Łukasz Bratasz

Numerical modelling of moisture movement and related stress field in lime wood subjected to changing climate conditions

Numerical modelling was used to follow the evolution of the moisture content gradient and the stress field resulting from the restrained differential dimensional response across a wooden cylinder, simulating sculptures, in response to variations in temperature (T) and relative humidity (RH). Material properties of lime wood (Tilia sp.) were used in the modelling as this wood species was historically widely used. The allowable RH variations, below which mechanical damage will not occur, were derived as functions of the amplitude, time period and starting RH level of the variation. Lime wood can endure step RH variations of up to 15% in the moderate RH region, but the allowable domain narrows when RH levels shift from the middle range. The allowable amplitude of the variations increases when time allowed for the change increases. The stress field does not vanish even for slow, quasi-static changes in RH due to structural internal restraint resulting from the anisotropy in the moisture-related dimensional change.

Impact of Indoor Heating on Painted Wood - Monitoring the Altarpiece in the Church of Santa Maria Maddalena in Rocca Pietore, Italy

Abstract Triangulation laser displacement sensors were applied to the continuous in-situ monitoring of the dimensional response of the wooden altarpiece in the Church of Santa Maria Maddalena in Rocca Pietore, Italy, to variations in indoor temperature and relative humidity (RH) between December 2002 and March 2005. Further, a small inductive sensor was used to monitor the width of a crack in one of the elements. The measurements demonstrated that only the external layer of wood, several millimetres thick, continually absorbs and releases water vapour following external variations in RH. For massive elements, this leads to gradients in the moisture content through the wood, a restraint of the dimensional change and a development of stress, which is the main threat to the integrity of the wood and the decorative layer. Particularly strong RH variations and related high stress levels were produced by the intermittent heating system based on the inflow of warm air. To incorporate requirements for preservation, heating systems must provide a localized comfortable temperature in the area occupied by people without changing the natural climate of the church as a whole.

Allowable microclimatic variations for painted wood

Abstract Environmental standards for cultural heritage collections have been much debated in recent years. The interest in the issue has been driven by the growing movement towards green museums, that is, managing indoor museum environments in a responsible and efficient manner, especially in terms of reducing energy consumption and carbon emissions but at the same time maintaining high standards of collection care. Painted wood is among the category of heritage objects most vulnerable to relative humidity and temperature fluctuations. Therefore, scientific understanding of how changes in environmental conditions ultimately affect painted wood is crucial to the development of rational guidelines for the control of climate in museums and historic buildings. This review provides a systematic progression through two fundamental approaches to establish the allowable ranges of climatic variations – an analysis of the mechanical response of painted wood as a complex, multilayer system to climate variations, and an analysis of the historic climate to which the objects have acclimatized. The climate specifications and standards based on both these approaches are reviewed.

Laser Sensors for Continuous In-Situ Monitoring of the Dimensional Response of Wooden Objects

Abstract The application of triangulation laser displacement sensors to the continuous in-situ monitoring of the response of wooden cultural objects to variations in temperature and relative humidity in their environment is reported. The sensors are robust, fast and precise, provide non-contact measurements, and are capable of operating in the field. They have been applied to monitor the response of the altarpiece in the church of Santa Maria Maddalena in Rocca Pietore, Italy, to fluctuations in temperature and relative humidity caused by the use of the heating system. Complex short-, medium- and long-term responses of a variety of carved wooden elements have been recorded.

Acoustic emission for tracing fracture intensity in lime wood due to climatic variations

Acoustic emission (AE) monitoring was used to trace directly the fracture intensity in cylinders of lime wood subjected to variations in temperature and relative humidity (RH) in their environment. High-frequency components produced by mechanical fracturing were extracted from the raw AE signals using the wavelet transforms. The accumulated energy of these components depended on the magnitude and rate of the RH variations. The AE activity correlated well with predictions of the numerical modeling carried out as the first part of the present investigations. In particular, the AE activity became negligible below the allowable magnitude for the rapid RH variation predicted by the simulation, or when the time interval allowed for the RH variation was long enough. Furthermore, AE proved capable of tracing the progressive evolution of damage at the microlevel, which preceded failure of wood discernible from the macroscopic perspective.

