Greek column krater

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.  Initially I thought
they might be calcium carbonate and could be removed with a
combination of dilute acids and mechanical removal followed by
flushing the areas well with deionized water to remove all acid
residue.  However, after taking samples of the accretion (which was
very difficult to remove owing to the hardness and tenacity of the
accretion), I tested it with concentrated nitric acid, expecting to
observe the evolution of bubbles of carbon dioxide.  I also tried
concentrated phosphoric acid with no results.

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).  Any and all
suggestions are welcome,

Susan White
White Conservation Services


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                  Conservation DistList Instance 24:53
                   Distributed: Monday, May 30, 2011
                       Message Id: cdl-24-53-018
                                  ***