Today Yesterday Q. Quite a bit of plaster was stuck on the fireplace bricks. If the plaster is chunky, you should be able to scrape some of it with a chisel or other bladed pusher, to get most of it off. Treat the remainder with muriatic acid and water.
Plaster contains lime and the acid attacks the lime.
Mix equal parts of water and acid. When it stops fizzing, scrape or scrub with a wire brush and rinse. Repeat as needed. Be extremely careful with acid, and wear skin and eye protection. Q. I have to make a big patch in a plasterboard wall.