I am doing alterations inside a converted 18th century barn. It has involved removing recent stud walls that joined old original York stone walls. This has left a line of plaster on the stone where the stud wall joined it. I want to remove this. I have tried using a new wire brush (leaves a black mark), a wire attachment on a drill (same effect), water and scrubbing brush (leaves all the area looking smudged), vinegar (little effect) and scraping with a sharp knife (good for larger areas but doesnt get inside the pores of the stone). Any suggestions?