Most of the degradation of works of art in historic buildings is caused by unfavourable indoor climate conditions. The most important works of art receive invasive conservation treatment, called direct action, but this treatment is very expensive. To avoid invasive conservation treatments and ensure that works of art are protected for now and for the future, indirect action to mitigate the deterioration process is strongly advised. This holds that exposure to unfavourable indoor climate conditions should be avoided, as far as is compatible with its social use.
To improve the indoor climate of the historic building, adaptations to the building or its systems are often necessary. Building performance simulation may be used in predicting the effect of a retrofitting strategy on the indoor climate, and in designing the adaptations to the building. In a previous project (FWO G.0420.05), a coupled 3D-HAM-CFD-model has been developed which predicts the local temperature and relative humidity variations of the indoor air as well as the hygrothermal interaction with hygroscopic materials like sculpture, panel paintings, books,…. Although these results are useful to evaluate the risk of moisture related damage, evaluating the indoor climate of an entire building with CFD is very time-consuming because of the computational cost, certainly if a long time period has to be analysed. Furthermore, besides evaluating preservation conditions, also other criteria, like annual energy use and comfort criteria are of interest. In this case Building Energy Simulation (BES) methods are more appropriate as they are able to simulate the indoor climate dynamics of the whole building for a long time span relatively fast. This allows to simulate, analyse and compare multiple retrofitting strategies and define the most ‘optimal’ solution.
To assess the preservation conditions properly, it is necessary to take typical conditions in monumental historical buildings into account in the simulation study. In this work, the main focus is placed on the presence of moisture in heavy building walls and the occurrence of hygrothermal gradients (stratification) in the often very large interior volumes due to the limited control by (older) climate installation system. This PhD-dissertation, part of FWO-project G.0448.10, aims at developing a simulation strategy to estimate the predicted risk for works of art, taking into account these typical boundary conditions of an historical building. The emphasis lies on developing a fast calculating modelling approach intended for practical use.

Preventive conservation of works of art in historic buildings : assessment and modelling

Moisture is a driving factor in the long-term mechanical deterioration of canvas paintings, as well as for a number of physico–chemical degradation processes. Since the 1990s a number of publications have addressed the equilibrium hygroscopic uptake and the hygro-mechanical deformation of linen canvas, oil paint, animal glue, and ground paint. In order to visualise and quantify the dynamic behaviour of these materials combined in a painting mock-up or reconstruction, we have performed custom-designed experiments with neutron radiography and nuclear magnetic resonance (NMR) imaging. This paper reports how both techniques were used to obtain spatially and temporally resolved information on moisture content, during alternate exposure to high and low relative humidity, or in contact with liquids of varying water activities. We observed how the canvas, which is the dominant component in terms of volumetric moisture uptake, absorbs and dries rapidly, and, due to its low vapour resistance, allows for vapour transfer towards the ground layer. Moisture desorption was generally found to be faster than absorption. The presence of sizing glue leads to a local increase of moisture content. It was observed that lining a painting with an extra canvas results in a damping effect: i.e. absorption and drying are significantly slowed down. The results obtained by NMR are complementary to neutron radiography in that they allow accurate monitoring of water ingress in contact with a liquid reservoir. Quantitative results are in good agreement with adsorption isotherms. The findings can be used for risk analysis of paintings exposed to changing micro-climates or subjected to conservation treatments using water. Future studies addressing moisture-driven deformation of paintings can make use of the proposed experimental techniques.

/pdf/distribution-of-moisture-in-reconstructed-oil-paintings-on-hj5shxxi93.pdf

Distribution of moisture in reconstructed oil paintings on canvas during absorption and drying: A neutron radiography and NMR study

