Painting on photographic base

I am working on an oil painting over photograph that needs
conservation treatment for an exhibition. The painting is a portrait
painted with oil over a photograph (dated from 1860) and the
photographic paper has been glued to two layers of canvas and
stretched into a strainer. The painting appears to have some type of
coating/varnish that shows some discolouration in the clear colours
of the portrait (face and white collar)

The main problem with the painting is that, apart from some marks,
small dents and minor damages, all the dark areas (that is the whole
of the painting apart from the face of the sitter) are slightly
bloomed, and the whole painting looks very dull as colours have lost
their brightness. I have tried to surface clean it without much
success as the painting has a surface that repels any water based
mixtures.

I have done some tests with many solvents and gels to remove the
discoloured coating but the painting seems to be sensitive to all
solvents and even white spirit removes part of the colour. Solvents
also seem to leave a mat surface after evaporating.

I would appreciate any advice from other conservators with
experience in treatments of painted photographs about the treatment
of this painting in general and, more specifically, about the
following:

    What types of coating/varnishes are normally found on oil
    painted photographs?

    Can a varnish be safely removed without affecting the paint, the
    photographic emulsion and the underlaying photographic paper?

    If the varnish cannot be removed, what type of  coating could be
    applied to saturate the colours again, considering that the
    paint is soluble in solvents like xylene and white spirits?

Mar Gomez Lobon
Paintings Conservator
Queen Victoria Museum and Art Gallery
Wellington St, Launceston
TAS 7250 Australia
+61 3 6323 3756
Fax: +61 3 6323 3770


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                  Conservation DistList Instance 19:23
                 Distributed: Friday, October 28, 2005
                       Message Id: cdl-19-23-026
                                  ***