Zinc soap formation aligned with wood grain pores on

Zinc soap formation aligned with wood grain pores on two late 19th century oil paintings Marta Félix Campos, Leslie Carlyle and Maria João Melo Abstract Landscape with apple trees in blossom and peasant woman (c.1881-93) and Beach at Póvoa de Varzim (1881) are two oil paintings on panel by the Portuguese artist Silva Porto (1850-1893). Both paintings exhibit paint losses revealing the underlying wood where crumbled ground residue surrounds wormlike raised lines of zinc soap following the wood grain. In addition, the Landscape painting exhibits raised paint in horizontal ridges aligned with the panel’s wood grain. These deformations are associated with 2-12 mm paint losses which show no relation to the paint’s colour or thickness. Both paintings have the same ground construction consisting of a first layer of zinc white directly on top of the panel, and a second layer consisting of a mixture of lead white, barium sulphate and calcium carbonate. The panels are thought to be commercially prepared. Materials were identified with a combination of optical microscopy, micro-Raman, and scanning electron microscopy with energy dispersive x-ray spectroscopy on embedded cross sections, and with micro-Fourier transform infrared spectroscopy on micro-samples. The presence of zinc soaps concentrated along the wood grain is discussed in relation to a previous study identifying lead soap concentration aligned with the wood grain on a much earlier panel. The zinc soap containing material in the two 19th century paintings appears to be contributing to paint losses due to a volume change associated with the concentrated zinc soap. We aim to provide analytical evidence that the paint losses and associated surface deformations are being caused by zinc soap formation within the ground layers and disseminate this form of metal soaps degradation. A scheme illustrating possible steps leading to deformation and paint loss is provided. This problem is causing extreme instability and risk of further loss. Currently, treatment options are unclear. Keywords Zinc soaps . Ground layers . Paint loss . 19th century . Oil paint . Infrared spectroscopy (µFTIR) . Optical microscopy (OM) . Scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS)

Introduction António Silva Porto (1850-1893) was a Portuguese Naturalist painter and a key figure in the introduction and dissemination of outdoor painting in Portugal. While studying a collection of 19 of his works in oil paint from Casa Museu Dr.

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Anastácio Gonçalves (in Lisbon, Portugal), it was discovered that two of the paintings exhibited a peculiar form of paint loss. Landscape with Apple Trees in Blossom and Peasant Woman (c. 1881-93) on a wooden panel (36.5 x 55.5 cm) showed extensive paint losses ranging from 2 to 12 mm wide (Fig.1 top). The losses included both paint and ground layers, leaving the wooden panel almost bare with the exception of worm-like raised lines of aggregated material along the wood grain and crumbled residue. These losses were associated with horizontal deformations throughout the entire surface which appear to follow the wood grain and are independent of the paint colour or thickness (Fig.2). Beach at Póvoa de Varzim (1881), also on panel (31.5 x 55cm), showed the same type of paint loss, all the way to the wood panel with raised lines and crumbled white material left behind, but in this case the losses were restricted to just two areas each approximately 3 mm wide (Fig. 1 bottom). Beach at Póvoa de Varzim did not show horizontal deformations in the paint.

Fig. 1 Top: Landscape with Apple Trees in Blossom and Peasant Woman with locations of detail images. Details 1 & 2 show paint losses along the wood grain with whitish material left behind. Also visible is the crumbled residue from the ground. Bottom: Beach at Póvoa de Varzim with location of detail images. Detail 3 shows paint loss along the wood grain with raised worm-like material left behind as found in 1 & 2 above. Detail 4 shows the crumbled ground residue.

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Fig. 2 Details from the Landscape: normal light (top) compared with raking light (bottom): the extensive paint deformations following the wood grain are not evident in normal light photography, but can easily be seen in raking light. These deformations occur throughout the entire surface of Landscape. No surface deformations were evident on the Beach.

