From the beginning of civilization, wood has played an indispensable role in human survival. It is therefore not surprising that wood retains a prominent place in our cultural heritage. In the decorative arts wood has routinely been utilized because of its aesthetic virtues. In contrast, when wood is used for painting panels, where the surface appearance is obscured, the choice of wood reﬂects the universal availability of the resource as well as the working and performance properties of the timber. As an engineering material wood is strong and stiﬀ for its weight and has density and hardness in the range suitable for conversion with hand tools. Wood is chemically stable when dry, and its surfaces oﬀer a compatible substrate for paint application. The use of wood is not without its pitfalls, however, and requires the understanding that it is anisotropic— that is, it exhibits properties with diﬀerent values when measured in diﬀerent directions—as well as hygroscopic—adsorbing and releasing moisture readily. It is also dimensionally unstable and subject to deterioration by fungi and insects.
In painting panels, cupping is perhaps the most commonly encountered form of warp and can result from a variety of causes, singly or in combination. Uneven moisture change in opposite faces of a panel may cause a slight, and usually temporary, cupping concave to the drier face, which may disappear as moisture equalizes through the thickness of the panel. Growth-ring placement within a board is an important factor in the determination of cupping potential. Quarter-sawn (radially cut) boards tend to remain ﬂat as MC changes. Flat-sawn (tangentially cut) boards, however, routinely cup, or attempt to cup, as they season (Fig. 13). Cupping results from the diﬀerent components of tangential and radial grain orientation across opposite faces of the boards. Panels fashioned from ﬂat-sawn boards will tend to cup additionally as MC varies. If ﬂat-sawn boards or panels are held ﬂat and restrained from attempted cupping, cracking may result along the grain into the concave face. A less obvious source of cupping in painting panels is compression shrinkage. The causal mechanism is typiﬁed by a panel painted only on the face side, its unpainted back therefore exposed to much more rapid moisture sorption. In the case of such a panel originally coated with gesso and painted when the wood was at a fairly low MC, subsequent exposure to high humidity causes the wood at the back surface to adsorb moisture and go into compression set. If the panel were mounted by fastening at its edges, the expected cupping concave to the painted surface would be largely restrained. Upon restoration of a normally low humidity condition, the rear of the panel now manifests its compression shrinkage and shortens; the panel then cups concave to the unpainted surface. This mechanism is commonly the real source of cupping that has been attributed to tangential/radial shrinkage and “drying out.” Uneven compression shrinkage can overshadow the eﬀects of tangential/radial shrinkage and can also produce cupping in radially cut panels that would otherwise remain ﬂat under simple moisture cycling.
These systems relate to earlier work carried out in the conservation studios of the Vatican Museum. Maintaining the historic integrity of the object by returning original components to functionality is very satisfying, however it is not known whether the compression set curvature has reached stability, or if it is likely to progressively increase, putting increasing stress on the panel over time where the battens restrain it. This is an aspect of progressive compression set requiring investigation.
In Portugal, Garcia (2011) also utilises a cross-batten support of flat aluminium bars sliding over wooden ‘bridge’ blocks, held in place by thin aluminium brackets. Brewer (1999) discusses several different types of non-original cross-batten of differing flexibility, discussing the effect these have on the panel. The restraint that a crossbar should put upon a panel has always been difficult to judge or predict, excessive restraints block movement and facilitate split formation, whilst too little restraint can allow too high a level of deformation.
Unattached Supports Several unattached support systems are in use, generally for smaller thinned panels that can make use of the relatively small perimeter to surface area ratio. The National Gallery in London has made use of a foam cushioned tray for some years as a support mechanism for display and handling of fragile paintings (Brough 1984). The panel is held against a profiled foam rebate using a series of shaped foam buttons within an inert tray. This system is a good example of minimal intervention successful within a museum environment. However within an uncontrolled environment the restraint exercised by the foam pads as the panel deforms can be easily misjudged and block out of plane movement. The flexible unattached support developed at Ebury St Studio, London within a similar tray structure performs along similar lines to the flexible attached support and is likely to be more successful in uncontrolled environments (Bobak 1998), however the technique is far more involved and time-consuming. Work carried out at the Kunsthistorisches Museum, Vienna is devised along similar lines to the spring support mechanisms. Initially points of attachment were made with Velcro and Beva 371 film, but the technique was developed to allow unattached support pads. In this case both conical and leaf springs were used to support the panel within a tray structure, allowing in and out-of-plane movement, and some measurement of movement with integrated strain-gauges (Hopfner 2011). The idea of monitoring the panel movement within the support structure, if allowable within the display limitations is useful in tracking the reinforcement, although imaging techniques have now developed to such an extent that they might be a more straightforward full-field solution.