The Mechanisms of Fatigue of Viscoeleastic Multilayer Paint Systems on Wood

In 2010 an international roundtable discussion, entitled " The Plus or Minus Debate", was held between 600 conservators, scientists and collections care professionals to explore and re-evaluate the environmental guidelines, advances in environmental research and the implications for collections, archives and libraries. The impetus for this meeting was the realisation that efficient environmental control has become essential in the light of the future energy crisis, the worldwide economic downturn, and a rising awareness of green technology. For over four decades the environmental guidelines for museums and institutions have been defined within narrow parameters. Conditions for multi-layer painted wooden objects in particular are amongst the most tightly controlled. We have empirical evidence (warping and splitting wood, cracking and delamination of paint) that these objects are vulnerable to continual environmental changes mainly because of the hydroscopic response of wood. However, we have yet to establish a correlation between environmental changes, the variations in the original preparation layers and the resulting different crack patterns or delamination at particular interfaces. Nor do we have sufficient data to reliably use crack patterns as indicators of particular mechanical failures within the structure.
This project aims to highlight the mechanisms which lead to initiation and propagation of cracks as a result of environmental conditions in painted wooden cultural heritage, and how these eventually lead to delamination of the painted surface or underlying layers. This damage can lead to loss of the image or motif, resulting in changes to the aesthetic of the work, change in meaning and appreciation of the viewer. Compositional differences in the preparation and paint layers mean that the possible interfaces at which cracks can initiate are considerable. In the past it was assumed that if the environmental conditions do not cause deformation of the object beyond its ultimate tensile strain then no permanent damage will occur. However, fatigue is a possible long term problem where objects are continuously subjected to small environmental changes even within a limited range of temperature and relative humidity. It is therefore timely to undertake research to understand under what conditions environmentally induced fatigue could lead to delamination of painted surfaces in wooden objects.
The methodology will be established considering multiple paint systems on wood. These systems are also found on polychrome sculpture, painted musical instruments, ethnographic objects and contemporary art. This will be achieved by an interdisciplinary project which will include determining the history of cyclic strain based on moisture and thermal deformation and the induced failure in different layers. The temperature, moisture and strain rate dependent (viscoelastic) properties of the constituent materials of the objects make this research a particular challenge both for the modelling and experimental testing. Published data and data collected from specific collections of environmental fluctuations, plus measured deformations of panel paintings, will be used as parameters for experimental fatigue testing. This simulates real fluctuating conditions but at a higher frequency: to a first approximation, this is equivalent to the induced deformations caused over hundreds of years of environmental changes. These results will be used to validate the modelling. Finally, accurate predictions for the lifetime of the painted panels will be made and compared to the Bizot (a group of the world's leading museums) 2015 guidelines for environmental control to ascertain what effects they might have on the condition of these objects. The research will provide experimental and simulation data of fatigue lifetimes for panel paintings and related cultural heritage that can be used to inform strategies for environmental control and collections care.

In 2012, the Getty Foundation and NOW (Netherlands), reported on the state-of-art in scientific research and future direction needed within the field of panel paintings conservation. A clear imperative was the need for a greater understanding of the mechanical properties and mechanisms of deterioration for these complex composite objects. Although a small percentage of the world's cultural heritage, panel paintings are a significant part of the history and development of Western art from the 13th century to 18th century. Major public collections within the UK hold significant works by both Italian and Dutch artists, institutions such as the National Trust also possess works of international acclaim and works of importance to our own cultural heritage. These works need to be accessible through display, loans and scholarly interpretation. The collections also contribute to attracting tourists to the UK. These benefits have to be set against the imperative to reduce the carbon footprint and use of energy. Thus efficient sustainable collections care needs to be underpinned by good science.
The proposed research focuses on understanding the physical mechanisms which drive crack propagation and subsequent delamination in composite painted wooden objects, focusing on panel paintings. It will provide accurate experimental data on the properties of the constituent materials and the composite, as well as experimental and simulation data of fatigue lifetimes. These outcomes will inform preventive and access strategies; and can be used with risk management models. Risk management is a relatively new approach within collections, aiding them to make decisions on the best use of resources through strategic planning. The National Trust, National Gallery and Akzo Nobel (see letters of support) will directly make use of our results, maximising the impact of our work in the fields of cultural heritage as well as the paints industry.
By understanding the fatigue properties, crack propagation and modes of delamination, scientists and conservators will be able to develop better, sustainable conservation treatments. Importantly it will enable the future commercial development of innovative materials, in particular adhesives with improved fracture toughness and stability, for cultural heritage. This research has wide ranging applications within cultural heritage beyond panel paintings: it is relevant to historic interiors, ethnographic collections, musical instruments, sculpture, and contemporary art.
The experimental methodologies and viscoelastic fatigue crack models developed under this project will be applicable to polymeric coatings research; especially in the development of coatings and adhesives with enhanced mechanical properties for automotive, naval, aerospace and wood industries: with consequential benefit to the UK economy. It compliments current activities in Mechanics of Materials at Imperial whose aim is to provide new tools towards novel product development.
Dissemination and Knowledge Transfer
a. Joint papers in academic journals and conferences representing the physical sciences and humanities disciplines.
b. Podcasts and articles for the general public (e.g. New Scientist, SkyArts, Radio 4).
c. A workshop will be held at Imperial to demonstrate the use of the results in predicting fatigue lifetimes and how it can be applied to cultural heritage.
d. A database of the experimental data and numerical model codes will be set-up via the Courtauld and Imperial College web sites. Research Space will be used as a platform for the dissemination and utilisation of the results to the research community.
e. The case studies of panel paintings at Knole House (National Trust) will provide an opportunity to explain the science through their public engagement programs.
f. The steering group, detailed in the Case for Support, ensures that the results can be applied to real problems within cultural heritage and the coatings industry.