Several factors reduce the ability of this process to prevent the
development of warping in panel paintings. In the first place, many cycles
ofmoisture change are short-term; therefore, the important changes do
not penetrate to the core of the panel—thus, compression at the back sur-
face is the predominant effect. Second, under extreme conditions, wood
structure may be more easily altered in compression, where the structure
can collapse in various ways; while in tension, structural changes are more
difficult, and rupture can result before significant deformation is reached.
Finally, studies of mechanosorptive creep have shown that it is difficult to
reach a limit in compressive creep, but in tension, a limit does seem to be
present (Mohager and Toratti 1993; Rice and Youngs 1990). The implica-
tion of these findings is that the warp in a panel painting can develop sim-
ply from the reaction to moisture cycles.
As an illustration, the author has produced a warp of this type.
Six samples of poplar were coated on one side with a moisture barrier, and
a strain gauge was applied to this side across the grain of the wood. The
samples were then exposed to various moisture conditions at which their
weight, dimensional change at the strain gauge, and warp were recorded.
In two cycles where the samples were equilibrated to very high relative
humidity (RH) and then equilibrated to lower levels, only a slight change
in the measured warp was found. They were then exposed for shorter peri-
ods to high RH and equilibrated to the lower levels. After these shorter
exposures, the warp of the samples increased noticeably. The strain gauge
measurements suggest that this warp was due largely to dimensional
change at the concave surface, which is analogous to shrinkage at the back
of a panel painting. This study emphasizes the potential of short-term
moisture changes to induce warping and, therefore, the important func-
tion that moisture barriers and environmental control serve. From this rea-
soning, one might infer that even if the sliding members are not restricted
intheir movement, a cradled panel could develop the typical “washboard”
conformation because of the continued buildup of compression at the
back surface. Of course, many panels, both cradled and uncradled, appear
to have survived many cycles of environmental change with little or no
warping. This fact emphasizes how difficult it is to generalize about a
material when so many variables separate one sample from another.
Because it serves as a moisture barrier as well as a mechanical
restraint, the balsa backing should protect the painting against the increas-
ing stress or warp that can develop from exposure to short-term moisture
fluctuations.When the balsa backing is used to reduce a warp in a panel, the potential
for introducing compression at the paint surface increases, bringing the
risk of insecurity between paint and support. Therefore, if one can pro-
duce deformation in tension at the back surface, this method may reduce
compression at the painted surface that could aggravate paint insecurity.
For example, Figure 11 shows that a warped board forced flat will develop
planes ofstrain in which compression increases toward the formerly con-
vex surface, and tension increases toward the formerly concave surface.^2
Thus, in a panel held flat by a cradle in elastic strain, one would expect
substantially increased compression at the paint surface. Can this risk of
compression be reduced? One way to do so would be to thin the panel
prior to flattening. By reducing the distance between the neutral plane andFlattening ofPanels
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