Greek column krater

Susan White <smwhitewhite [at] aim__com> writes

>I have received a Greek Column Krater which is covered with patches
>of extremely hard and opaque white accretions which are quite
>unsightly and cover important design areas.
>...
>After some careful research I discovered that the accretions had to
>be gypsum which doesn't respond to concentrated.acids.  Does anyone
>know of a safe way to remove gypsum accretions from terracotta? (I'd
>expected to see sulfur dioxide bubbles if the accretions were
>calcium sulfate, but then realized that the hydrated form of calcium
>sulfate, or gypsum, is invulnerable to strong acids). ...

A few thoughts: Mechanical reduction to thin the deposit, followed
by prolonged soaking in water should soften it enough for gentle
mechanical removal, though that may also leach trace minerals out of
the ceramic, so if any elemental analysis is desired, it should be
done first.

There was some research into the use of sulfate-reducing bacteria
for conservation purposes, but I don't think that has been pursued
as a treatment method for gypsum deposits.

If the object cannot be soaked, there is a patent out for a solution
that is supposed to quickly dissolve plaster residues from skin, and
its main ingredients apparently are water acidified with vinegar or
citric acid (pH between 2-5--that is about 2% by weight, according
to the document), mixed 50:50 with alcohol, to which an emollient is
added to protect the skin.  I would substitute a detergent for the
emollient to help fully wet and break up the gypsum.  This could be
poulticed as needed and cleared afterward.

Caution: the citric acid may leach iron from the ceramic, so this
would have to be considered (perhaps a buffering agent?) No
promises, since I was not aware that gypsum could be affected by
acids.  However, it might be worth a shot, as I don't think anyone
has yet found an improvement over water and time!

Linda Roundhill
Art and Antiquities Conservation, LLC
Woodinville, WA
425-481-0720


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                  Conservation DistList Instance 25:1
                  Distributed: Wednesday, June 8, 2011
                        Message Id: cdl-25-1-004
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