This thesis investigates mainly the use of nanocellulose-based treatment for the consolidation of degraded cotton canvases of modern paintings and includes within this some case studies on linen canvases (sized and unsized) and 19th cent. historical samples from paintings. It. uses a multi-scale analytical approach where primarily controlled relative humidity dynamic mechanical analysis (DMA-RH) was used to evaluate the effect of the novel nanocellulose based preparations. It aims at quantifying the advantages, disadvantages, and limitations of their application. Initially, the baseline viscoelastic response to RH variations of a degraded cotton canvas was measured by DMA-RH. This technique was used further together with SEM to assess morphologically and mechanically 6 traditional consolidants including natural such as animal glue and synthetic materials. Following the same protocol, two solutions of nanocellulose-based consolidants developed in the frame of the Nanorestart project were assessed. These materials consisted of nanocellulose dispersions in water or water/ethanol and nanocomposites of nanocellulose-reinforced cellulose derivatives in polar/apolar solvents. Overall, higher consolidation at lower weight added was measured for the nanocellulose-based treatments tested when compared to the traditional consolidants. The penetration of the consolidant in the canvas also shows to greatly differ between treatments with the nanocellulose showing low penetration. Higher mechanical response to RH was also measured after treatment in particular with the water-based treatment. The results demonstrate how the adhesion, measured here at the nanoscale, and consolidant penetration into the canvas are dominant factors for the development of consolidation treatment for painting canvases. The assessment of the novel consolidants was finally carried out on historical canvases. Most treatments show to perform well on historical paintings in terms of handling properties, penetration and surface appearance and consolidation. Preliminary time-resolved neutron radiography with new purpose built sample chamber and RH controller provided visual information on time-dependent moisture response of the samples.

Multiscale approach in the assessment of nanocellulose-based materials as consolidants for painting canvases

https://www.dora.lib4ri.ch/psi/islandora/object/psi%3A21187/datastream/PDF/Mannes-2017-Design_and_applications_of_a-%28published_version%29.pdf

Design and Applications of a Climatic Chamber for in-situ Neutron Imaging Experiments ☆

The preservation of early Netherlandish panel paintings gets a lot of attention as they are an important part of the cultural heritage of the lower countries. Indoor humidity fluctuations are the main cause for mechanical damage of these artworks. To estimate the risk of mechanical damage, the intention of the authors is to model the moisture transport in the artwork related to a simulated indoor climate. Because the hygrothermal response of the panel is controlled by the vapour permeability of the wood and the barrier effect of different material layers on top and on the back of the wooden panel, hygric material characteristics of all layers are needed to perform reliable simulations of the local moisture content gradients in the object. In the literature, several values were presented which were used to model the mechanical behaviour of the panel painting. The driving potential in these studies is the moisture content of wood. However, for the coupling of a HAM tool, which predicts the moisture transport in the object, with other simulation tools (BES, CFD, etc.), it is more straightforward using one unique and material-independent driving potential in both tools, such as vapour pressure. The conversion of characteristics presented in the literature using vapour pressure instead of moisture content is not straightforward as information about converting the characteristics is missing. Another difficulty is that the obtained quantitative results in the literature are valid for a specific application method. Consequently, it is disputable whether these characteristics may be applied to early Netherlandish panel paintings. Therefore, the main goal of this study is to measure the moisture properties of early Netherlandish panel paintings. The desired characteristics were determined using inverse modelling of a 1D moisture transport and finding the best fit between the calculated diffusion process and the measurements.

Evaluation of the diffusion coefficient and sorption isotherm of the different layers of early Netherlandish wooden panel paintings

Easel paintings on canvas are subjected to alteration mechanisms triggered or accelerated by moisture. For the study of the spatial distribution and kinetics of such interactions, a moisture exposure chamber was designed and built to perform neutron radiography experiments. Multilayered sized and primed canvas samples were prepared for time-resolved experiments in the ICON cold neutron beamline. The first results show that the set-up gives a good contrast and sufficient resolution to visualise the water uptake in the layers of canvas, size and priming. The results allow, for the first time, real-time visualisation of the interaction of water vapour with such layered systems. This offers important new opportunities for relevant, spatially and time-resolved material behaviour studies and opens the way towards numerical modelling of the process. These first results show that cellulose fibres and glue sizing have a much stronger water uptake than the chalk–glue ground. Additionally, it shows that the uptake rate is not uniform throughout the thickness of the sized canvas. With prolonged moisture exposure, a higher amount of water is accumulating at the lower edge of the canvas weave suggesting a decrease in permeability in the sized canvas with increased water content.

I. Cerjak

Papers

Neutron radiography for the study of water uptake in painting canvases and preparation layers