MEDIUMS AND ADHESIVES 15
requiring a large amount of pica greca, with or without the addition of a soft resin,
it is known that colophony was used in Italy as early as the IX century.
Congo Copal (see also Copal and Resins) is derived from the tree, Copaifera
Demeusi Harms., in the Belgian Congo. It is found as a fossil resin in deposits
from six inches to three feet underground, although some is still obtained by
tapping the trees. It is considered the standard fossil resin. Except for colophony
and shellac, Congo copal and resins from the Dutch East Indies constitute the
bulk of natural resins in present-day manufacture of varnishes for general use.
Like all the copals, it has no definite melting point but is fused at from 180° to
200° C. It must be heated before it will dissolve in oil, thus forming an oil varnish,
and this process makes the resin darker. It has probably not been much employed
as a picture varnish.
Copaiba (Copaiva) Balsam (see also Balsam and Resins) is an oleo-resin
obtained from the tree, Copaifera landsdorfii^ in South America. It is a deep
brown, viscous liquid with a peculiarly fruity odor and a high content of essential
oil. It is soluble in fatty and essential oils as well as in alcohol. It was formerly
much esteemed by restorers. Max von Pettenkofer (see Doerner, p. 125) used
this balsam in the so-called ' Pettenkofer treatment' for the brittle, dried-out
* linoxyn ' skin of old oil paint. When combined with ammonia, it is less harmful
to a paint film than is a strong alkali solution (see Varnish). Helmut Ruhemann
(* A Record of Restoration/ Technical Studies, III [1934], p. 7) mentions using
it to make a mixture of petroleum spirit and ethyl alcohol in the process of re-
moving surface varnish from a Flemish painting.
Copal (see also Resins) is the general name given to a large variety of hard
resins. They are obtained as fossils and are also taken directly from living trees.
The fossil resin, found three or four feet underground, is the harder kind and is
the most valuable and widely used of all the resins. The copals vary much in their
origin, in their degrees of hardness, and in their solubility. They are products,
also, of many different species and even genera of tree. It appears, from the re-
searches of Tschirch and his associates, that copals consist of * resenes,' neutral
compounds containing oxygen, and of resin acids. The oxidation of these'resenes
by contact with the air, and the resultant increase in the acid number and de-
crease in iodine absorption, have been illustrated by experiment. The finer the
particles of the resin and the more porous they are, the higher will be their acid
number.
There is a wide range in the solubility and fusibility of copals according to
their origin and age. The melting points vary from 180° to 340° C. For conversion
into a soluble form, they are heated at a temperature of 200° to 220° C. for several
days, or are distilled dry, at a temperature of 380° to 400° C., or until 25 per cent
of copal oil has passed over. The benefits from the increased solubility by dis-
tillation are counteracted by the color which darkens in proportion to the tem-
perature or time of heating.