loosening tan-colored thread-lock compound on black-iron pipe
That's probably old gas/water thread compound, and not a thread locker. It could also be plumber's putty, which does tend to get quite hard. 

Solvent probably won't do you any good, and you've presumably had some heat in it already. You could try heating it locally with a torch. What remains after that is ever-larger wrenches. 

Good luck.