Water soluble coating and glazes on oil painting

While doing cleaning tests for the removal of surface dirt from a
badly damaged 18th century oil painting (using deionized water),
I've noticed that the yellowed coating (which resembled a yellowed
varnish) is completely water soluble.  The appearance and solubility
of the layer varies slightly with the underlying colour.  Red and
blue draperies are finished with red, blue and brown water soluble
glazes.  Interestingly, the paint sample cross-sections show strong,
resin-like UV fluorescence in red and blue glaze layers, while the
samples from other parts of the painting reveal a transparent
yellowed top layer with no fluorescence observable at all.  There is
no indication that this coating is a later intervention; moreover,
drapery folds and shadows are painted using only this water soluble
glaze.  I'd like to know has anyone had similar experiences 18th
century paintings (or paintings from other centuries) and if
chemical analyses were carried out (at the moment we have no
possibility of a precise media analysis in these particular layers).

Jelena Zagora
Intern at the
Conservation Department, Split
Croatian Conservation Institute


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                  Conservation DistList Instance 28:21
                 Distributed: Friday, October 24, 2014
                       Message Id: cdl-28-22-018
                                  ***