I use glass bottles, both amber and clear, for liquid photochemical storage. The caps themselves are a hard plastic (phenolic?), some black and some green. There are (at least) two types of cap liners available - metal foil on fiber/cardboard and PTFE. I think vinyl liners may also be available. Over the years I've learned the hard way that metal liners are eventually attacked by working-strength stop bath, so they're clearly a poor choice for acids and I use PTFE there. That's the easy part. How about for basic solutions like developers and some fixers? Is there any practical difference between the black and green cap bodies? Would I be better off with metal foil or PTFE in terms of forming an oxygen barrier? Would adding layers of plastic kitchen wrap between cap and bottle add any value in terms of extending solution life? I'm curious because sometimes it can be months between my b&w printing sessions and I don't want to needlessly waste chems. [Some might say that my prints are a waste of chemicals, that's a separate issue...] Gerald? Ron?