You may be looking at gypsum stains formed by calcite in the slate in
reaction with water.  Such stains are an indication that the slate is
beginning to break down, since the molecule of gypsum are larger then the
calcite molecules and promote delamination. I guess you should deterimine
whether you are looking at gypsum first before thinking about cleaning these
slate.
Christopher Tavener
ctavener @eypae.com

-----Original Message-----
From: Robert Buckle [mailto:[log in to unmask]]
Sent: Friday, February 26, 1999 12:01 AM
To: [log in to unmask]
Subject: A clean slate


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.