The liquid solvents essentially include aromatic compounds such as xylene, toluene or alkylnaphthalenes; chlorinated aromatic or chlorinated aliphatic hydrocarbons such as chlorobenzene, chloroethylene or methylene chloride; aliphatic hydrocarbons such as cyclohexane or paraffines such as mineral oil fractions, as well as alcohols such as butanol or glycol and the ethers and esters thereof; ketone