Examples of the solvents usable are alcohols such as methanol, ethanol and isopropanol; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; mono- or di-alkyl ether derivatives of ethylene glycol or propylene glycol; esters such as ethyl acetate and butyl acetate; and aromatic solvents such as toluene and xylene; and these are used each alone or as mixed solvents