one side, a differentoverall stress distribution develops that depends on envi-
ronmental interactions. With larger panels, the total (elastic) stress in the
panel structure is accordingly greater.^42
For the Mengs, the combination of thinning and disjoining
appeared to have reduced the physical equilibrium of the individual
planks. Once disjoined, they responded to the internal stress with defor-
mation. The plank edges at the joints, originally parallel, became con-
toured to the irregular grain direction of their respective planks. Thus,
the joint gap varied by millimeters along some disjoined sections (Fig. 9d),
and the joint faces no longer met continuously or squarely. Such potential
damage from wood’s reaction to stress release should discourage the thin-
ning of panel paintings.
During the rejoining process, wood inserts were fitted to the gaps
so that no original wood was removed.Treatment considerations
Planning is important for any large panel treatment. The order and choice
of treatment steps should be logical in relation to the treatment as a whole
and should not foreclose later treatment options. The greater scale of treat-
ment for large panels usually makes backtracking difficult and costly. A cau-
tious, considered approach, in which each stage is tested, should be adopted.
As an example, radiographic examination of the Palmezzano did
not reveal enough of the condition of the wood, prior to treatment, to
ascertain the full extent of damage, since the battens were obscuring the
panel wood. The possibility that the panel might require extensive wood
replacement was anticipated with a more thorough facing than the panel’s
apparent condition warranted. Halfway through a batten or cradle removal,
with splits all around, one cannot easily move alarge panel to apply a fac-
ing that should have been anticipated earlier.
Photodocumentation is important for the back of the panel, as
well as the front.^43 It is therefore necessary to have larger panels disposed
so that necessary photography can be done at any treatment stage. Even
with the best photographic resources, adequate space is required for the
necessary distances and angles. Also, large panels invite strong lighting,
especially for overall photographs, so heat effects should be considered
(Wolters and Kühn 1962). Short-duration electronic flash units have aless
drying effect than the heat associated with continually lit tungsten lamps.
Though not easily achieved, relatively constant humidity should
be maintained to minimize stresses from warping movements during treat-
ment. Rejoining can take days for larger panels because thicker joints and
less-absorbent, higher-density woods require longer drying periods.
It is sometimes better to allow sufficient time for the panel struc-
ture to equilibrate during treatment, to avoid stress that might precipitate
damage. When the Mengs was rejoined, for example, the balsa reinforce-
ment was removed in stages between which the exposed panel was allowed
to equilibrate to a more stable curvature (prior to rejoining) unimpeded
by restraint caused by the reinforcement or its moisture-barrier effect. If
rejoined before equilibration, joints could fail again prematurely.
It is possible to manipulate humidity to facilitate some proce-
dures. Larger panels especially can bind or “lock” sliding reinforcements
because ofgr eater total movement and rigidity. It may be possible tem-
porarily to raise or lower the humidity slightly in order to loosen slidingP A S C L P P 463