Support for painted veneer

Stephan Schaefer <s.schaefer [at] visualbyte__com__br> writes

>Our studio has been entrusted with the conservation and restoration
>of a painted wooden panel (dated 1939) by one of the most famous
>Brazilian artists. After taking it out of it's frame we realized
>that it is not a solid wooden panel as we were previously told by
>museum personnel, but that during previous restoration, the painted
>layer of an original three-ply plywoodboard (3 mm thickness per
>layer) has been glued onto a wooden cradle board (of 18mm thickness)
>utilizing a wax-resin mixture. The wood grain of both runs in
>parallel direction.

As I understand the problem, a single veneer of what was originally
a three-ply panel has been bonded to a solid wood secondary support.
The grain of both runs in the same direction.  Contraction or
expansion perpendicular to the grain of the secondary backing panel
has caused  fracturing and separation of the original.

Before deciding on a suitable course of action it would be advisable
to gain a better understanding of the environmental conditions,
which are the primary cause of damage, by monitoring the display
conditions.

I would guess that the damages mentioned are probably also
accompanied by some curvature which would indicate whether
conditions of RH were high or low.  If the panel viewed from the
front is concave you can assume that the RH was high, but if as I
suspect, any curvature is convex, it would indicate that the RH in
the display environment was low.

Although the ambient conditions in Sao Paulo are perhaps high
(compared to UK), air conditioning may cause the RH to drop
dramatically to levels where damage may occur, especially in the
type of composite structure you describe. It will be possible to
relieve some of the stress by first stabilizing the panel under
controlled conditions before removing the secondary support timber.
This could be done by careful chiselling and planing. Repairs to the
original can then be completed after which it should again be
monitored. No doubt there will be a tendency for the panel, now only
3mm thick, to assume a high curvature. If end-grain profiles are
recorded, (simply by marking them from the face of the panel on to a
piece of card), the response of the panel can be monitored relative
to the variation in display conditions. This should provide the
information required to decide on the appropriate strength of a
support.

I would doubt whether it is necessary to bond it to a solid board,
as this would almost inevitably lead to further problems in the
future. It may be possible to design a cradle, similar to the
conventional type with sliding battens, which would not restrict
dimensional change and would not result in high stresses developing.
If you look at the

    "The development of a flexible attached auxiliary support"
    The Structural Conservation of Panel Paintings
    Proceedings of the symposium at the J. Paul Getty museum 1995

the article describes the design of a flexible cradling system.
Don't be put off by the calculations, the basic principle is fairly
simple and allows a panel to respond to environmental fluctuations
without the dimensional restrictions that are often the cause of
damage. Since writing that article the idea has been refined and
"cradles" successfully used to replace damaging supports on a wider
range of panels including  some light thinned panels.  It does
depend though on how responsive the panel is, the nature of the
damages and the strength of the panel, as to whether it would be
suitable, this can only be assessed by monitoring the panel after
repairs while it is free to respond without restraint.

I would be happy to correspond further if you think I can help on
this project.

Ray Marchant


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                  Conservation DistList Instance 14:15
                 Distributed: Thursday, August 24, 2000
                       Message Id: cdl-14-15-002
                                  ***