MEDIUMS AND ADHESIVES 53
holes, are cut in the bark. In some cases the incisions pierce to the heart of the
tree. At first, the flow of resin is very small, giving at most, a ' tear/ as in mastic
and sandarac. In the conifers, producing colophony and turpentine, however, the
injury seems to stimulate the growth of the canals and resin flows profusely at
certain seasons.
Resins are considerably modified on exposure to air and light so that the
collected product rapidly changes in consistency and color from its natural state
within the tree. Handling and adulteration often change it still more; a wide
variety of resins, though all from the same species of tree, may be found on
the market. Much early research on the properties of resins has had to be dis-
carded because the samples could not be accurately traced. This difficulty of
obtaining pure, more or less uniform, samples still presents a problem to inves-
tigators and accounts for differences in many of their results. Several general
characteristics are found in resins as a whole: they exhibit an amorphous struc-
ture, rarely crystalline but often glassy; they soften with heat, and finally melt
to a more or less sticky fluid; they burn with a smoky flame. All resins are
insoluble in water, but generally they can be fairly easily dissolved in alcohol,
ether, and carbon disulphide. Unlike fats and oils, they do not leave a greasy
mark on paper, and they are resistant to reagents and to decay.
Resins form the basis of all natural varnishes. They may be used in the raw
state fresh from the tree, or in mixtures (see Varnish). With the exception of
amber, the fossil and hard resins called by the generic name * copal' come from
Africa, South America, New Zealand, and the East Indies. Amber is found chiefly
on the shores of the Baltic Sea, and in northern Europe. These hard resins are
combined with a drying oil to make oil varnishes, and are extremely strong,
resistant, and usually dark in color. From the East Indies and the eastern and
southern shores of the Mediterranean Sea, come the soft resins, dammar, mastic,
and sandarac. These are the spirit-varnish resins, the resin or a combination of
resins being dissolved usually in turpentine, and they constitute the most gen-
erally used picture varnishes. The balsams or oleo-resins come from coniferous
trees widely distributed throughout Europe and America. Lacquer is the exudate
of a tree indigenous to1 China and Japan. Shellac, the most common resin in
commercial varnishes for woodwork, is not a product of a particular tree, but
is the excretion of an insect feeding and breeding on a variety of trees in India,
Siam, and Burma.
Chemically, the natural resins differ from one another very widely, and
inquiry has not been carried far enough yet to make possible a satisfactory
arrangement of the resins from a chemical viewpoint, Barry (p,, 25) classifies the
essential constituents as: