Structural condition
When the cradle was removed, the extent of the weaknesses and damage
to the panel could be fully appreciated. One fracture, which ran almost the
whole length of the panel, had occurred in an area of worm-damaged sap-
wood adjacent to a join. The fracture was so severe that one of the board
sections was virtually hinged to the main body of the panel only by a
number of small areas of intact fibers.
There were also traces of two 10 cm wide cross-grain channels
across the panel that could just be discerned in the thinned surface. These
traces indicated that battens may have been present before the cradle was
fitted. Exposed dowels showed that the panel’s original thickness had been
reduced by about halfwhen it was thinned. It was likely that this panel
had had a history of structural problems long before the cradle was fitted.
After structural repairs, rejoins, and consolidation had been car-
ried out, the panel’s cross-grain profile was monitored and recorded sev-
eral times during a period when the relative humidity (RH) was allowed to
vary widely. To judge the panel’s response to likely extremes of environ-
mental conditions, its profile was recorded at 40%, 55%, and 75% RH, and
its condition was reassessed. Monitoring was carried out with the panel
standing vertically on its endgrain.
Released from the cradle, the panel’s profile altered considerably,
becoming convex when viewed from the front and responding quickly to
even small changes in RH. Because of its thinness, the strength-to-weight
ratio, although improved by repairs, was so poor that it could be handled
only with great care. If laid horizontally, it was subject to the risk of frac-
ture ifany attempt had been made to lift it by one of the long-grain edges.
Another cause for concern was an area of severe worm damage,
again in a band ofsapwood extending across the board, close to the bot-
tom, supporting edge. This weak edge would be subject to damaging
forces imposed by the weight of the panel bearing on it and the need for it
to move to accommodate changes ofcurvature. If this natural tendency to
warp were again restricted by a rigid secondary support, further damage—
caused by compression and/or tension perpendicular to the grain—would
be likely to occur.Identifying the need for an attached auxiliary support
It was apparent that if the panel were to remain stable without suffering
further damage, a method ofsupport other than those normally used was
needed. Because the problems presented by this panel were known to be
difficult to resolve satisfactorily, it was decided that the options should
be considered very carefully before a course of action was decided upon.
Conventional techniques of support and retention of RH-responsive
panels include sprung-metal clips, secured within a frame rabbet; a foam-
cushioned panel tray support (Brough and Dunkerton 1984); and un-
attached auxiliary flexible supports (see Bobak, “A Flexible Unattached
Auxiliary Support,” herein).
In these examples of unattached supports wefind a common prin-
ciple: retainers exert pressure on the back of the panel, and this pressure—
frequently concentrated around the perimeter or on the line of the central
long-grain axis—is balanced by the reaction of the lip of the frame rabbet
acting against the edge of the face of the panel.T D F A A S 383