solvents and diluents	199
varnish industry. Great care is being taken to prepare petroleum thinners for
specific purposes. Impurities of sulphur compounds, which formerly caused
trouble, have been largely eliminated.
Plasticizer. This term has come into use recently to describe any solid or
liquid substance added to a film material in order to give that material elasticity
and pliancy. More specifically it stands for any one of a series of substances,
largely liquids with high boiling point and low volatility, that are used to reduce
the inherent brittleness of lacquer and synthetic resin coatings. Ordinary solvents
and diluents, even though fairly volatile, may be retained in small quantities as
part of the lacquer or varnish film during some weeks or months. With their final
escape, however, most organic films become brittle and friable, lose their pro-
tective value, and, through the formation of minute crevices, become dull.
Linoxyn, the dried film of linseed oil, remains plastic for some years, and that is
why it is used as a plasticizer in resin-oil and spirit varnishes, but it, too, eventually
changes into a brittle and crumbling state. Lack of permanent flexibility seems to
be a shortcoming of most organic films, particularly those made from cellulose
derivatives and resin-like polymers. Celluloid is nitrocellulose plasticized with
camphor; castor oil also has long been used to plasticize nitrocellulose. The syn-
thetic or chemical plasticizers are mainly esters of high molecular weight, but
they include a few amines, ethers, aromatic ketones, and even hydrocarbons.
Most of them boil above 25o°C. and have a vapor pressure less than o.i mm*
mercury (see Gardner, pp. 576-578). The various plasticizers of modern industry-
have been put into two groups: gelatinizing—those which have a solvent or
swelling action on the material to be plasticized—and non-gelatinizing—those
which have no solvent or swelling action but merely fill intermicellar spaces as
water does the pores of a sponge. Probably the best known of these plasticizers are
dibutyl phthalate and tricresyl phosphate.
Reagent Solvents are those which are used to dissolve water-insoluble mate-
rials by chemical action upon them. Most of them are acids, but some are alkalis.
They react with materials to form water-soluble salts. Hydrochloric and nitric
acids and sodium hydroxide are examples. Almost without exception none of
these reagents can be used directly on artists' paint. Their activity is apt to be
violent and destructive.
Restrained A word restricted almost entirely to the terminology of picture
restoration, it was formerly applied to a solvent which had a low activity on
resinous surface films and which was used in combination with a solvent having a
high activity. In some cases they were mixed together. In others, they were
applied alternately to the film. The theory was that the slow-acting or non-solvent
material would serve to dilute and make less concentrated the fast-acting one so
that its solvent effect could be controlled. Turpentine and alcohol, the former as
the restrainer, were regularly used in this way before a wider variety of solvent
materials became available.