Well, a universal solvent would be the ultimate paradox, wouldn't it?...since it would also have to dissolve itself.
Am I missing something here? If "alchemy" is the 'science' of turning (transmuting) lead into gold (or some other base metal into precious metal), wouldn't an "Alkahest" simply be the solvent which effectuated that particular transmutation? In other words, an "Alkahest" would be a solvent only for the metals it acted upon transmutationally. It follows that an "Alkahest" could be collected in a glass beaker, a leather pouch, or, perhaps, even a metal container which happens to be all base metal or all precious metal, but not both.