- to flatten and smooth the back surface, a procedure usually fol-
lowed by the attachment of battens or laminates to restrain
the panel in a flattened state; - to lighten the panel for easier handling or transport (which
may also have the opposite effect because of increased
fragility); - simply to take action.
It could be tempting to lighten large, heavy panels such as the
Mengs. From a strictly practical viewpoint, sheer size would be reason
enough for cutting away some wood to provide a flat surface to more
easily fit a new auxiliary support. To respect and conserve the entire original
object,however, it is possible, for example, to build up an even surface of
balsa wood for battens to bear upon (Buck 1962) without removing origi-
nal panel wood to achieve the same purpose. The three remaining reasons
cited above for thinning are unjustifiable with respect to preservation.
The most recent thinning ofthe Mengs panel appears to have been
directed at obtaining a flat surface to allow adhesion of relatively large
balsa planks. The use of power tools is evident from the parallel kerf marks
of a circular saw, power-planer blade marks, gouge marks chipped deep
into the irregular walnut grain, localized rasp marks, and other damages.
Despite the large scale of work that seems to justify the use of
power tools on larger panels, the use of manually controlled hand tools is
preferable. Some power tools are “double-edged swords” that can speed
work but also easily outstrip the intention and control of the user.^37 A
higher speed of treatment, for whatever reason, should not endanger the
painting. In this regard, responsibility for the rateof treatment and its
effects extends beyond the conservator to all custodians ofcultural prop-
erty—administrators, curators, dealers, and owners.
In an effort to prevent buckling of the Mengs, strips of slotted
metal had been screwed into the edges of the panel and balsa laminate
(Fig. 12). Obviously, even though cross-laminated, balsa did not prove
sufficiently rigid to prev ent buckling when the panel was upright. The
metal edging provided a relatively rigid outer framework that met the
immediate reinforcement need but that had serious consequences for
the painting.^38
Unrestrained, the panel would expand and contract as a unit, the
top moving upward and downward with changes in MC. With such a large
panel, lateral movement across the wood grain could be on the order of
50 mm, if fully equilibrated over a 30% change in RH.^39 However, the
entire panel could not move as a unit. Instead, the planks were individually
constrained to expand and contract around the wood screws at each end.
At lower humidities, the panel would contract across the grain, and either
the wood had to split or the joint adhesive had to give way, depending on
whichever was weaker. Though casein is normally a strong adhesive, the
walnut wood was stronger, even across the grain, so the joints failed in
tension across the adhesive layer. They probably opened catastrophically,
as zippers sometimes do, especially ifthe panel was subjected to relatively
rapid and large changes in RH.^40 One joint near the center of the panel
had completely parted.
Environmental history affects the stress distribution in wooden
panels.^41 Seasoned planks develop a particular stress distribution before
being assembled. Once the planks are joined, grounded, and painted on462 Brewer
Figure 12
Anton Raphael Mengs, Noli Me Tangere. Screw
holes in the panel edge where metal reinforce-
ment strips were attached. The panel is on the
right. Note the layers of balsa/wax-resin and
fabric laminate and the saw marks in the
panel where the balsa was carelessly trimmed.