Solvents include paraffins such as pentane, hexane, heptane, octane, decane; toluene and xylene, paraffinic naptha fractions, kerosene; and cycloparaffin hydrocarbons having from about 5 to 10 carbon atoms, e.g., cyclohexane and so forth other inert solvents, especially ethers such as dioxane and tetrahydrofuran may be effectively used.