In general, the restraining bars are placed parallel to and approxi-
mately halfway between the joint and each parallel panel edge. The bars
may be made of padded wooden strips of sufficient rigidity. They should
barely contact the panel surface or lie slightly above it. Spacer blocks are
placed beneath each end of the bars, and the ends are clamped.
If necessary, the alignment frame is repositioned, after which the
screw clamps are lightly snugged to the panel surface. The foot pieces are
isolated from the panel and glue with a release film^11 and padded, if neces-
sary, with small pieces of mount card or blotting paper. Glue should not
contact the card or paper, as it could seep beneath and damage the painting.
Out-of-plane alignment of the joint edges is usually best attained
with the least number of screw clamps and with the least pressure applied
at the least number of pressure points. The conservator determines the
arrangement by trial, repositioning the clamps until the desired effect is
achieved. The procedure is usually to move each joint edge alternately, and
about equally, until alignment is achieved. One edge should not assume all
the strain. Many splits and disjoins realign with ease when the simplest
appropriate arrangement of pressure points is used.
Out-of-plane alignment and the best overall curvature may be
determined by several methods. These include (1) passing of the finger-
tips across the joint, (2) repeated passing ofthe palm across the general
area ofthe joint, (3) use of raking light cast across the joint from both
sides, (4) sighting of panel edges at the ends of joints (if appropriate),
(5) checking the gap with backlight, and (6) use of raking light or back-
light, with straight edges placed over the joint.
During use of these techniques, the joining pressure is tested, a
process that previews how the panel shape will change under the antici-
pated pressure. Alignment pressure may have to be adjusted slightlyin
accordance with a shape change, and further precautions may be neces-
sary. For example, thinner panels may bend in plane when joining pressure
is applied to a joint that is gapped in the middle (Fig. 6). The joint edges
contact near each end while the gap is reduced. This type of bending
increases as joining pressure is concentrated across the gap. It may occur if
the padded bars are not sufficiently rigid—a deficiency that causes pressure
concentrations where the bar-clamp stops make contact.
To control these effects, it may be necessary to shim the curved
edges of the panel (Fig. 7). Very small movement can have a significant
effect on final alignment and bond strength. Shims can be used to concen-
trate joining pressure to close or reduce slight joint gaps.Glue application and rejoining
After successful completion of the dry rehearsal discussed above, the
conservator can proceed with the application of glue and the actual rejoin-
ing. To allow access to the joint for gluing, the top right-angled-section
beam(s) and screw clamp(s) may be entirely removed from above the
panel, or a bolt may be removed from one end only and the beam(s)
hinged up and away. Alternatively, each top screw clamp could be backed
offthe panel—a maneuver that may be preferable and wastes little time
during repositioning after or during glue application. The bottom clamps
provide a sufficiently fixed datum if the panel is relocated exactly. Another
option would be to mark each screw’s position with an ink line across the
screw thread and screw housing, back offthe clamp, apply the glue, andS R M P P 425Figure 6
The effect of even joining pressure (indicated
by arrows) on a disjoin with a central gap
(exaggerated). The dotted lines show move-
ment of the panel under pressure as it bends
in-plane.
Shims
PanelWooden beamFigure 7
Shims placed in gaps along an uneven edge,
left, spread the joining pressure (indicated by
arrows) evenly along the joint.