B. Rachwał’s research while affiliated with Polish Academy of Sciences and other places

Numerical modelling was used to follow the moisture movement and strain in a composite system – an unrestrained, single wood panel coated with a layer of gesso, in response to cyclic sinusoidal variations in relative humidity (RH). The allowable magnitude of the variations, below which physical damage of the gesso layer on the wood does not occur over a selected time of exposure, was derived as a function of cycle duration, panel thickness and moisture diffusion configuration. The dimensional response of wood substrate becomes subject to restraint by the applied layer of gesso. The panels do not respond significantly to diurnal fluctuations or shorter irrespectively of the panel thickness. The panels respond more and more significantly when the duration of the fluctuations increases until a certain critical period at which the panel fully responds to each cycle. The analysis of the data obtained indicates that moderate RH variations within the approximate range 50 ± 15% are safe. This safe range was derived using the extremes of conservative criteria of the gesso’s fatigue fracture and assumption of worst‐case wooden substrate response. The reduction of allowable amplitude of RH cycles because of decrease in the gesso’s modulus of elasticity and thickness is discussed.

: The finite element method was used to model the moisture movement and strain in the wood supports of panel paintings, in response to changing climate conditions – temperature and relative humidity (RH). The material properties of lime wood (Tilia sp.), determined experimentally, were used in the modelling. Critical amplitudes of cyclic sinusoidal RH fluctuations generating strain of 0.002 in the most responsive tangential direction of the unrestrained, single wood panel, which the pictorial layer was assumed to endure without damage, were derived for the mid-RH region as a function of cycle duration, panel thickness and diffusion configuration. Panels do not respond significantly to diurnal fluctuations or shorter. The panels respond more and more significantly when the duration of the fluctuations increases until the panel fully responds to each cycle. These fluctuation periods are 14 and 90 days at 20 °C for a panel thickness of 10 and 40 mm, respectively, with two faces of a panel diffusively opened. Sinusoidal RH variations bringing about wood’s full response have the critical amplitude of ±6% RH, that is strain of 0.002 endangering the pictorial layer is produced at such amplitude in the tangential direction of the unrestrained panel.

Painted wood is a multi-layer structure composed of materials which swell or shrink differently in response to the sorption and desorption of moisture. The allowable levels of strain of the gesso layer were determined experimentally by subjecting specimens imitating historic panel paintings to mechanical stretching, and monitoring the development of cracks. These strain values were translated into the magnitudes of cyclic RH fluctuations allowable for unrestrained panel paintings, which depend on cycle duration, panel thickness and the configuration of moisture exchange by a panel with the environment. Panel paintings do not respond significantly to diurnal or shorter fluctuations irrespective of the panel thickness. The panels respond more and more significantly when the duration of the fluctuations increases until the panel fully responds to each cycle. The approach to calculate the accumulated fatigue damage from real-world climatic variations being combinations of climatic cycles of various duration and amplitudes was demonstrated

General' moisture sorption and swelling/shrinkage patterns which would apply as a first approximation to any wood species constituting cultural objects were proposed. They were obtained from the experimental data measured for 21 historically important wood species used in the past for panel paintings and woodcarving. Information on further wood species of interest to the wood conservation community can be added to the database and used to constantly improve the general relationships.