This is my first post on this forum so I hope I do everything right.
I am restoring a french farm house. The stone around the windows are in a standard granite. The granite was covered in old cement which I have been cleaning with concentrated (at least at to Consumer standard) Hydrochloric acid. The acid bubbles up green when in contact with the cement. This worked fine and I repeated the process many times with no problems, until the tempreture increased and under direct sunlight it has left a very definate yellow stain on the granite. Now I cannot get rid of it. I am not sure what the stain is: a safety dye in the acid or from the reaction with small amounts of cement still in the granite.
Can anyone suggest a course of action? This yellow was caused chemicaly, surely it can be chemicly undone. Note that granite porros and the yellow is several milimeters deep into the stone.