In the two paintings, the paint losses involve both the ground and paint layers, with material left behind: a crumbled residue and lines of material oriented along the larger pores of the wood grain. Raking light shows that the material along the grain has considerable volume and appears raised (worm-like) (Fig.3). Paint losses in the Landscape are ongoing since 1990 when the painting was last restored. There were many fresh losses visible in 2013 (Fig.1). Evidence of previous losses which had been restored indicate that the Landscape has had a history of flaking and loss. On the contrary, previous restoration on the Beach is associated with contact and abrasions at the inside edge of the frame and not due to paint deformation in ridges. However, although these losses are due to abrasion, delamination has occurred at the ground/panel interface suggesting lack of cohesion within the ground layers. In order to investigate the cause of the paint losses and deformations in the Landscape’s paint, various analytical techniques (OM, SEM-EDS, µRaman, µFTIR) were used to identify the material found in the paint losses on both paintings and the intact double ground on both panels. Within the paint losses, micro-samples were taken of the material which was concentrated at the wood grain and that are present as a crumbled residue on the wood surface. For the grounds, micro-samples were taken of the paint and ground composites from areas around existing paint losses.

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Materials and Analytical Methods Micro-samples taken from the paint and the ground (separately) as well as the material found in the wood grain (raised lines) and the crumbled material on the surface of the wood, and were analysed with micro-Fourier transform infrared spectroscopy (µFTIR). A Nicolet Nexus spectrophotometer was coupled to a Continuum microscope (15x objective) with a MCT detector cooled by liquid nitrogen. Spectra were obtained in transmission mode, between 4000-650 cm-1, with a resolution of 4 cm-1 and 128 scans, using a Thermo brand diamond anvil compression cell. The spectra for the material in the wood grain and reference sample (see below) are shown (Fig.3 right) as acquired, without corrections or any further manipulations, except for the removal of the CO2 absorption at ca. 2300-2400 cm-1. For stratigraphical analysis, a selection of 13 micro samples were collected, both in areas of paint loss and intact paint, and were embedded in polyester resin (Resina Cristal, MR Dinis dos Santos©) for cross-section examination with Optical Microscopy (OM), µRaman spectroscopy and Scanning Electron Microscopy with Energy Dispersive Xray Spectroscopy (SEM-EDS). OM was carried out on a Zeiss Axioplan 2® microscope equipped with a HAL100 halogen lamp (under crossed-polar illumination) and an HBO100 mercury ultraviolet lamp (Zeiss UV Filter set 2; excitation G365; emission 420). Images (Fig.4) were taken with a Nikon DXM1200F digital camera coupled with the microscope. Image acquisition and post-processing was done using the ACT-1 software. Micro-Raman spectroscopy was carried out on the previously mounted crosssections for pigment identification in the different layers. A Labram 300 Jobin Yvon spectrometer was used, equipped with a He-Ne laser of 17mW power operating at 632.8 nm. Spectra were recorded as an extended scan. The laser beam was focused with a 50x Olympus objective. The laser power at the surface of the samples was varied with the aid of a set of neutral density filters (optical densities 0.3, 0.6, 1). SEM images were obtained with a FEI Quanta 400 SFEG ESEM, which uses a Schottky emitter field emission gun, operating at low vacuum conditions and at 15 kV, equipped with an EDAX Genesis X4M detector. Images were acquired using secondary (SE) and backscattered (BSE) electron detectors, but only the latter are shown here (Fig. 4, bottom). EDS point analysis was used to determine the composition of the ground and paint layers in the cross sections.

Results With the help of an infrared reference database of metal carboxylates developed in the Department of Conservation and Restoration at Universidade Nova de Lisboa (Otero et al. 2014), it was possible to confirm that the material concentrated at the wood grain consists of zinc soap, either a zinc stearate or a zinc palmitate or a

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mixture of both (Fig. 3, right), characterised by the sharp peak at 1540cm a(COO) (Robinet and Corbeil 2003; Hermans et al. 2015).

