The Structural Conservation of Panel Paintings

Central bearing

The thickness and area of the central bearing is determined by the maxi-

mum curvature expected (Fig. 12a), the available depth within the frame

and tray, and the desire to make the bearing as short as possible to reduce

the “hard point” in the center of the flexible batten and the back spring.

Bases

These form a bridge enabling the back spring to function. Their area and

depth are guided by the same factors as the central bearing. The bases are

glued at the ends of the back spring and consequently will reduce the

effective span of the back spring by their length.

Only one ofthe two bases should be fixed to the backboard. If

both were glued, then a rigid arch would be formed, and the flexibility of

the back spring would be greatly reduced. The maximum height of the

bases must allow for the further expected deflection of the back spring. In

more recent developments, Plastazote foam,^10 sandwiched between timber,

has been used in the bases to allow movement in the back spring when

both bases are fixed to the backboard (Fig. 12a, b).

By taking profiles at frequent intervals with the RH constant at 55%,^11 it is

possible to monitor the curvature of the panel and record its profile when

it has reached equilibrium. This curvature, if any, is taken to be the neu-

tral position. When thereis regular slight curvature, it is not necessary to

shape the battens to the panel, since a small preload is desirable to keep

the panel in position against the tray rabbet and to bias the panel’s move-

ment in the preferred direction.

Where the curvature is large or uneven, the batten will need pack-

ing to the surface profile of the back of the panel. This is done by the use

ofshort, shaped sections ofbalsa wood glued to the front of the batten.

The balsa grain running at 90° to the batten grain will minimize any

change in flexibility (Fig. 13).

Safe deflection of the panel

In order to establish safe deflection, test samples and model panels can be

made. Although they must not be relied upon to give analytical informa-

Matching the Support

to the Panel

378 Bobak

a

b

Plastazote foam

Plastazote foam

Figure 12a, b
Critical clearance points (marked byarrows)
for maximum (a) and minimum (b) curvature.