20th- and 21st-century oil paintings present a range of challenging problems that are distinctly different from those noted in paintings from previous centuries, most perplexing of which is the de­velopment of water, solvent and mechanical sensitivity which can render them untreatable. These are often, but not exclusively, associated with unvarnished, unprotected paint surfaces.

The CMOP project aimed to further our understanding of the causes of sensitivity in oil-based works of art, enhance low risk options for conservation treatment and inform appropriate preservation strategies for the care of works and collections. Specific project objectives included in-depth investigations into causes and mech­anisms behind the formation of paint sensitivity, the evaluation and optimisation of soiling removal sys­tems, the development of methodologies for cleaning of sensitive modern and contemporary oil paint­ings, and the application of these methodologies to conservation treatment of selected case study works of art.

The project was designed around six work packages: Management and engagement (WP1) and dissemi­nation and knowledge transfer (WP6), and the main tasks centred around: inventory, selection and preparation of model paint samples (WP2); chemical and physical analysis of degra­dation phenomena of modern oil paints and paintings (WP3); the development, testing and evalu­ation of cleaning systems (WP4) and case study conservation treatments (WP5).

Project partners were University of Amsterdam (Project Leader), the Cultural Heritage Agency of the Netherlands, Tate, The Courtauld Institute of Art, University of Pisa. Associate Partners inclu­ded the Getty Conservation Institute, Gemeentemuseum Den Haag, Stedelijk Museum Amster­dam, Opificio delle Pietre Dure and the Department of The Royal Danish Aca­demy of Fine Arts, Schools of Architecture, Design and Conservation/Centre for Art Technolog­ical Studies and Conservation. Pro­gress was evaluated through regular consortium meetings, conference calls, staff exchange, reporting, train­ing and a range of dissemination activities, which were largely reported to the public via the CMOP website.

With­in WP2, several inventories were produced including: the degra­­dation pheno­me­na asso­cia­ted with modern oil paints and paintings, archival paint samples and swatches from Talens and Winsor & Newton paint manufacturers, and the range of model samples created within the project. Many of these samples were used for analytical studies in WP3 and cleaning trials in WP4.

WP3 focussed on elucidating the chemistry and physical charact­eristics of sensitive oil paints and works of art via analytical sur­veys of archival samples and over 50 works of art held in consortium col­lect­ions. These studies were augmented by the preparation and investigation of model samples where curing beha­vi­our and the development of solvent sensitivity was explored with respect to the presence of different oils (e.g. linseed and safflower), artists’ modifications, addi­tives (e.g. stearates) and various pigments, in addition to the role played by environ­mental conditions.

WP4 centred around the sys­tematic modification and comparative evaluation of novel and established soiling remo­val systems applied to sensitive model oil paint samples and donated works of art. Meth­ods trialled included various gels, micro­emul­sions, pickering emulsifiers, solvents and tailored waters, in addition to exploring the effects of appli­cation methods, clearance strategies, solvent retention within paint films and cleaning material residues. Several of these were also trialled and evaluated on case study paintings as part of WP5 including one work by Asger Jorn from the Gemeentemuseum in Den Haag and two works from the Stedelijk Museum, by well-known international artists Robert Ryman and Karel Appel.

The interdependencies between each work package facilitated a targeted, inter-related scheme of work, which has delivered results that are highly contextualised and directly accessible, and which together form a substantial resource for further research. The direct link between paint formulation, ambient environment and the formation of water-soluble epsomite in Winsor & Newton oil paints has been explored across a significant body of works of art in international collections. Combined scientific studies on naturally and accelerated aged paint films has confirmed that paint constituents – e.g. oil type, pigments, additives, and artists interventions – can be key to the drying, curing and degradation behaviour of oil paints in addition to ambient environment. Compromised curing can result in the phase separation of polar (largely acidic and hydroxyl-lipidic) molecules at paint surfaces, and the development of a compromised polymer/ ionomeric network, which can result in mild to severe paint sensitivity.

By investigating the degradation products of oil paints, specific degradation pathways –mainly related to oxidation reactions – have been shown to dominate in the presence of different, often recurring, pigments, which can directly affect the paint’s response to solvents, mechanical action and the ambient environment. In contrast, paints made from pure linseed oil and pigments such as lead white, zinc white and titanium white remain largely unaffected by solvents due to the influence of these pigments on the formation of a well-developed polymer network. To understand these molecular changes, new analytical procedures were developed to investigate the molecular composition of these materials with high accuracy and sensitivity, and new analytical methodologies have been employed to investigate the physical properties and behaviour of model paints.

This research has led to the proposal of a model for the interpretation of the development of water sensitivity in modern oil paintings. This has directly informed both the interpretation of painting surfaces and the development of approaches to soiling removal treatment, which included key modifications to aqueous conditions, micro-emulsions, silicone emulsifiers and application methods, all of which lower the inherent risks associated with soiling removal treatments. Collectively, the research outcomes offer conservators valuable guidance, enhanced tools and methodologies that are more appropriate for use on these paints. In addition, the extended treatment of three case study works of art by well-known international artists have served as useful examples of this newly developed approach for conservators to take forward into studio practice.

The knowledge generated by CMOP is significant to heritage research and practice and will have an international impact in the following areas: new interpretation of development of conservation issues in oil-based works of art, tailored approaches to conservation treatment, new scientific methodologies and research streams, and the creation of continuous professional development and student training courses and workshops for conservators.

Over the course of the project, the collaboration has become strong and productive, and these key professional relationships will continue to build. Thus far, project outputs have reached international audiences across several fora including the CMOP webpages and a 4-minute documentary film produced for a general, non-specialist audience that outlined the key challenges and aims of the CMOP project. A total of 13 peer reviewed articles and 8 other texts were published, during the project with many more papers published post-project. CMOP project partners contributed 35% of the total content for the Confe­rence on Modern Oil Paints event held at the Rijksmuseum, Amsterdam, 23-25 May 2018, which attracted 326 delegates from 31 countries, from private conservation studios and institutions, museums and galleries, and research institutions. This highly successful conference proceedings were published as a volume through Springer, entitled Conservation of Modern Oil Paintings, in 2019 (see Publications).

Klaas Jan van den Berg, Lise Steyn, Digna van der Woude (University of Amsterdam); Ilaria Bonaduce (University of Pisa); Bronwyn Ormsby, Luigi Galimberti (Tate); Aviva Burnstock (Courtauld Institute of Art)

May 2018; last updated June 2021.