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These results are in keeping with the discovery by Osmond et al. (2012) who have identified a preferential pattern in the formation of zinc soaps on reconstructed reference samples. Exposed top surfaces exhibit a range of different carboxylates with an infrared absorption band between 1640-1505 cm-1, whereas unexposed areas tend to form zinc stearates and palmitates, characterized by the asymmetric COO- stretch at 1540 cm-1. This latter case corresponds to the zinc soaps identified in Silva Porto’s Landscape and Beach paintings.

Fig. 3 Detail in a paint loss in the Landscape, top left in normal light, with location of the sample (red arrow), versus bottom where raking light clearly shows the volume of the zinc soap lines on top of the wood grain. Right: the zinc soap (in red) was characterised using infrared spectroscopy compared with a reference sample of zinc stearate (in black).

The crumbled residue at the wood surface on both paintings was identified by µFTIR and µRaman as a mixture of zinc white oil paint and zinc soap. As detailed above, the lines of material present in the wood grain on both paintings were identified as zinc soap. In the Landscape, the lines appear to be associated with the extensive and disturbing raised horizontal deformations in the paint, implying that this feature is related directly to the zinc soaps. Since the Beach does not show deformations, only paint loss with similar zinc soap presence in the wood grain, it is thought that this painting may not be at the same stage of degradation. Zinc soap is the only material which remains in bulk on the surface of the panels within the paint losses. It is concentrated in areas directly above the large pores of the wood grain (Fig. 3 left). The extensive horizontal deformations on the Landscape painting appear to have resulted from the presence of the zinc soap

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below, since these raised ridges continue on either side of losses in alignment with the material in the wood grain (Fig. 2). The current thinking is that the deformations result from a change in volume during the formation of the zinc soaps, see discussion below. Analyses (OM, µRaman and SEM-EDS) of cross-sections from both paintings revealed the same stratigraphy with two distinct oil grounds: first, a layer of zinc white alone; and over it, a layer composed of mixture of lead white, barium sulphate and calcium carbonate.

Fig. 4 Two cross sections from the Landscape; At the top: OM normal light, in the middle: UV light, at the bottom: SEM. Left image: a cross-section from a green area where a regular layer structure is easily visible: a translucent zinc-based layer on the bottom (1), with its characteristic fluorescence under UV light, followed by a second ground layer consisting primarily of lead white mixed with smaller quantities of barium sulphate and calcium carbonate (2); and finally a layer of light green oil paint (3). Right image: a cross-section taken next to a paint loss with equivalent ground layers (1 and 2), several paint layers (3) and a zinc soap aggregate forming within the first ground layer (arrows).

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Fig. 4, shows two cross-sections from Landscape: on the left, a sample taken from an unaffected area; and on the right, a sample taken close to a paint loss. The first and more translucent ground layer (1) shows the typical UV fluorescence of zinc white. On the cross-section on the right, this fluorescence is less visible suggesting a saponification of the metallic pigment. In the embedded cross-section on the right, an inclusion of material is visible in normal (crossed polar) and ultra violet light in the zinc layer (red arrows). Based on appearance, which conforms to images in the literature (e.g. Keune (2005), Osmond (2005), Helwig (2014)) this is thought to be a zinc soap aggregate. In Fig. 4 bottom right, the SEM backscatter image (BSE) shows a corresponding area of what appears to be predominantly organic material which may represent zinc carboxylate. The second ground layer (2) on the right appears to already have been distorted by this aggregate formation. This could then lead to deformations in the paint layers (3), similar to the ones in Fig. 2.

