6th Indoor Air Quality 2004 Meeting (IAQ2004) Padova, Italy, November 2004 Impact of daily and seasonal Temperature and Relative Humidity cycles on wooden artworks Dario Camuffo, Emanuela Pagan National Research Council (CNR) Institute of Atmospheric Sciences and Climate Padova, Italy

EMC in wood vs. temperature & RH Wood is strongly dependent on RH, weakly on T RH changes are dangerous, but T may change RH Two regions are visible: 0<RH<80% & 80<RH<100% Equilibrium Moisture Content (%) Relative Humidity (%)

In conservation, the specific problems of each individual object are much more relevant than knowing average properties of materials and establishing hypothetical well- being areas. Dimensional Change (%) Relative Humidity (%) A relevant class of hygroscopic materials, e.g. wood, parchment, ivory, has a Moisture Content that is in equilibrium with RH. Changes in RH affect MC and generate dangerous shrinkage/swelling.

Cycles in T and RH are responsible for dimensional changes and internal tension to the wooden coffered ceiling. In the long-run, the tension may have a cumulative effect, or even in some occasions it may exceed the threshold after which some structural part break. This is not only a consequence of the width of the cycles, but also of the synergism between cycles, if these are repeated before wood has relaxed. A fundamental role is played by the frequency of the largest cycles, supposing that the rare extremes which had occurred in the past have not yet concluded their action to adapt the material (i.e. to break it) to respond to their intensity or repetition.

Effect of ‘cold’ lamps and hot spotlights on the historical coffered ceiling of the Giant Hall, Padova Radiometric measurement of the ceiling before a concert, showing the effect of cold lamps only After the concert: the contribution of hot spotlights is evident

Trend of Equilibrium Moisture Content (EMC) of the coffered ceiling in the cold season Padova, Giants Hall. In the cold season, the EMC changed from 13 to 4%, with some fluctuations. The seasonal trend had superimposed some minor cycles with quasi-monthly average period, and average amplitude around 4%. The coffered ceiling is composed of square oak panels, 2m side, thickness 1-2 cm. 9% change in EMC implies a dimensional change by 3%, i.e. 6 cm in the direction tangential to the tree rings.

Instantaneous Tangential Deformation (TD) representative of the surface layer of the coffered ceiling (pine) and 7- and 14-day running average representative of two deeper layers Padova, Giants Hall ott01-nov01-dic31-dic30-gen01-mar31-mar Deformazione Tang. del legno (%) Deformazione Tangenziale dal valore medio Media Mobile su 7 giorni Media Mobile su 14 giorni Tangential Deformation (%) dimensional change by 3% in tangential direction 3% Date

Surface Stress = Surface Tangential Deformation (TD) – Inner layer (7- and 14-day running average) Padova, Giants Hall Surface Stress (%)

Scatter diagram Values falling outside this interval might be considered in a risky area. The practical “safety area” was based on laboratory tests on material samples, and was represented with ‘well being’ rectangles as in this example. However, this approach does not consider that wooden artworks may adapt to the ambient variability with cracks. Every new crack constitutes an adaptation to a wider environmental variability. Cumulated damage displaces thresholds for unsustainable variability. Scatter diagrams represent the history of the past T and RH cycles that have interacted with the artwork. The interval of safe variability in T and RH lies between 0 and a critical threshold in T and RH. safety area  T(°C/day)  RH(%/day)

Yearly daily cycles Padova, Giants Hall, 1 m Most of the points lie outside the safety area and this explains why so many cracks were found in the coffered ceiling. The interval 6% for RH and 1.5°C for T determined from laboratory tests can doubled after field survey. In fact, these values are close to the modes of the observed data and have been experienced many times. New cracks have damaged the ceiling, widening the area of environmental variability  T(°C/day)  RH(%/day)

The histograms show the frequency of distribution of the daily variability either in T, or in RH, grouped by classes of intervals. The mode M represents the most frequent cycle to which artefacts have adapted (with cracks). The upper tails UT (i.e. right side) in plots are constituted by rare and risky departures from the typical values. These have not yet concluded their potential impact, and should be carefully avoided. Yearly frequency of T and RH daily cycles Padova, Giants Hall T RH M M1M1 UT M2M2

Uffizi Gallery,Florence Heating with exceeding moisture compensation The best situation is found during the closure days when the HVAC is off Heating without moisture compensation

Yearly frequency of T and RH daily cycles (1m height) Uffizi Gallery T RH M UT M  T(°C/day)  RH(%/day)

Yearly frequency of T and RH daily cycles at 4.5m height near the altar The histograms show the frequency of distribution of the daily variability either in T, or in RH, grouped by classes of intervals. Daily cycles at the mode M are typical and the material has adapted, possibly with cracks. Cycles from M to 2M may be still sustainable, but they fall in the attention area because they are not so frequent. They might still deepen existing cracks not yet concluded and fatigue may be accumulated for new ones. T RH M1M1 M1M1 M2M2 M2M2

In the case that past cycles were not sustainable, they caused some cracks in the critical constraints to adapt the artwork to the environmental T and RH cycles. Cracks created new degrees of freedom to respond to the environmental variability. The sustainable T and RH span is widened at the expenses of a worsen and worsen damage to artworks. Conclusion 1

Daily cycles at the mode M are typical and the material has adapted, possibly with cracks. Cycles from M to 2M may be still sustainable, but they fall in the attention area because they are not so frequent. They might still deepen existing cracks not yet concluded and fatigue may be accumulated for new ones. Conclusion 2 Daily cycles greater than two times the mode are quite exceptional and may be responsible for the largest cracks that are visible, or may provoke new ones. M 2M