Scientific American 11, 10.9.1864
When a piece of wet cloth is hung up in the air, the water which it had absorbed is evaporated, and floats away in the atmosphere. The cloth is dried by the removal of the moisture. But in the drying of paint there is no removal of moisture. The linseed oil absorbs oxygen from the atmosphere, and is changed from a liquid oil to a solid rosin. In the place of losing by evaporation, it gains in weight. It is, perhaps, hardly more proper to call the process "drying," than it would be to speak of iron drying when it cools from the liquid to the solid state, or of water drying when it freezes into ice.
Sprits of turpentine is a solvent of linseed oil, and when it is mixed with paint it renders the mixture more fluid, thereby facilitating the spreading of a thin and even coat over the surface. It is very volatile, and soon evaporates, but a small portion is converted into resin by the absorption of oxygen, the same as the oil, and therefore it adds slightly to the body of the paint. In evaporating it opens the paint to the action of the atmosphere, and thus hastens the process of solidification.
The lighter portions of those hydrocarbons which unite to form petroleum, are solvents of linseed oil, and therefore serve to render paints more fluid. They are also volatile, and evaporate from the mixture when exposed to the air. But none of the hydrocarbons of petroleum absorb oxygen, or change to resin. Naphtha, therefore, adds nothing to the solid body of paint.
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