Discussion and Conclusions A similar phenomenon of metal soap concentration along the wood grain of a panel painting has been described (Noble et al. 2005). In that case, lead carboxylate was found along the wood grain on a 17th century panel. This had resulted in dark lines of greater transparency in the paint following the wood grain. Nobel et al. proposed that the lead carboxylate concentration was related to the larger pores of the wood being filled with more chalk during the application of the ground. It was proposed that the larger proportion of binder associated with the deeper chalk ground-filled pores supplied the fatty acids needed to form lead carboxylates in the imprimitura. In Silva Porto’s two paintings, it is notable that the zinc soaps have also formed along the wood grain, perhaps in a similar mechanism with the deeper wood pores providing the source of triglycerides from the zinc white ground build up in the pores. Alternatively, it is possible that the wood panel was initially sealed with a drying oil prior to the ground application. Although this method was seldom described (Carlyle 2001: 180; Stols-Witlox 2015: 99), it would presumably also leave a residue of oil in the wood grain as a source of fatty acids. We propose that these aggregates are forming in place and provide a scheme illustrating a possible sequence for the resulting paint loss (Fig.5). The first ground layer fills the large pores of the wood grain; the oil binder, and possibly oil already present from sealing the wood, provides a source of fatty acids which leads to the formation of zinc soap aggregates. In the next step, the increased volume of the aggregates creates raised deformations and cracks in the upper layers. Cracks in the paint/ground are potential sites for greater ingress of water/moisture possibly further encouraging the expansion of the aggregates. The increased volume of the

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zinc soap aggregates forces the more brittle paint/ground composite upwards leading to detachment and paint loss with the softer aggregated zinc soap material left behind.

Fig. 5 Scheme of the proposed degradation process. Wooden support followed by the first layer of zinc ground (light grey); next a layer of lead white with barium sulphate and calcium carbonate in oil (darker grey); finally, the top layer (yellow) representing the paint.

Unlike the usual manifestation of metal soaps that can aggregate, migrate to the surface and erupt in the form of scattered white protrusions, the zinc soaps studied in these paintings form worm-like structures (raised lines) along the wood grain of the panel and do not protrude through the paint surface. Instead, it is the zinc white ground layer itself that is detaching from the support leaving lines of zinc soap (with small amounts of ground residue surrounding). Recent research on the chemical and physical properties of zinc white oil paint and zinc soaps could help to account for the low cohesion within the zinc ground and its poor adhesion to the wooden support. Rogala et al. (2010: 105) state that zinc white pigment compromises the structural stability of the oil paint matrix, “rather than a well formed paint layer consisting of a uniform cross-linked network, a paint containing zinc oxide consists of plate-like layered ‘islands’ held together by only a few cross-links”. Following up on a study by Jacobsen and Gardner (1941), Maines et al. (2011) found that the lamellar structure of zinc white in oil paint keeps the hydrocarbon chains tightly packed, consequently making them more difficult to oxidize. This leads to unusually high amounts of unsaturated oleic acid (C18:1) in fully cured zinc white oil paints, which would normally have been oxidized into azelaic acid (C9). This zinc–hydrocarbon–zinc layering, that is restricting the cross-linking of the oleic acid, likely contributes to interlaminar failure (Rogala et al. 2010: 105) The lamellar structure of zinc white in oil which traps the oleic acid is also thought to restrict the movement of zinc soaps (Rogala et al. 2010: 111), which

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may help to explain the concentration of zinc soap at the panel surface. The reduced oxidation of the paint system with its laminar morphology and unreacted oleic acid could account for the presence of the crumbled ground residue at the surface of the panel since this points to a lack of internal cohesion in the zinc ground layer. Internal cohesion issues in zinc white oil paint were also referred to by Helwig et al. (2014: 169). None of the remaining 17 oil paintings studied exhibit the same kind of paint loss or this form of metal soap degradation. When zinc grounds are present, they were found only on canvas supports with no other ground layers on top and exhibited the more common white protrusions on the surface. Other paintings on panel had lead white ground layers. Why one of these two painting is more affected than the other remains uncertain and requires further study. In the case of Landscape, this problem is causing extreme instability and there is a risk of further and/or total loss should this condition progress. Unfortunately, at present, treatment options are unclear. Acknowledgments The authors would like to thank Casa-Museu Dr. Anastácio Gonçalves for the opportunity to study their collection of Portuguese naturalist oil paintings. We would also like to acknowledge Centro de Materiais da Universidade do Porto (CEMUP) for the SEM-EDS analysis. Marta Félix would like to thank designer Ana Pedro for the scheme presented in Fig. 5. This work was supported by the PhD grant SFRH/BD/75123/2010 from Fundação para a Ciência e Tecnologia (FCT-MCTES).

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