Subject: 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 *** Conservation DistList Instance 14:15 Distributed: Thursday, August 24, 2000 Message Id: cdl-14-15-002 ***Received on Tuesday, 22 August, 2000