MEDIUMS AND ADHESIVES 51
oil, but Doerner (p. 109) recommends using it in its natural state, for he says that
the bleaching does not last. Poppy oil has been known from classical times (see
Oils, history in painting), but it was not until the XVII century that it came,
in Holland, into general use for painting. It is used today in the preparation of
tube paints, especially with the light pigments, because of its pale color. Some
authorities (e.g., Eibner) object to its use because of numerous disadvantages,
chiefly, poor drying. Owing to its high linoleic acid content (its iodine number
is in the neighborhood of 150), thin layers of poppy-seed oil dry, but, linolenic
acid being absent or present only in small amount, the film formed melts at about
100° C., and is softer and more soluble in ether than is a linseed oil film. It does
not yellow much on aging but has a tendency, especially in a closed space, to
resoften (' synaeresis '). Poppy oil has a greater tendency to crack than has lin-
seed oil, especially if it is not thoroughly dried or if it is too quickly painted oven
Its properties as a paint oil are improved by polymerization.
Protective Varnish (see also Resins and Waxes). The problem of coating a
paint film in order to seal it from destructive agencies of all kinds is one that has
challenged painters, restorers, and technologists for many centuries. In spite of
innumerable studies and experiments, no single solution has been found, and, in
view of the diversity of paint materials, it is doubtful if any one film substance
could be expected to provide safely for the covering of all pictures. The difficulty
is to find a film material which is highly impermeable, enduring in itself, harmless
in application to paint, capable of safe removal from paint, and possessed of such
optical properties that it does not distort the subtle tone relations of a pictorial
design.
Permeability to moisture has been studied experimentally by Gettens and
Bigelow with concluding evidence in favor of waxes, natural soft resins being
next the waxes in having this property. A comparison of these results with other
properties of a series of film materials used for pictures was recently made by
Stout and Cross. Their observations tended to support a practical suggestion
made by Helmut Ruhemann (see Stout and Cross, p. 249) that a thin coating of
resin followed by a coating of wax be taken as an effective means of protection
for the usual type of European painting. It is an old rule, and one supported by
theoretical studies, that a resinous film should not be put over paint until the
paint has had many months in which to become thoroughly dry.
Pyroxylin (see Cellulose Nitrate).
Resins (see also Synthetic Resins) are secretions or excretions of certain
plants. Most of them are the products of living trees, but some copals are from
trees long dead, and amber is from a plant known only as a fossil. According to
researches by Tschirch, Stock and others (I, 93), nearly all resins and balsams
are formed in special secretory glands. The resin may exude naturally to some
extent onto the surface of the bark, but it is generally collected by wounding the
tree. The trunk is ' tapped *—small incisions, either vertical slashes or triangular