Removing sulfate from azurite polychromy

Susan White <smwhitewhite [at] netscape__net>

>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. ...
>...
>... 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?

Have you tried a sequestering agent solution, such as 10% w/v
tribasic sodium phosphate in deionized or distilled water?  Given
that you also have iron oxide in the mix, you might want to add a
small amount of tetrasodium EDTA to chelate the iron.  I have used
these reagents in poultices to remove sulphate layers from
polychromed sherds in order to reveal the decoration without
removing or damaging it.  In the study that I did, the sequestrant
solution along with pre-consolidation of the pigmented areas worked
successfully.  I know that other workers didn't have the same
results on their objects, so I suggest that you try it on a small
area first and see how it works in your particular situation.

The objection to using sequestrants and chelating agents in the
poultice might be that you could breakdown the pigment layer, but I
would think that if the sulphate layer is thick enough, and if you
are careful about monitoring the poultice, you should be able to
soften or dissolve the layer, then rinse the polychrome enough to
stop any chelation of the pigment.

The reference for the article that I mentioned is Neely, J.A. and
Storch, P.S., "Friable Pigments and Ceramic Surfaces: A Case Study
from SW Iran", in Journal of Field Archaeology, Volume 15, 1988, pp
108-114.  If you have difficulty in locating the article, let me
know and I can send out a copy. Hope this helps,

Paul S. Storch
Senior Objects Conservator/Section Head/Internal Unit Preparation
    Specialist
Daniels Objects Conservation Laboratory (DOCL)
B-109.1, Minnesota History Center
345 Kellogg Blvd. West
St. Paul, MN  55102-1906
651-297-5774
Fax: 651-297-2967


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                  Conservation DistList Instance 16:2
                  Distributed: Thursday, June 20, 2002
                        Message Id: cdl-16-2-001
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