Michał Łukomski

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.

Dynamic response of earlywood and latewood within annual growth ring structure of Scots pine subjected to changing relative humidity

Abstract Scots pine (Pinus sylvestris L.) was subjected to relative humidity (RH) changes, and the dynamic strain field on the surface and in the bulk wood was monitored by digital speckle pattern interferometry and X-ray computed microtomography assisted by digital volume correlation. If a freely shrinking specimen was subjected to an RH decrement, earlywood (EW) and latewood (LW) at the surface layer were deformed in the opposite directions at the beginning of drying due to moisture gradient across the specimen. As a result, the surface and core behaved as independent sub-components, with the surface restrained in its response by the dimensionally unchanged core. With time, both LW and EW shrank as moisture content (MC) became uniform across the specimen. When an entire wood specimen was restrained from movement and desiccated in ambient RH, EW was stretched to compensate for the considerable shrinkage of LW. Knowledge about surface deformation at the annual ring level as a function of varying RH may be helpful to assess the risks associated with the damage of paint layers caused by fluctuations of ambient RH.