Len, I have always found marble to be such a porous material that removing
deeply imbedded stains is almost impossible. The greatest success I had
in cleaning the surface used a mixture of acetone and denatured alcohol,
applied with 0000 steel wool, and wiped with water as a final rinse.
For bleaching, I had moderate success on Carrara marble using oxlic acid
mixed with whiting (something with a calcium carbonate base) and applied
overnight as a poltice.  Please do not sand, you will never restore the
appearance of the original polish, and you will open the marble for
further staining in the future.  Clean the surface of accumulated dirt, and
accept the results as an honest representation of aged marble.
Gary M. Turnquist



conservator
Appalachian Cultural Museum
Boone, N>C>