The Structural Conservation of Panel Paintings

tion, they are nevertheless helpful in establishing the broad range of likely

forces. Test samples can never represent the exact structure of the panel,

its paint and ground layers, or the weaknesses of irregularities and aging.

Gently flexing the panel can help to verify sample data and must be done

with the greatest possible prudence. A rig using a spring balance or small

weights can also relate load to movement.

Once the total safe load for the panel is established (including a

safety margin to ensure that the support will yield to the panel), the safe

load is divided by the number of elements in the support to find the load

per element, which produces a determined deflection. The flexible batten

is then thinned to give the determined deflection at that load. The number

of elements in the support will be determined by the area of the panel and

the length of the panel along the grain. In practice, most panels have

spring elements with centers between 100 mm and 150 mm. The back

spring must be just stiffenough to ensure that the flexible batten is held

fully engaged, in contact with the back of the panel, thus providing an

ev en support. It must not be so stiffthat the concave movement ofthe

panel is restricted.

Effect of batten curvature on panel curvature

To summarize, if the neutral curvature profile of the flexible batten is less

than that of the panel, then the panel will be moved toward a flatter plane.

Ifthe neutral curvature profile ofthe flexible batten and that of the panel

are the same, then the panel will have no tendency toward either concave

or convex movement. Thus it is possible to tailor the spring system to

encourage a panel toward a flatter plane.

The timber used is Sitka spruce (Picea sitchensis). It is straight grained,

largely free of faults, light, elastic, and of consistent density. It has been

used in aircraft construction for more than ninety years and is available

with aircraft release notes.^12 The face of the flexible battens touching

theverso ofthe panel can be covered with felt or cotton tapes to protect

the panel from abrasion. The panel tray is constructed from hardwood

(mahogany or a similar wood) to achieve rigidity for a minimum size of

section. Saw kerfs^13 can be cut in the mitered corners and hardwood

tongues glued in place to increase rigidity (Fig. 14). This method is espe-

cially good for small trays when the timber section is insubstantial. In

some cases there may be insufficient space for a tray within an existing

frame; it may be possible to use the frame as the basis for the shaped slip

pieces and to build up the sides of the rabbet so as to make the frame

effectively into the tray. The backboard and spring supports may be fitted

by the same method.

All four sides ofthe panel have their profiles taken, and these are

transferred onto the tray edging section. It is generally easier to construct

the shaped profiles and glue them into a basic section than to carve them

Construction

A F U A S 379

Figure 13
Part of a flexible batten showing cross-grain
balsa-wood packing pieces shaped to fit the
back ofan irregular panel.