Calcium oxalate on easel paintings

Flavia de Souza <fluksy2000 [at] hotmail__com> writes

>I am a conservation student currently writing my thesis on calcium
>oxalate film encountered on easel paintings. ...
>...
>... In my review I intend to
>summarize the analysis methods performed for identification of
>calcium oxalate ...

With reference to Calcium oxalate, in the case of films on stone it
has been discussed extensively as both a natural and artificial
protective layer, (the latter characteristically described as
scialbatura), and is widely found on monuments, within crusts or
between the crusts and rainwashed areas, and as a yellowish
discolouration/patina. On stone it's derived either from the
incomplete combustion of fossil fuels, the transformation of
previous restoration treatments using oils and waxes, and/or due to
the bio-infestations which produce oxalic acid that reacts with the
underlying carbonate matrix.

For example, one mechanism is where saxicolous lichens are reported
to produce up to 50% of their total weight in oxalic acid which
reacts with marble to produce calcium oxalate.

Scialbatura consists mainly of calcium oxalate in two forms:
monohydrated whewellite and dehydrated weddelite. Both are
chemically pure, although impurities are bound up in the crust
during their formation. Calcium oxalate is itself colourless but
when organic fragments, mainly lichen, and inorganic fragments of
quartz, feldspar and other minerals are present they give the patina
a yellowish brown colour. Calcite crystals become isolated creating
a mixed oxalate/calcite band with no clear definition between the
carbonates and oxalates, making it difficult to detect in
cross-section, and this layer can be less than 100 um thick, or as a
heavy encrustation. Calcium carbonate is significantly more soluble
than calcium oxalate at pH 7. As the solubility of the carbonate
increases until pH 5, beyond which irreversible decomposition occurs
the oxalate is far more acid resistant and not as susceptible to the
degradation caused by acid gases.

Non invasive portable Raman spectroscopy can be used to identify
monohydrated whewellite and dehydrated weddelite

One of the things I'd like to investigate but have never had the
time is developing some kind of portable test kit (and I too would
like to know of anything that can be adapted for testing stone
surfaces/samples non-invasively). Three things I have been aware of
are:

    The possibility of identifying calcium oxalate crystals using
    techniques developed around alizarin red S staining as a spot
    test.

    The re-examinination of the Raspail spot test, a test developed
    in 1827 by Francois-Vincent Raspail in which he originally
    demonstrated the presence of calcium oxalate in the starch of
    monocotyledon plants. Nowadays the test is standardised only as
    a method for detecting rosin in paper

        <URL:http://www.modernmicroscopy.com/
            main.asp?article=69&page=2>

    There might be something to investigate in using a standard
    silver nitrate stain test on samples

        <URL:http://www.histosearch.com/
            histonet/Jul07A/Re.HistonetYasuesSilverNiA.html>

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

Jonathan Kemp
Senior Sculpture Conservator
Sculpture Conservation
V&A Museum
Cromwell Road
South Kensington
London SW7 2RL
+44 2079422121


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                  Conservation DistList Instance 22:56
                  Distributed: Tuesday, March 31, 2009
                       Message Id: cdl-22-56-004
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