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Beneath the surface: paint losses due to zinc soap formation within ground layers from a late 19th century oil painting



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 worm-like 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.
Marta Félix Campos1,2, Leslie Carlyle1, Maria João Melo1,2
1 Department of Conservation and Restoration, Faculty of Sciences and Technology, Universidade Nova de Lisboa, Portugal
2LAQV-REQUIMTE, Faculty of Sciences and Technology, Universidade Nova de Lisboa, Portugal
Beneath the surface: paint losses due to zinc soap formation
within ground layers from a late 19th century oil painting
Landscape with apple trees in blossom and
peasant woman (c.1881-93)is an oil painting on
panel by the Portuguese naturalist painter Silva Porto
(1850-1893). Scattered throughout the painting’s
surface, paint losses, ranging from 2 to 12 mm,
appeared to have no relation to the paint’s colour, a
particular pigment, or the thickness of the paint layer,
Through the analysis of painting cross-sections,
we were able to ascertain the painting has two
distinct grounds: first, a layer of zinc white; and over
it, a mixture of lead white and barium sulfate.
Metal soap formation in oil paints has been
recognised as the cause of a range of paint defects,
including the formation of protrusions at the surface.
Even when not visible, they can be found as individual
rounded aggregates in paint cross-sections [1-5], as
seen here in the optical microscopy and SEM-EDS
images (on the right).
References Acknowledgements
[1] Boon, J J, et al. (2002), ICOM-CC 13th Triennial Meeting Preprints, volume I, pp. 401-406.
[2] Noble, P, et al. (2005) ICOM-CC 14th Triennial Meeting Preprints, volume I, pp. 496-503.
[3] Keune, K, Boon, J J (2007) Studies in Conservation, 52, pp. 161-176
[4] Shimadzu, Y, et al. (2008) ICOM-CC 15th Triennial Meeting Preprints, volume II, pp. 626-632.
[5] Centeno, S A, Mahon, D (2009) The Metropolitan Museum of Art Bulletin, pp. 12-19
[6] Otero, V, et al. (2013), Journal of Raman Spectroscopy, 45 (11-12), pp. 1197-1206
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 thank Centro de Materiais da Universidade do Porto (CEMUP) for the SEM-EDS analysis. Marta Félix would
like to thank the designer Ana Pedro for her work with the scheme presented here.
This work was supported by the PhD grant SFRH/BD/75123/2010 from Fundação para a Ciência e Tecnologia (FCT-MCTES).
Unlike other metal soap aggregates, these are
not migrating to the surface and protruding. Rather,
the aggregates have formed within the zinc white
ground where it is caught in the large pores of the
wood grain. As they increase in volume, they deform
the upper layers and the ground loses its
cohesiveness. The mechanical strain then forces the
upper layers to detach, leaving 2-12 mm paint losses
(see scheme on the side).
Another panel painting by Silva Porto with the
same ground layer construction is also suffering paint
losses due to zinc soaps following the wood grain,
however its condition is much less advanced. Why
one painting is more affected than the other remains
uncertain and requires further study.
In both cases this problem is causing extreme
instability and, at present, treatment options are
panel almost bare except for some remaining material
in the larger wood grain pores. Raking light shows
that this material has considerable volume.
With the help of an infrared reference database
of metal carboxylates developed in the department
[6], it was possible to indentify this material as zinc
A similar process has been described [2], with
metal soap formation following the wood grain in a
17th century panel. It was proposed that this was
related to the larger pores of the early wood grain
becoming filled with more chalk ground during
preparation. The ground-filled pores then supplied
the triglycerides needed to form carboxylates.
In the Landscape by Silva Porto, zinc soaps have
formed along the wood grain, and are pushing the
paint and upper ground layer into long ridges
eventually creating losses.
The whole surface exhibits deformations in the
paint in horizontal striations that follow the wood
grain. Associated paint losses have occurred at least
since 1990,date of the painting’s last restoration
when this phenomenon was already occurring. All
paint layers as well as the ground are lost, leaving the
Saponification of
fatty acids, formation
and aggregation of
zinc soaps
volume increase
Paint layer
Lead white + barium sulfate
Zinc white
Wooden support
H2O / high RH
The first ground
layer applied onto
the support fills up
the wood grain
fissure and
detachment of the
upper layers
Cross-sections: Optical microscopy images in polarized light (top) and UV light
(middle); SEM-EDS close ups showing the layering structure of the ground
layers (bottom left) and the formation of zinc soaps within the first ground layer
(bottom right)
Landscape with apple trees in blossom and peasant woman (c. 1881-93)
Silva Porto
oil on panel, 370 x56 mm
Paint losses highlighted in red
Normal light vs. raking light: the visible deformations
grow along the wood grain of the panel
Paint losses: all paint layers as well as the ground are lost, leaving the panel almost bare.
