A problem becomes apparent when desiccation of the panel is attempted.
A drop from 70% to 62% RH causes tensile yielding, and a drop to approxi-
mately 38% RH can cause cracking of the wood. Increasing the RH to
approximately 74% induces yielding in compression. The panel cannot
tolerate the much larger variations in RH that are possible with a panel
equilibrated to 50% RH, as seen in Figure 3. This narrow range of RH
must be considered when evaluating the risks of lending panel paintings
acclimated to high RH.
In the past, some panels have been treated with water or large
amounts ofwater vapor in an attempt to flatten them. Battens or cradles530 Richard, Mecklenburg, and Tumosa
Starti ng RH (% )0 20 40 60 80 100110
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0EndingRH(%)Res trai ned at this RHTens ion res pons eCo mp res sion res pons e
Yield 5 0.004Yieldsin co mpres si on at this RH
(compres sio n set begins)Zero stres s or strainYield s in tens ion at this RHCrack s in ten si on at thi s RHYield 5 0.004Failure110
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0
0 20 40 60 80 100Com pres sio n res pons eStar ti ng RH (% )EndingRH(%)
Yield 5 0.004Yi eld 5 0.0 04Failu reTens ion res pons e
Res trai ned at thi s RHCrack s in tens ion at thi s RHYieldsin te nsio n at this RHZerostres s or strainYield s in com pres si on at this RH
(c omp res si on set begins)Figure 3
Calculated reversible RH range of fully
restrained, tangentially cut white oak versus
ambient RH. A yield value of 0.004 was used
as the limiting criterion in both tension and
compression. The values of the dotted lines
are for stress-free wood that has been fully
equilibrated to 50% RH.
Figure 4
Calculated reversible RH range of fully
restrained, tangentially cut white oak versus
ambient RH. A yield value of 0.004 was used
as the limiting criterion in both tension and
compression. The wood has been fully equili-
brated to 70% RH. The allowable RH range
has been severely reduced in comparison to
woodequilibrated to 50% RH.