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 have usually find white, hard accretions on Greek pots to be
insoluble salts of some short (most probably carbonates or
phosphates). According to your description, it is very unlikely the
accretions on your pot to be gypsum since gypsum is a relatively
soft mineral (it has a 2 on the Mohs scale of hardness) and you
could scratch it with your finger nail.

Try to perform the spot test again, this time by using HCl instead
of HNO3. If carbonates are present it should effervesce. If not try
to do a spot test for phosphates using ammonium molybdate. Make sure
the water you are using for your spot tests is de-ionized.

In general I have found very hard to impossible to remove accretions
like these solely by mechanical means, it is very possible that the
surface will get damaged. The acid bath is the safest way, in my
opinion, as long as the object is very very carefully rinsed
afterwards. But in order to perform the acid bath you have first to
be definite on the accretion's nature.

If you do have gypsum then you should be able to wet it (since it
gets softer) and mechanically remove it.

Konstantina Tsatsouli, MSc
BSc Archaeological Conservation
MSc Conservation


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                  Conservation DistList Instance 25:1
                  Distributed: Wednesday, June 8, 2011
                        Message Id: cdl-25-1-003
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