--- I would appreciate your help on a potential job. I’m looking @ a floor in a lobby that has been crystallized in the past. Please see attached pictures. Anyway they want the discoloration on the grout lines removed. Easy enough grind the floor.  Or perhaps someone has another recommendation. The open area is white thassos and it has been ground flat in the past. Now the bordering stone is granite and serpentine which has not been leveled. I don’t feel comfortable grinding those stones especially when are next to a wall. Any suggestions in removing the crystallizer to clean up the grout lines.  Would using straight oxalic acid remove the junk? It will not hurt the granite and I don’t believe it will have any effect on the serpentine. I'll be restoring the white thassos. The next question should I put is disclaimers in my proposal pertaining  to the possibe condition of the stone once I remove the crystallizer. Again any recommendations appreciated!