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). ...

I would be very surprised if the accretions/deposits are gypsum.
Gypsum is a form of calcium sulfate and is sparingly soluble in
water and I have rarely seen it as a deposit on Greek vases. I think
it is more likely that you have a complex silicate deposit
("chabazite", a calcium-sodium alumino-silicate), referenced in
Bethune Gibson's 1971 Studies in Conservation article, "Methods of
removing white and black deposits from ancient pottery", pp. 18-23.

These are very hard deposits, sometimes looking like root deposits
(which they are). They are very common on Greek vases, especially
from burial soils in Italy and Sicily. Calcium sulfate would slowly
dissolve simply by soaking the vase in changes of deionized water.

Chabazite can be removed mechanically, which is risky, or by the hot
sodium hydroxide/EDTA mixture Gibson recommends (also risky). I
would never use phosphoric acid to clean a Greek vase. Phosphoric
acid can severely damage the glaze and clay body. Contact me
directly for more information.

Stephen Koob
Chief Conservator
The Corning Museum of Glass
One Museum Way
Corning NY 14830
607-438-5242


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                  Conservation DistList Instance 25:1
                  Distributed: Wednesday, June 8, 2011
                        Message Id: cdl-25-1-002
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