involves the minute structure that Barkas has defined as a gel and that has
the potential to revert when exposed to high or cycled moisture content.
The understanding of plasticity and elasticity presented by Barkas
has been somewhat modified by evidence that places even stronger empha-
sis on the importance of moisture movement in wood in these behaviors.
Since it is the cycle of moisture change that we know to be of concern for
panel paintings, it is useful to consider this evidence.
Much work has been done by wood technologists on the phe-
nomenon of creep. When wood is placed under load, it will slowly
deform; the extent of this deformation depends on the stress and, in par-
ticular, on the moisture content. Beginning in the early 1960s, many stud-
ies have shown that cycling of moisture content greatly increases the rate
and extent of creep (Armstrong and Christensen 1961). It has become
clear that the movement of water in the wood structure is of primary
importance in this behavior. The creep development that relates to mois-
ture movement has come to be called mechanosorptive creep (Grossman
1976), while creep unrelated to moisture change is referred to as visco-
elastic creep. As this second designation implies, creep unrelated to mois-
ture movement is substantially elastic. Creep developed under moisture
change also has elastic aspects. When the load is removed, the wood
recovers somewhat, but if the sample is also then cycled through high-
moisture content, there will be additional recovery. It has become appar-
ent, however, that the permanent plastic deformation involved in creep
depends primarily on moisture change.
A closely related phenomenon in wood is stress relaxation. Ifwood
is placed under fixed strain, the stress will gradually decrease. Although
much less work has been done on this behavior with cycled moisture than
has been done for creep, moisture movement can also increase the potential
for stress reduction (U.S. Department of Agriculture 1974:4–37). It should
also be noted that there is some evidence that in stress relaxation, the
potential for plastic change is as great in tension as in compression; under
conditions of room temperature and moisture content below the fiber satu-
ration point, it may even be slightly greater (Youngs 1957).
It seems then that moisture change and internal stresses may be
significant in the development of warping. To elaborate on the function-
ing ofthe uniform backing, it is useful to consider the development of
warping in panels and the potential for stabilizing or reversing it. The
work done on creep seems to imply that moisture changes in the wood
structure facilitate strain, which manifests in the direction of stress. In
panel paintings, the typical convex warp can be the result of at least two
factors. When a painting is brought into a drier environment than that of
the original fabrication, the back surface can shrink, but the paint and
ground layers restrain the wood on the front, and an initial warp devel-
ops. Subsequently, cycles of moisture change influence the back surface
preferentially, and compression shrinkage develops as the outer layer of
wood tries to expand against the restraint of the inner core, due to the
uneven moisture gradient. One might wonder why the reverse process
does not neutralize this effect during the cycles. Ifa panel equilibrated to
a high moisture content dries preferentially at the back surface, would not
a moisture gradient develop and a strain in tension reverse some previ-
ously established compression shrinkage? This outcome certainly could
happen, as the well-known phenomenon of case hardening in the lumber
industry illustrates.R B: T D U B B P P 297