Removing sulfate from azurite polychromy

I am treating a 14th c. marble Madonna for the Duke University
Museum and have a situation where a thin, discolored (by iron oxide
and carbon) sulfate layer covers original azurite polychromy. The
museum director would like to have this layer removed for both
aesthetic reasons and because the blue pigment distinguishes her as
the Madonna (and not just a generic female saint). Sulfate corrosion
also covers unpolychromed areas and I've been able to swell it
sufficiently with methyl cellulose and deionized water poultices to
remove it carefully with a scalpel. This is not possible in the
areas of polychromy because the paint is soluble in water (very
little original binder remains). Does any one have a suggestion as
to how best to attempt the removal of the sulfate layer over the
azurite so as not to risk loss of the pigment?

Susan White
White Conservation Services
New York NY



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                  Conservation DistList Instance 16:1
                   Distributed: Friday, June 14, 2002
                        Message Id: cdl-16-1-012
                                  ***