Blistering on painting

I am presently working on a varnish removal from a painting (on oak
panel) by the Leiden painter M.Naiveu at Glasgow Museums. What under
the very thick, dark varnish appeared to be some raised paint layer,
presents itself now as small blisters in the paint layer. Apparently
there must be some connection relating to the wood grain as well, as
the blisters are rather vertically stretched and sometimes form
little vertical rows (the wood grain being vertical as well).

My first idea as a cause for such a damage was heat/fire, however
the paint doesn't look particularly darkened. Technical problems in
the ground layer could offer another explanation, but no evidence
for this is visible. The ground layer seems undisturbed and in no
way unusual, wherever it is visible through a broken blister.

Maybe the following offers some further hints (or confusion?):
Several areas of overpaint can be spotted. Most of the overpaint
comes off easily. However within the dark background some spots of
older (strangely wrinkled) varnish remain, underneath those the
paint appears quite vulnerable. Areas with islands like crocodile
skin of old varnish are also present. Altogether mainly the dark
areas of the painting have a very lean appearance.

Has anybody experienced anything similar. I would be grateful for
any hint towards a possible cause of such a damage.


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                  Conservation DistList Instance 19:3
                  Distributed: Thursday, June 23, 2005
                        Message Id: cdl-19-3-020
                                  ***