The Mechanisms of Fatigue of Viscoeleastic Multilayer Paint Systems on Wood

Lead Research Organisation: University of Glasgow
Department Name: School of Culture & Creative Arts


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.

Planned Impact

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.
Description We are beginning to understand the surface characteristics of our cultural heritage materials which lead to failure when subject to external loading.
1.We are able to measure the areas of high strain within arts works painted on wood and how these lead to damage.
2.We have developed a method of making reconstructions (with a thermal ageing protocol) of panel paintings which mimic many of the physical properties (increased elastic modulus and reduced ductility) of historic panel paintings. The results have been compared with data from the published literature, theoretical models and with 30-year-old samples of cold-pressed linseed oil lead white paint. The final methodology provides the research community with a viable way to produce samples that can be used to understand the behaviour of a (simplified) but complete multilayer. See publications.
3. We have 2D models which enable the time to crack initiation to be calculated. Crack growth rates as well as crack initiation times are output from the 3D models. The results show that the channeling crack initiates earlier and grows faster than the interfacial crack. This means the channeling crack is the dominant reason of paint fracture under environmental stress. As expected, both the results of 2D and 3D models show that a stable environment can help preserve the painting by delaying crack initiation.
system. The protocol and design notes for the models are now open access on project website.
4. We have developed a robust protocol for the fatigue testing of painted wooden objects subject to environmental fatigue.
Exploitation Route Recognition of vulnerable surfaces and development of suitable adhesives.

This methodology for the use and ageing of alkyd paints is also relevant to the study of fracture and delamination in modern works of art containing layers of alkyd, oil and acrylic media.

Use of methodology for the artificial ageing of oil paint and production of surrogate materials for cultural heritage to obtain data for damage criteria. This all feeds into Risk Assessment and Sustainability issues with cultural heritage institutions and policy makers.

Testing methodologies and data for Cultural Heritage Protection research in ways to mitigate global warming (specifically increase variation in environmental conditions)
Sectors Aerospace, Defence and Marine,Construction,Energy,Environment,Culture, Heritage, Museums and Collections

Description To explain to the public and conservation professionals the impact of the environment on the preservation of cultural heritage. Inform preventive conservation practice at The Hunterian Art Gallery We are sharing our expertise and knowledge of complex polymer mechanics with colleagues in the James Watt School of Engineering at Glasgow University. I have returned to Knole House (National Trust) twice to continue measurements of the panel paintings, as they have requested help because of potential instability and deformation of the paintings. Using the extrapolated data and the measurements I will be able to make recommendations for their safe display in the future. As yet this have been preliminary recommendations as the is a large amount of data still to process and interpret. This will continue over the next three months and be disseminated to the National Trust directly and through publications.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Education,Environment,Culture, Heritage, Museums and Collections
Impact Types Cultural,Societal

Description Collections Management
Geographic Reach Local/Municipal/Regional 
Policy Influence Type Influenced training of practitioners or researchers
Impact Finding simple solution to preventive conservation for the transportation of works of art.
Title Environmental Fatigue Subroutine 
Description The modelling subroutine outlined in this note is an implementation of the cohesive zone model for use in finite element analysis to simulate the initiation and propagation of cracks in a layered system. 
Type Of Material Computer model/algorithm 
Year Produced 2021 
Provided To Others? Yes  
Impact The model allows other researcher and conservation scientists with cultural heritage institutions to model the influence of environmental fatigue. It also provides a starting pint to further develop the model for specific scenarios. 
Description Knole House 
Organisation National Trust
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution A better understanding of how the environment is changing a Knole House. Our research has erased new questions about the original materials used to create the case study paintings we are studying.
Collaborator Contribution Contextual information into the nature of the collections at Knole House. A clear understanding of the criteria that need to met by The National Trust in terms of access and scholarly research.
Impact This is a multi-disciplinary collaboration between engineers, conservators, curators and conservation scientists. Our output to date has ben in form of public engagement while testing is going on in-situ and soon our webpages will go live.
Start Year 2017
Description The Hunterian 
Organisation University of Glasgow
Department Hunterian Museum and Art Gallery
Country United Kingdom 
Sector Academic/University 
PI Contribution Our research is providing further knowledge about the collections and preventive conservation issues associated with them. We are developing further technical analysis projects for the Hunterian collections. With the CapCo grant we have been able to provide analysis on coins, miniatures, paintings and manuscripts. I am helping to writing proposals for further funding for technical facilities and external funding to support a new research centre. The equipment funded under the present grant has further enhanced the facilities and support for the collections.
Collaborator Contribution The Hunterian have provided space for the research to take place so that we are now integrated with the collections. The curators have shared their knowledge of the collections. The collections management team have provided access and support with art handling, access and collections data management.
Impact The collaboration is building a strong multi-disciplinary working relationship between engineering, art history and professional museum practice. My RA has given a public engagement talk in the Hunterian Museum relating our research to objects in the museum. We have secured another grant (with a third collaborator Heriot Watt University) which being interdisciplinary in nature relies on the combination of access to the collections and appropriate scientific facilities in-situ. We have held combined seminars which engage the academic community with Glasgow University and the professional community of the Hunterian, external professionals working in the field of the conservation of paintings and visiting researchers from Yale University. In 2022 "Roman usurper or a hoax? Shedding light on Sponsian via non-destructive analysis of gold coins in the Hunterian Collection" accepted for ICOM-CC 2023 Whistler Pastels Project in collaboration with The Hunterian, Colby and the Launder Foundation. Technical Analysis in support of the conservation of Gavin Hamilton Hectors Farewell to Andromache as part of the Getty Conserving Canvas Training Initiative. Preventive Conservation advice for loan, storage and display of the Hunterian collections
Start Year 2017
Description The National Gallery 
Organisation National Trust
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution We have provided data and advice for the planned temporary storage of the paintings we are studying as part of this research. We have provided additional art historical knowledge of their collection and interpreted data from previous researchers.
Collaborator Contribution The National Trust have provided access to an important collection of paintings which are acting as the main cases studies for this research. They have also provided data of the environmental conditions at Knole House that are invaluable for one of the work packages in our project. Contextual information and discussions have also beheld that help us ensure that our research will have impact.
Impact Based on the initial collaboration we have developed content for our website.
Start Year 2017
Description Insight Talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Explanation of research and its relevance to the collections.
Year(s) Of Engagement Activity 2018
Description James Watt Anniversary 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact The activity which took place in the Hunterian Museum as part of Glasgow Science Fair. It was themed in the celebration of James Watt and the steam engine. But focus on the research of the project which is essentially the effects of moisture on panel paintings- It was called "Flaky Paint". It used Pink Panther wafers and a model steam engine with the steam produced by an ultrasonic mister. The general public could pick a wafer hold it over the funnel and see how all the layers delaminated. The didactic sheets, myself and two MUSE students from Glasgow University explained to the everyone what and why the layers separate and why it is important n the museums to control moisture.
Year(s) Of Engagement Activity 2019
Description Online Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Held on 10th December 2021, our online workshop brought together researchers and practitioners from academia and museum professionals to share the IMPASTOW project findings and to promote wider discussion of challenges and applications related to cultural heritage.
Year(s) Of Engagement Activity 2021
Description Science on the Sofa 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact A video and twitter session on the website of " Science of the Sofa _Glasgow Science Festival" of our testing in situ giving an insight into what we do and why we are doing the research.
Year(s) Of Engagement Activity 2020