Thanks for the reply!

We worked at it with a grinder with a 60 grit pad, and it does the trick. We have a 5/8" layer to work with, and the grinder does a pretty good job (with a steady hand!) at taking off the top layer. It almost appears that it's calcification, it is so much harder than the surrounding plaster.

We have had no success with chemicals at all, without getting drastic. Acid scares me, at least with the grinder we have some control.

Thanks again!