John in NM
Leave a puddle on it for a few hours and it might just pop off in one piece with a little persuasion and a sharp chisel. That would be better than spreading the residue. I posted before reading yours, but I do think water is probably a better bet after reading what you said.
I've heard siliceous rocks called acidic before, though the term is very old and no longer used much. Is silica actually acidic?