The influence of dovetailed cross beams on the dimensional stability of a panel painting from the Middle Ages

Abstract

This paper describes experiments carried out to determine the influence of dovetailed cross beams on the dimensional stability of a panel painting from the Middle Ages. Besides a serious cup (transverse distortion), the panel featured an unusual longitudinal distortion, a bow, mostly originating from bending in the proximity of the middle cross beam. Because of its localized occurrence, bending appeared to be caused by the crossbeam somehow pushing on the walls of its channel. Thanks to the cooperation offered by the curator and the restorer, some non-damaging tests were performed to measure the distortions of the panel, with and without the cross beams, after conditioning it in different climates. Processing and analysis of the measurement results demonstrated that the bow of the panel increases as the wood moisture content decreases. This led to the conclusion that the bow is actually produced by forces exerted by the cross beam along the panel's length, according to a mechanism named thrust transformation. By this mechanism the cross beam partly transforms the panel's tendency towards cupping, into a bow distortion.

Keywords:

• Bowing
• Cupping
• Deformation
• Dimensional stability
• Dovetailed cross beam
• Panel painting

Acknowledgements

The authors thank Patrizia Bertini, Giovanni Cabras, Renato Castorini, and Barbara Schleiker, restorers at Palazzo Pitti Restoration Laboratory and the curators Ornella Casazza and Alessandro Bagnoli.

Notes

1 The measurements and subsequent data processing have been performed for only two RH values (54 and 78%), typically providing EMC of 10 and 17% for the wood of the panel; although no evidence is available for other moisture contents of the panel, neither in equilibrium nor in transient conditions, it appears reasonable to assume that in the range between the two tested values the panel would show an intermediate, approximately linear behaviour. Therefore, for the sake of simplicity, relationships between RH, cup, and bow have been analysed and plotted as linear correlations; however, such simplification is in no way indispensable for the explanation of the studied phenomena; this explanation would hold even if the relationships were not linear.

2 Anisotropy is the difference of the shrinkage coefficients existing between tangential and radial direction of wood, producing a cup distortion in a flat sawn board when the MC changes.

3 Such asymmetry produces a transient cupping with a typical trend. It tends to disappear when a new steady-state equilibrium is reached.