A coating to provide a nonporous gas and/or vapor barrier, as for example, a coating, a co-extruded plastic such as polyethylene and polyethylene terephtalate, or a laminate of these materials and others such as polypropylene, polyvinylidene chloride, cellophane, and the like, may be applied to the outer surface of support sheet 14 as a sealant to prevent entry of vapors or water, or evaporation o