of warp-prone areas is also a factor. For example, planks cut tangentially
are more prone to warp movement. If a warp-prone area is located
toward the middle of a panel, the movement will be transmitted to the
panel’s (longitudinal-grain) edges so that the overall deflection may be
somewhat greater than that ofthe central plank (Fig. 1).^5 Since this is an
angular relationship, deflection of the panel edges may be almost instan-
taneous, especially for larger, thinner panels that are more flexible and
therefore more responsive.^6
Treatment RH should be similar to that of the panel’s normal or
destined location (Fig. 2). If not, after the panel is relocated there will be
further movement opposing any restraints imposed by rejoining, reinforce-
ment, or framing. Effective treatment should lessen potential stresses in
the painting structure as much as possible.
Proportional increase in total wood movement has other implica-
tions for panels of larger cross-grain dimensions. The development of
end-grain splits or checks is well-known in the drying of commercial oak
timber, especially larger sections. This is partly due to much higher mois-
ture permeability through end-grain, where oak’s large-diameter vessels
play a part, than through side-grain surfaces.^7 Asimilar phenomenon
seems evident with respect to wood movement in oak panel paintings
where cyclic compression sets and tensions provoke end-grain fractures
(Desch 1956:93–95) and disjoins. These effects are proportionally greater in
wider planks. Like oak, walnut has relatively high density and large vessels.
Figure 3 shows a joint between wide walnut planks that had parted several
times, developing an ever-increasing gap, evident from the stratigraphy of
three or four putty layers.^8Structure of larger panels
Structure determines many aspects of conservation. Tree species that
grow larger and yield larger planks have usually been used in large panels.
White poplar (Populus albaL.), oak (Quercusspp.), and Scots pine (Pinus
sylvestrisL.) are examples. Large panels are sometimes made from rela-
tively small planks, as in some of Rubens’s larger landscapes (Brown,
Reeve, and Wyld 1982).P A S C L P P 449Dimension 1
Dimension 2Deflection 2 Deflection 1Figure 1, above
Apanel, consisting ofthree planks, viewed
from slightly above one end of the planks. A
greater cross-grain dimension would “mag-
nify” the deflection of the middle plank in the
diagram. This is shown as the difference
between deflection 1, for a smaller panel of
dimension 1, and deflection 2, for a larger
panel ofdimension 2.
Figure 2, right
Atent enclosure for treatment of a large
panel, built to sustain RH and temperature at
approximately the same levels as in the panel’s
normal environment.
Figure 3
A gap in a joint toward one end of the adjoin-
ing planks shown after several strata ofover-
lying nonoriginal layers and putty were
removed. Since disjoining, the plank edges
had developed a step that had been subse-
quently abraded to the same level. The length
of joint shown is about 300 mm.