What removes calcium plumbate from a cast iron fireplace?
As you have, no doubt, found calcium plumbate sticks very well to some metal surfaces and can be difficult to remove. A further complication is that the high lead content rules- out some obvious mechanical solutions since this would cause contamination.
A chemical-based stripper may be your only hope and, unfortunately, this can be a case of trial and error because some work better than others.
I would avoid some of the ‘extra-powerful’ options because they can react badly with metal surfaces – causing severe oxidation – so the proprietary brands tend to be the safest option, although not the most effective.
To put it simply, it can be a slow and tedious process but try a few different brands and, to be safe, do a small test area first.
Sorry, I can’t be of more help.