-, PAINTING MATERIALS
1. Aromatic acids.
2. Aliphatic acids.
3. Resinols and resino-tannols. (Alcoholic Constituents.)
4. Resin acids, (Acidic Constituents.)
5. Resenes,
6. Essential oils.
Aliphatic and aromatic acids, aside from the characteristic resin acids, are not
common. Acids of the benzoic and cinnamic acid series occur in a few resins but
mainly in those that are of minor importance commercially; succinic acid, which
occurs in amber, seems to be the only aliphatic acid found. The resin acids may
occur free or combined with resinous bodies which Tschirch divides into two
groups: the resino-tannols, which give the tannin reaction, and the resinols,
which do not. The acids which have been most investigated are abietic acid,
QoHaA, and its congeners. The resenes are inert substances and are very re-
sistant to chemical reagents. They appear to be of an aromatic nature. In some
cases, the oxygen content is so small that they might be hydrocarbons of high
molecular weight. Contrary to expectation, their chemical inactivity seems to
have no direct relation to the hardness and durability of varnish films. Zanzibar
copal, for instance, a very hard resin, has 10 per cent resene; dammar, of medium
hardness, has 60 per cent; and colophony, a very soft resin, also has 10 per cent.
Two investigators, Haber and Von Weiman (see Barry, p. 29), agree that, as the
resenes interfere with crystallization, they tend to keep the resins in an amorphous
condition.
The amount of essential oil in resin varies. The considerable amount present
in the fresh exudation from the tree either evaporates or changes chemically on
exposure to air. Thus, the balsams are practically solutions of resin in essential
oil, the elemis and soft copals contain sufficient oil to make a pasty consistency,
and most of the fossil resins contain only an extremely small amount. Following
is an arrangement of some of the natural resins according to hardness.
COPALS—OIL VARNISH RESINS
Hard Medium Soft
Zanzibar Sierra Leone Manilla
Kauri
SPIRIT VARNISH RESINS
Dammar Sandarac
Shellac Colophony
Mastic
Resins do not have definite melting points because the fusing is a continuous
process and may extend over a considerable range of temperature. Hard resins,
particularly, have a long transition stage, the * upper melting point ' being reached