Something funny about those aluminum caps

[bvscfanatic]

I'm hoping that somebody can explain to me why a black residue forms on the threads of the aluminum caps that are being used on many non-bourbon products (rum, brandy and other products). I first saw this residue form around the mouth of a bottle of Jagermeister (which I can't stand, but I keep in the house for friends who seem to enjoy it), but I've also seen it on bottles of Kraken Rum and E&J Brandy. I have a bottle of cheap Scotch which shows no sign of this residue. Does anybody know what it is that I'm seeing? Is it related to the aluminum in the cap in any way? More importantly, is it an aluminum compound? It concerns me because I know the danger of aluminum as a toxin. Anybody know? I wrote to Kraken, but they never wrote back. I guess that pretty much nails Sailor Jerry as my favorite rum, since it has a plastic cap.