He also demonstrated that while moisture barriers can slow the reaction
of panels to environmental change, they probably cannot eliminate it
(Buck 1961). Although wood structure, physical or chemical deterioration,
environmental history, and so on, can affect the panels, in general, dimen-
sional change follows moisture change. Many variables determine how this
change manifests. In turn, these variables can be used to modify behavior
in particular cases.
Wood can react to stress with changes that take the form of elastic
or plastic deformations. By definition, elastic deformation will be reversed
ifthe stress is removed, while plastic change will remain. In wood, how-
ever, the relation of these can be complicated, with moisture levels,
moisture gradients, internal stresses, and external loads or restraints con-
tributing significant variables. Buck, whose work was based in particular on
that ofW. W. Barkas (1949), stressed the importance of the potential for
plastic change to take place below the fiber saturation point (Buck 1972).
Barkas and Buck placed strong emphasis on the spring-and-dashpot model
of elasticity and plasticity (Barkas 1949:80; Buck 1972:3). This description
uses the spring to represent the totally recoverable elastic element and the
dashpot (a plunger moving through a viscous material) to represent the
plastic aspects of wood movement. The interaction of these two aspects is
complexand highly dependent on moisture content and other variables.
Buck has argued: “As the moisture content approaches the fiber saturation
point, the bound water becomes almost a lubricant, permitting actual slip-
page ofelements past each other under stress, as the much weakened
bonds break and change partners. This kind of behavior is plastic. It creates
none of the tensions that cause elastic reversal” (Buck 1972:4).
Barkas considered wood as a gel material and stressed the impor-
tance ofmoisture level and moisture movement in determining elasticity
and plasticity in wood: “Wood fibres would behave elastically both longitu-
dinally and transversely for strains which do not exceed the limit of bond
recovery, but plastically for those strains which involve a change of hydroxyl
partners. Also, if the moisture content were lowered while the displacement
was maintained, the newshape would become ‘frozen in’ by the formation
ofa new set of direct hydroxel linkages in place of water bridges. But if the
distortion were also to involve elastic strains, these would also be frozen in
by the structure thus leading to the recovery when the wood is rewetted,
even after a considerable lapse of time” (Barkas 1949:82).
While much work remains to achieve an understanding and theo-
retical model of these relationships, this passage reminds us of the impor-
tance of moisture movement in the development of elastic and plastic
deformation. It is also important to remember that elasticity and plasticity
can be dependent on defined conditions. For our purposes, for example,
the elasticity released by very high moisture content and temperature
could for all practical purposes be considered a stable situation in a panel
painting; thus, the separation between plastic and elastic deformation
becomes somewhat ambiguous.
Wood structure can retain two types of elasticity that can be
undetected until released by mechanical or environmental change. The
first type is differential stress in the larger wood structure. For example, if
a case-hardened board is sawed, the two sections can show a pronounced
warp due to the release of elastic strain. If such discontinuities are present
in a panel painting, they could be released mechanically by thinning or
environmentally by moisture change. The second type of elasticity296 Horns