One thing that I've noticed about nearly all my meads that are in the "bulk aging" phase is that they develop some sort of filmy sort of residue up at the top of the carboy on the surface of the mead. Whenever I thieve out a sample for tasting or analysis and the volume level drops a little bit, the residue leaves a little "ring-around-the-carboy" mark. Is this normal? Doesnt necessarily look too appetizing...