There’s a very noxious chemical present in sludge metal that separates it from all other forms of transgressive music. Even when placed side by side with contemporaries from every god-forsaken, drop-tuned fashion known to the underground, the crooked spoon fury of bands declaring themselves to be sludge stand out like a sore thumb. If punk rock is rebellion and heavy metal is hedonism, for instance, then sludge is a terrible nothingness that is all too human to recognize yet exemplified by sounds which are inhuman. That may sound like a bit of mental gymnastics, but no other subgenre of music has resonated with the…