The Function of Drying Oils as Varnish Ingredients,
i.e., as "Vehicles" for Resins.
chief function of drying oils as ingredients of varnishes is
first of all to dissolve the resin so as to bring it into the
fluid condition; here again colza and other oils would act
similarly, but the solution of resin in colza oil would never
dry, and the only use for such a product would be for axle
grease, and even for that purpose it would not be very fit, and
not only so, it would be a very costly way of making axle
Here again the
function of the linseed oil is to dry, and in so doing to cement
together the particles of resin by an elastic binding agent
consisting of the product into which the linseed oil is resolved
The greater the
quantity of linseed oil and the fewer the particles of resin
which it has to cement together, and which are present in a
"dried" coating of varnish, the more elastic and durable will be
If a linseed oil
substitute consisting of hydrocarbides, such as rosin oil, be
substituted for a drying oil, e.g., linseed oil, whether in a
paint or varnish, this durability and elasticity is lost, the
dried product dissolves in weak solutions of alkali, and even in
warm solutions of household soap, as is seen in the case of the
front door of the housewife who must always be scrubbing.
newly painted and varnished front door in a few months looks as
if it had not been painted or varnished for as many years.
melting point of the dried product of linseed oil is very high,
as any one may satisfy himself by trying to melt linseed oil
skins, but the melting point of the " dried " (sic) product of
hydrocarbide oils, e.g., rosin oil, is no better than that of
dried product of such oils or varnishes is soluble in the
original varnish, especially in the sun, when the former melts,
a fact which gives rise to many exasperating difficulties when
it is desired to apply a second coating of the same paint or
varnish on the same article.
In the cool of
the evening the coating may be as "dry as a bone," in the heat
of the day it is simply a liquid pitch, and if the coating does
eventually so dry and harden as to withstand the heat of the
sun, it is such a mass of cracks and furrows that
those on an elephant's hide might well be compared to them.
These cracks and
furrows are produced by the difference in temperature between
day and night, producing hundreds of alternate liquefactions and
solidifications of the coating, into the composition of which
this linseed oil substitute enters so largely.
Moreover, in the
cool of the evening the coating seems to dry, but it is merely
surface drying, underneath is a layer of liquid pitch like that
of a lake of asphaltum, and it only requires the heat of the sun
to melt-the surface crust to liberate streams of the liquid
function and characteristic property of a drying oil is to yield
on drying a coating of high melting-point, great elasticity,
great imperviousness and great durability, and resisting both
natural and artificial reagents and solvents.