Gillian Osmond, Paintings Conservator, QAGOMA

Metal Soaps in Art
14–15 March 2016
Rijksmuseum, Amsterdam

Arriving at the Rijksmuseum in Amsterdam on 14 March 2016, the main entry was dominated by a large banner proclaiming the Metal Soaps in Art conference. It was wonderful to see such a prominent display of institutional support for a conference on what is arguably a fairly abstract degradation process for many within the museum profession, let alone the wider public. Yet the conference was fully subscribed with 250 participants – testimony to the increasing levels of interest and concern on this emerging topic.

Intended for conservators, scientists and art historians, the conference was structured around papers by six invited speakers grouped together with poster pitches on related topics. Each afternoon concluded with a workshop program where parallel small group sessions discussed specific themes before regrouping for concluding panel review.

After welcoming formalities, participants were given a contextual introduction by Petria Noble, Head of Paintings Conservation at the Rijksmuseum. Petria, formerly of the Mauritshuis, is well known for her foundational work with molecular physicist Jaap Boon in the late 1990s attributing disfiguring surface protrusions and pitting in Rembrandt’s The anatomy lesson of Dr Nicolaes Tulp to lead soap formation. The Netherlands Organisation for Scientific Research has made serious investment in this field through several multiyear funding allocations, most recently in launching the Netherlands Institute for Conservation, Art and Science, a multidisciplinary research institute with a focus on the origin and the life of works of art through time. Petria has played a crucial role in negotiating for ongoing research commitment and support with her ability to communicate complex ideas and scientific concepts with great clarity and based on extensive practical conservation experience.

As an invited speaker, I was tasked with providing an overview of zinc oxide reactivity and consequences of soap formation in oil based paintings. While most early research focussed on lead soaps, documented incidences of zinc soap formation are becoming increasingly prevalent and concerning. The issue has particular relevance in Australia where our collections are dominated by nineteenth and twentieth century paintings – a period during which zinc oxide has been a frequent component of both artist and house paints.

Zinc soaps may manifest similarly to lead soaps forming aggregates or lumps – with textural and optical consequences – but also display a tendency to concentrate at surfaces or interfaces. A significant number of poster presentations detailed associated structural impacts. Case studies included ongoing investigations of early 20th century paintings by Mondrian where flaking is linked to layers mixed with zinc white (Ruth Hoppe, Gemeentemuseum, The Hague, and Laura Raven, Kroller Muller Museum and University of Amsterdam) and of extreme interlayer cleavage in mid 20th century Abstract Expressionist paintings (Corinne Rogge, Museum of Fine Arts, Houston; Dawn Rogala, Smithsonian MCI). An interesting incidence of flaking in 19th century ground layers linked to the grain of the wood panel support was presented by Marta Felix (Universidade Nova de Lisboa). Paintings by Francis Picabia were the subject of two presentations including increased transparency in surface zinc oxide based Ripolin paint layers which has altered reading of multilayer build-ups in 1920s–30s paintings (Joyce Townsend, Tate), and in 1950s paintings where expanding zinc soaps in underlying layers have penetrated through cracks and broken the surface (Jaap Boon, Amsterdam). Johanna Salvant (Northwestern University/Art Institute of Chicago) presented work where zinc soap protrusions in paintings dating between 1920 and 1950 from the Georgia O’Keeffe Museum are central to a multi-analytical study informing development of a non-invasive long term monitoring protocol with broader potential application.

Aviva Burnstock (Coutauld Institute of Art) spoke of metal soap phenomenologies and challenges observed in paintings from the 18th to the 20th centuries, focussing on changes in surface appearance, notably implicating lead, zinc or other metal soaps in formation of insoluble efflorescent crusts and commonly observed spotting which may otherwise have been attributed to ‘oil spots’ or ‘fly dirt’. Increased difficulty of cleaning caused by migration of metal ions into oil-resin varnishes was presented by Sally Higgs (Courtauld Institute of Art). Tommaso Poli  (University of Turin)  discussed the reactivity of various metals/pigments with natural resins to form oxalates, finding colophony to be the most reactive resin and zinc white and smalt the most reactive pigments studied. Four poster presentations discussed metal salt crusts involving oxalates which present their own cleaning challenges. Some success with removing insoluble complex inorganic metal salt crusts was described by Carol Pottasch (Mauritshuis) using carefully designed chelating agents to target metal ions in the crusts while controlling parameters to minimise risk to the paint layers.

Joen Hermans (University of Amsterdam) outlined the challenges and requirements for modelling the complexity of oil paint to allow meaningful examination of the dynamic processes involved in metal soap formation in oil paints. His work strongly suggests zinc and lead ions from pigments bind to carboxylate functionalities on the polymerised oil network as an intermediate stage in paint ageing. Synthesised versions of this model are allowing controlled study of diffusion of metal ions and fatty acids to explain different manifestations of metal soaps in oil paint structures.  

