Blistering on painting

Sylvia Krauss <sylvkra [at] web__de> writes

>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.

I read your query and thought that I may contribute some thoughts on
the matter.  I am a painting conservator and work in Valencia,
Spain. I recently restored a retable by Juan de Juanes, (Valencian
Mannerist paintings).  There were many panel paintings that had been
exposed to a big fire during the Spanish civil war. Although your
oak panel may be a thinner support than the 3cm thick mediterranean
pine panels, I imagine that in the end their behaviour in proximity
of a fire would be somewhat similar.

The natural resins that occur in the wood structure heat up and
change from a solid to a liquid state.  This liquid seeks a way to
seep out of the painting.  The fissures or joints in the wood may be
natural causeways for the resin to flow down.  The joints may even
originally have been "glued" together with a resin filler.  When the
painting then cools down, small clusters or drops of this dark
cooled resin may become trapped below the paint layers.  It may also
seek out onto the surface of the painting and ooze down the surface
taking with it all that it finds in its course--a bit like volcanic
lava.

In the areas of blistering you say that where the blisters are burst
it is clean underneath?  This is probably due to a previous
restoration.  The combination of burst blisters and then blisters
that may be full of resin and can not be flattened in the
consolidation process, reminds me of the sort of problems that we
encountered in these fire damaged panels.

We also found that the darker colours were more damaged because of
the greater amount of binder which is present.  The lighter colours
survived better, they have a higher pigment content.  Colours that
had resin coatings especially the greens where charcoal like.

This resin was easily dissolved in alcohols, so it's possible that a
previous restoration, especially with recent easy removable
retouching on top, has changed the complete effects of the heating.

A closer look at the back of the painting may help.  Although if the
heat source was at the front of the panel and not on the wall side
then the resin will tend towards the heated side of the picture
(these were our findings). We did not find a solution to returning
those blisters to their original position.  Retouching was
labourious and not that good for the eye-sight! I hope my comments
help to shed some light on the subject.

There is a comprehensive catalogue published in January of this year
on the work we did, only in Spanish though (bookshop online of the
Generalitat Valenciana).

Christabel Blackman


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                  Conservation DistList Instance 19:5
                  Distributed: Saturday, July 9, 2005
                        Message Id: cdl-19-5-003
                                  ***