I am looking for info on cleaning slate. Slate is being removed from a
collapsed building. It is to be reused on  an adjacent building. It has
some staining - white which may be a salt. The roof is near the ocean. The
slate appear to be in good condition other than some staining. A chemical
cleaner was recommend but I think this may not be required.  From local
sources it appears the slate was installed in the 40s (second roof of a
1907 wood frame builing...poorly constructed. Slate is attached with
galvanized roofing nails.