Examples of suitable materials include resins, such as polystyrene, co-polymers of polyvinyl chloride and acrylonitrile, silicone lacquers, polyvinyl alcohol or its derivatives, with a leachable plasticizer and made insoluble by formaldehyde, ethyl and methyl celluloses or cellulose esters, such as a solution of cellulose acetate in acetone, rendered insoluble, gelatin rendered insoluble with solu