The Structural Conservation of Panel Paintings

out of the solid wood. Many panels have a propeller-like twist in addition

to any curvature. Such twisting must be carefully considered when estab-

lishing datums for construction within the tray in order to balance the

diagonal distortion evenly.

All visible edges of the tray can be toned, gilded, and distressed

to match the existing frame. Frequently the sight size of the frame is too

large, and consequently part of the tray’s edge can be made to project

beyond the rabbet and become visible as an inner slip.

The backboard is made of marine-quality plywood,^14 which may

be obtained with good-quality veneers. Other stable sheet materials may

be used, the choice being made on rigidity, thickness, weight, and appear-

ance. The backboard must be strong enough to withstand the loads

imposed by the support with little deflection.

The panel is placed in the tray and retained by the spring sup-

ports and backboard. The backboard is then screwed into the tray edge

section. Finally, the completed assembly is fitted into the frame with brass

strips and screws.

There are disadvantages to the flexible unattached auxiliary support sys-

tem: it is only suitable for a limited range of panels; the assessment of

forces and panel strength is largely empiric; some panels and frames will

not accept a deep tray without the result appearing ungainly; and it is not

possible to see the back of the panel without the removal of the back-

board and support. Despite these limitations, there are several advantages

to the system. For example, there is a minimum of interference with the

original panel; concave and convex movement of the panel is possible

without overstressing; known forces are applied to the panel; the panel’s

movement within the tray indicates changes in RH and alerts conservators

to inadequate RH control; RH changes are buffered by the tray and back-

board; and physical protection ofthe panel is provided—an especially

important consideration when the panel is out of the frame.

1 I 5 bd^34 12, where: I 5 moment of inertia; b 5 breadth (width); and d 5 depth (thickness).

With a constant thickness,d,ifthe width, b,is halved, the deflection for the same load will

double. Ifdis halved, the deflection will increase by eight times with the same load (see

Marchant, “Development of a Flexible Attached Auxiliary Support,” herein).

2 Teflon PTFE (polytetrafluoroethylene) is a tape with very good properties for reducing friction.

3 To humidify a panel, the RH in the room may be increased to 65–70% for several days to

reduce the stress within the panel if the convex curvature is expected to be large.

Notes

Assessment of the

Unattached Flexible

Auxiliary Support System

380 Bobak

Figure 14
The corner of a panel tray showing saw kerfs
in which hardwood veneers have been
inserted, to increase rigidity and the strength
of the joint.