Plaster reliefs

We are working in a mosque that belongs to 300 years ago and there
is fine decorative art, of gypsum. For some reason its surface is
covered by oily germs of candle.  Does anyone have any idea that how
can I treat this? I think ammonium or acetone can clear these germs.
If I do that, does the gypsum soak it and make it darker? Physical
methods are very slow and scratches the surface so, I prefer not to
use them.

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                  Conservation DistList Instance 20:17
               Distributed: Thursday, September 28, 2006
                       Message Id: cdl-20-17-010
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