Yes, I’d just commented about the old fluxes. Maybe those had a good shot of nitric acid in them? Some would eat through clothes if not cleaned off fairly promptly after the job was done.
Nitric acid is available at www.dudadiesel.com somehow it’s used in making biodiesel although I haven’t the slightest idea how. If you’re cleaning the inside of tubing that will do the job but don’t use it full strength because it will dissolve your tubing while emitting clouds of nasty brown nitrogen dioxide.