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). ...

First of all, where had the object been stored? The encrustations
may have been caused due to inappropriate storage conditions.

For example calclacite Ca(CH3COO)Cl . 5H2O (calcium acetate chloride
pentahydrate) is produced in inappropriate storage conditions,
formed on the surface of pottery and geological material after
exposure in wooden furniture, due to the chemical reaction between
the material of the object and the acetic acid of the wooden
furniture. Attempts to clean pottery with ammonia and hydrochloric
acid also result in the formation of this type of salt.

You mind wish to check

    Wheeler, George Segan; Wypyski, Mark T., 1993
    "An unusual efflorescence on Greek ceramics"
    In Studies in conservation, Vol 38, pages 55-62

    Anna Schonemann, Hartmut Kutzke, Bernhard Lendl, and Gerhard
    Eggerts. Combined Raman and FTIR Study of Thecotrichite and
    Related Efflorescence, available at
    <URL:http://publik.tuwien.ac.at/files/pub-tch_6392.pdf>

    Soluble Salts and Deterioration of Archaeological Materials,
    Conserve O Gram No 6/5, August 1998, available at
    <URL:http://www.nps.gov/museum/publications/
        conserveogram/06-05.pdf>

    Lieve Halsberghe, Lorraine T. Gibson, David Erhardt,
    A collection of ceramics damaged by acetate salts:
    conservation and investigation into the causes.
    Available at
    <URL:http://si-pddr.si.edu/jspui/bitstream/10088-8133-1/
        mci_icom14-2005.pdf>

    L.T. Gibson
    Acetic and formic acids emitted from wood samples and their
    effect on selected materials in museum environments Corrosion
    Science (2010)Vol 52, Issue: 1, Elsevier, Pages: 172-178

Prof. Dr. Gerhard Eggert (Staatliche Akadmie der Bildenden Kuenste,
Am Wiessenhof 1, D-70191 Stuttgart, Germany,
gerhard.eggert<-a t->abk-stuttgart< . >de) was doing a survey on salt
efflorescence in 2002 so you might wish to contact him.

    **** Moderator's comments: The above URLs have been wrapped for
    email. There should be no newlines.

I hope this helps.

Evangelia Kyriazi
BA Hons Conservation and Restoration
MSc Geography and Applied Geoinformatics


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                  Conservation DistList Instance 25:1
                  Distributed: Wednesday, June 8, 2011
                        Message Id: cdl-25-1-005
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