Representative examples of suitable non-volatile solvents or oils include synthetic esters and ethers, especially of fatty acids, for instance the oils of formulae R1COOR2 and R1OR2 in which R1 and R2 represent, independently of each other, a branched or unbranched hydrocarbon-based chain containing from 3 to 30 carbon atoms, for instance purcellin oil, isononyl isononanoate, isopropyl myristate,