Off gassing image on glass

I have just removed a water colour from its frame for packing off to
the paper conservator and noticed an after image on the glass where
something has been off gassing. The painting was mounted on a wooden
board and it was in direct contact with the glass. I've seen this
type of image before with oil paintings caused by VOC but what is
causing it in the water colour? Is it from the wood?

Ana Flynn
Conservation Officer
Collections
and Exhibitions
Tyne and Wear Museums
Laing Art Gallery
New Bridge Street
Newcastle upon Tyne
Newcastle NE1 8AG
+44 191 232 4275
Mobile: +44 775 247 5473


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                  Conservation DistList Instance 22:65
                    Distributed: Friday, May 8, 2009
                       Message Id: cdl-22-65-014
                                  ***