Acoustic emission monitoring of an eighteenth-century wardrobe to support a strategy for indoor climate management

Abstract The on-site monitoring of acoustic emission (AE) has allowed the direct tracing of climate-induced crack propagation in an eighteenth-century wardrobe displayed in the Gallery of Decorative Art in the National Museum in Krakow, Poland. The anti-correlation measuring scheme and frequency filtering allowed very low levels of physical damage to the wardrobe to be detected in spite of the high background noise typical of the museum environment. The total AE energy recorded during two years of monitoring corresponded to a fractured area of 12 mm2 or a total crack propagation of 1.2 mm for two10-mm-thick panels. Although the total damage recorded was minute, correlation between the events of fracturing and falls in indoor relative humidity (RH) in winter due to insufficient humidification was evident. The risk of damage, expressed in terms of crack propagation, was quantified as a function of the magnitude of the RH falls of the duration compatible with the response time of the object. The data allow acceptable RH falls to be identified if a conservation professional or a curator selects a ‘tolerable’ yearly propagation of the fracture, in other words the progress of damage considered insignificant.

Vibration as a hazard during the transportation of canvas paintings

Abstract The assessment of vibration levels that cause damage to canvas paintings travelling on loan has been hampered by the lack of direct measurements of the dimensional change of the canvas. The application of triangulation laser displacement sensors in this study has allowed direct, continuous in situ monitoring of the vibration of canvas during packing, handling and transporting by road of three nineteenth-century oil paintings from the collection of the National Museum in Cracow, Poland. The measurements showed various levels of vibration primarily influenced by the packing method: paintings wrapped in tissue and transported in soft cardboard boxes were better cushioned from the vibration than a painting rigidly fixed in a wooden case. However, increased levels of vibration were always measured during packing the paintings and transferring the cases or boxes to and from the road vehicle, which emphasizes the need to train staff to handle the paintings with care when they are still in the museum. The strains induced by the vibration levels recorded were much lower than the critical levels of tolerable strain for the nineteenth-century canvas paintings quoted in the conservation literature. Therefore the paintings are unlikely to be endangered by fracture or fatigue damage under the transportation conditions monitored

Mechanism of craquelure pattern formation on panel paintings

The drying shrinkage accumulation from exposure of freshly prepared gesso layers to relative humidity (RH) cycles was determined to elucidate the mechanism of craquelure pattern formation on panel paintings. The progresive drying shrinkage of the gesso is observed only under the cycles going to high RH levels which bring about transitions from brittle to ductile state of the material. The first incidence of fracture on the gesso layers occurred after a limited number of cycles ranging between a few and 100 for a range of layer thickness between 0.5 and 1 mm. The craquelure patterns stabilised also after a limited number of cycles (30 for the 1-mm thick layer). Upon increase in the gesso layer thickness, the strength of the layer is reduced and the spacing of shrinkage fractures increases. The study demonstrated that craquelure patterns, mimicking historical ones, can be realistically produced in laboratory conditions. Such studies would provide useful information for preparing specimens simulating historic panel paintings and would inform the current efforts on automatic, computer-aided classifications of crack formations on paintings.

Digital radiography (DR) and imaging analysis for evaluating the penetration and distribution of organic substances used in wood conservation

Consolidation treatments are commonly employed in museum conservation studios for treating biologically deteriorated wooden cultural heritage objects. Impregnation using either a solvent/resin or a polymerizing system is an example of such an intervention, which is often difficult to describe in terms of its behavior within the object’s structure. A new simple method has been devised to evaluate the effectiveness of these impregnation treatments in terms of spatial distribution of consolidant solution. A combination of digital radiography and imaging analysis has permitted to evaluate the degree of penetration of a consolidant and to determine its location within the artifact by studying the X-ray images taken before, during, and after treatment. The adequacy of polymer solutions or their effectiveness as wood consolidants is properties which are often difficult to investigate due to limited visual access to the interior of the object. The proposed method provides an alternative way of studying these parameters through analysis of X-ray attenuation recorded on two images: taken before and after the conservation treatment. The resulting image is then converted into a two-dimensional map of consolidation agent within the object using a straightforward calibration procedure.