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  The Manufacture of Varnishes... by John G. McIntosh, 1904
Vol. 1

Vol. 2

The Function of Drying Oils as Varnish Ingredients, i.e., as "Vehicles" for Resins.

The 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 grease.

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 on drying.

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 that coating.

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.

Her newly painted and varnished front door in a few months looks as if it had not been painted or varnished for as many years.

Moreover, the 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 common rosin.

Moreover, the 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 confined beneath.

The great 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.

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