Costanza Miliani (Instituto di Science e Technologie Molecolari, Perugia) spoke of work using macro and micro scale FTIR of zinc soaps comparing models and paintings and finding that zinc carbonate and zinc sulfide accelerated formation of crystalline zinc stearate. Models comparing the reactivity with zinc oxide of drying oils commonly encountered in 19th and 20th century paint formulations were presented by Francesca Casadio (Art Institute of Chicago). Annaliese van Loon (University of Amsterdam) described research using a C17 fatty acid rarely encountered in oil paint systems as a marker to investigate formation and migration of lead soaps through multi-layer models. Various advanced analytical studies of lead soap crystal and coordination structures were reported including the hypothesis by Sylvia Centeno (Metropolitan Museum of Art) that minium impurities in lead white are the source of lead for lead soap formation. The potential of synchrotron deep UV photoluminescence imaging for differentiating dynamic processes in lead and zinc soap formation at the sub-micron scale was introduced by Barbara Berrie (National Gallery of Art, Washington D.C.).

Sylvia Centeno further discussed the dynamics of lead soap formation in oil paintings in model systems where experiments demonstrate a linear increase in reaction rate with increasing RH, and an increase in mobile fractions of fatty acids and metal soaps with increasing temperatures. Evidence linking metal soap formation and exposure environment in actual paintings was suggested by Katrien Keune (University of Amsterdam) after comparing the extent of lead white pigment loss to saponification within painted wall hangings lining the four walls of an interior space where proximity to and exposure through windows seems a determining factor. Environmental fluctuations are also likely contributors to flaking in wall-mounted and plaster ceiling paintings investigated by Kate Helwig (CCI) where calcium soaps have formed between calcium containing ground layers and adjacent oil rich layers at the point of failure.

The final invited speaker for the program was Jaap Boon (Amsterdam) who discussed modern oil paint formulations incorporating semi-drying oils implicated in dripping from paintings with heavy impasto. In these paints only a portion of the oil has the capacity to form cross links while the remainder is susceptible to oxidation and formation of unanchored polar fractions. These internally soften the paint and are mobile, typically penetrating absorbent substrates or migrating to the surface of the painting where drips may result. The polar fractions may additionally react with zinc oxide in adjacent layers to form soaps, causing a volume expansion and weakening the interface between layers. Pauline Helou-de La Grandière (France) described related phenomena in numerous paintings by Pierre Soulages.

Concluding poster presentations dealt with metal soaps as intended components. Marine Cotte (ESRF, Grenoble and Sorbonne University, Paris) reported that metal soaps are currently one of the most frequent topics in worldwide research involving chemical analysis of paintings; although most commonly mentioned in reference to degradation, participants were reminded that metal soaps have often been an intended product formed during traditional paint making. In the context of modern paint formulations Klaas Jan van den Berg (Cultural Heritage Agency of The Netherlands) discussed metal soaps added as extenders or to modify handling properties. Rosie Grayburn (University of Warwick) described intentional soap formation at the surface of heritage metals being examined for potential application as protective coatings, while research overlaps were further considered by Solenne Reguer (Synchrotron SOLEIL, France) in advanced analytical characterisation techniques of iron and copper carboxylates to inform understanding of both deterioration in paintings and for applications as corrosion inhibitors in metal artefacts.

The second aspect of the conference program involved two afternoon workshop sessions where participants met in small focus groups to discuss specific topics and process information from formal presentations. Discussion topics ranged from links between soap formation and artist technique, and impacts on appearance and interpretation of artist intent, through to analytical challenges and our evolving grasp of underlying mechanisms and reaction dynamics. For conservators struggling to understand how the incidence of metal soaps may influence their practice, the likely effects of solvents, heat and moisture on soap formation were discussed. The prevalence and range of manifestations of zinc soaps attracted particular attention, with widespread concern expressed regarding high rates of associated structural failure in affected paintings and indications that conventional approaches to consolidation are ineffective or may contribute to ongoing instability. It was clear that while some success has been achieved in customising cleaning systems to address disfiguring surface crusts, significant challenges remain to address internal chemical and physical instability associated with mobile acidic fractions and reactive pigments. Current best practice remains to control exposure environments at moderate levels and invest in continuing research which may assist practical solutions to emerge.

For those interested in more detail of research presented at the conference, a book of proceedings is currently in preparation, planned for publication by Springer in 2017.

I would like to thank the conference organisers, Rijksmuseum and Queensland Art Gallery/Gallery of Modern Art for supporting my involvement and attendance at this event.