticizers which are usually high-boiling esters like
dibutyl phthalate and tributyl phosphate. Some of the synthetic resins serve as
plasticizers for cellulose coatings. (See also p. 199.)
Polymerized Oil or Stand Oil (see also Polymerized Resins). If air is excluded,
most oils can be heated to a temperature of about 250° C. without undergoing
any appreciable chemical change. Some oils (*.£., linseed oil) become pale in
consequence of the destruction of the dissolved coloring matter. When heated
above 250°, and up to 300° C, most drying oils (see Oils and Fats) undergo a
change which is essentially one of polymerization. The iodine number falls rap-
idly, **.*., a certain number of the ethylenic linkages become saturated, not by the
addition of hydrogen or oxygen but by polymerization. When the iodine number
has fallen to about 100 (^c thin stand oil *), the density has increased from about
0.935 to 0,966 and the oil has become somewhat more viscous. On further heating
at the same temperature, the iodine number falls lower, though more slowly, the
viscosity increases rapidly, and the oil becomes very thick but remains clear* In