In my experience, when nitric acid gets “tired” and seems less
chemically active in etching, it sometimes can be helped along by
very GENTLE heating. I use one of those heating trays for keeping
food warm at the table (often available at second-hand stores – buy
only the kind that has a little knob for “High-Low-Off” and only use
Low; I etch in a Pyrex dish). Do NOT under any circumstances allow
the acid to boil!!! Keep your eye on the acid – it sometimes seems
to have a mind of its own, in terms of activity. Usual
disclaimers… Judy Bjorkman