Advise please.
I need to clean an 18 century firebox that was removed from a building and has laid outside for thirty plus years. Some pieces laid face up, others laid face down while other pieces laid atop others on dunnage. All are now face up. There is staining where the dunnage made contact, some discoloration overall and the lintel that laid face up has a black residue on a portion of it. Same black found elsewhere on other stones. I tried a power washer, 1500 psi, Dawn detergent and ammonia worked with a wire bush. While the lichen and some of the discoloration has been removed there is an obvious unevenness in the look of the stones. I should add that the cheeks are one piece as is the lintel. Photos emailed showing conditions upon request.