The Structural Conservation of Panel Paintings

The screw clamps will be attached to the angle-sectioned beams

above and below the panel joint so that they are on either side and in line

with it. First, however, the alignment apparatus is positioned approxi-

mately and the bottom beam clamped to the table crossbars with small

C-clamps, which stabilize the apparatus.

Next, two straight wooden beams, of about 50 3 50 mm in cross

section, are placed on each side of the alignment frame(s). These may be

clamped to the table crossbars. Then, depending on the panel size, at least

two bar clamps are laid across the wooden beams and through the rectan-

gular frames. The beams above the lower set of screw clamps support the

bar clamps and the panel. The top surface of the bar-clamp rail should lie

in the middle of the alignment frame(s). This arrangement defines the

panel position in relation to the clamps.

For stability, all the bar clamps may be joined by some relatively

rigid means so that they are parallel to one another. In the diagram, two

standard threaded steel rods serve the purpose, passing through stop holes

and fixed with nuts to either side of each bar clamp. Fixing the clamps

together rigidly prevents accidental slips and provides a secure base for the

panel. Depending on the panel shape and the angle of the joint relative to

the panel edges, the clamps can be positioned at angles to the panel edges

rather than placed strictly perpendicularly.

Prior to placement of the panel in the apparatus, the effective

contact area of the stops of the bar clamps is extended and padded. A

length ofrelatively rigid bar (e.g., a strip of wood) is placed against the

line ofbar-clamp stops at each panel edge. A thin balsa plank or strip of

card is placed between the rigid bar and the panel edge.

These two pieces distribute the pressure more evenly along the

entire panel edge. The batten spreads the point pressures of the stops, and

the padding conforms to local irregularities. The padding material can be

carved or sectioned to apply pressure to the strongest surface while it

avoids weaker areas. The lengths of batten and padding are cut slightly

shorter than the respective panel edge. To permit judgment of curvature

during the procedure, they are positioned to allowsighting along the end-

grain edge of the panel.

Panel manipulation before rejoining

Before glue is applied, a dry rehearsal of the alignment procedure is con-

ducted. To bring both sides together squarely, it is critical to respect the

panel’s curvature during rejoining. Otherwise, a poor joint usually results,

with interruptions of the inherent contours of the panel surface at the joint.

If the panel is weak or warped, it should be supported in a state of curva-

ture that minimizes the bending stresses imposed by its own weight. This

can be done by placing wooden shims at intervals beneath the panel which

are cut to fill the gap between the panel back and the bar clamps (Fig. 5).

The panel is then slid horizontally, painted side up, onto the bar

clamps and through the rectangular void of the alignment apparatus until

the intended joint is approximately aligned with the line of screw clamps.

Conve x warps (viewed from the painted side) often promote buck-

ling when joining pressure is applied. Inherent warp and excessive side

pressure increase the tendency to buckle. This pressure can be redirected

through the panel toward the desired direction and across the intended

joint by the positioning of restraining bars above the panel.

424 Brewer

a

b

Panel

section

Restraining

bars

Shims Liquid

glue

Figure 5a, b

Views of the end-grain edge ofa warped

panel, with the curvature supported by shims

from below. Under joining pressure (indicated

by arrows), when the panel is unrestrained

(from above) from buckling (a), the joint

aligns poorly and a gap is created between the

joint faces, which will not be well bridged

when the glue shrinks and dries. Padded

restraining bars above the panel surface

(b)redirect the joining pressure through the

joint. The joint faces then meet more squarely

and squeeze glue evenly from above and

below, indicating a better joint configuration.