Raking light shows that the material within the larger pores of the wood grain has considerable
volume (right)
Infrared spectroscopy: sample taken from the larger pores
of the wood grain (black line) and reference sample of a
zinc stearate [6] (grey line)
Scheme of the proposed process: the first ground layer with zinc white (ZnO) fills the large pores of the wood grain; with the presence of water or high relative
humidity the zinc soaps form and aggregate, creating deformations and fissures in the upper layers; as the aggregates increase in volume, the ground loses it
cohesiveness and the upper layers crack and detach.
ResearchGate has not been able to resolve any citations for this publication.
Full-text available
The formation of metal soaps is a major problem for oil paintings conservators. The complexes of either lead or zinc and fatty acids are the product of reactions between common pigments and the oil binder, and they are associated with many types of degradation that affect the appearance and stability of oil paint layers. Fourier Transform Infrared spectroscopy (FTIR) reveals that a paint sample from The Woodcutter (after Millet) by Vincent van Gogh contains two distinct zinc carboxylate species, one similar to zinc palmitate and one that is characterized by a broadened asymmetric stretch COO band shifted to 1570-1590 cm−1 . Though this observation has been made in many paintings, it was previously left unexplained. It is shown that neither variations in the composition of zinc soaps (i.e. zinc soaps containing mixtures of fatty acids or metals) nor fatty acids adsorbed on pigment surfaces are responsible for the second zinc carboxylate species. X-ray diffraction (XRD) and FTIR analysis indicate that the broad COO band represents amorphous zinc carboxylates. These species can be interpreted as either non-crystalline zinc soaps or zinc ions bound to carboxylate moieties on the polymerized oil network, a system similar to ionomers. These findings uncover an intermediate stage of metal soap-related degradation of oil paintings, and lead the way to improved methods for the prevention and treatment of oil paint degradation.
Full-text available
Lead soap aggregateshave been-found in lead-containing oil paint layers in paintings from the thirteenth to the twentieth century. They severely affect the stability of the paint layers and disturb the surface of the paintings. Paint cross-sectionsfrom five paintings affected by lead soaps were selected to illustrate and investigate this degradation phenomenon with the analytical imaging techniques of Fourier transform infrared spectroscopy, secondary ion mass spectrometry and scanning electron microscopy combined with X-ray analysis. Examples aregiven of lead soapsforming in a mature paint system or, alternatively, in the early drying stage of the oil; lead soapsforming from various types of lead-containing pigments or driers; lead soapsforming in multiple paint layers; and lead-containing crystallization products inside aggregates. The phenomenon of lead soap aggregatesis multifaceted, and one general scenario describing theformation of lead soap aggregatescannot explain all aspects. However, the integration of the chemical information and its distribution among the paint layers leads to the proposal that reactivefree monocarboxylicfatty acids playa key role in lead soap aggregateformation. The availability and release of thesefatty acids depends on the original paint composition, the build-up of the layers, and the conservation/environmental exposure history of the painting.
Conference Paper
General results of the metal soap aggregation survey done between 2002 and 2005 are presented. From the data collected, it is clear there are several degradative phenomena associated with metal soap formation: aggregate formation, efflorescence and changes in transparency. We present a case study of localised darkening due to increased transparency as a result of metal soap formation on the wood grain in a leadwhite- containing imprimatura layer in a panel painting. Reduced scattering, as a result of the dissolution of the lead white particles, explains the darkening observed.
To characterize more fully the metal soaps found in paint films or on metal surfaces, several metal soaps were synthesized and their X-ray diffraction pattern and Fourier transform infrared (FTIR) and Raman spectra measured. Metal soaps were obtained from four different fatty acids found in drying oils - two saturated (palmitic and stearic acids) and two unsaturated (oleic and linoleic acids) - and from copper, zinc and lead, three metals that are typically found in metal alloys and paint systems. Data are reported for the following compounds: palmitic acid, stearic acid, oleic acid, linoleic acid, zinc palmitate, zinc stearate, zinc oleate, zinc linoleate, copper palmitate, copper stearate, copper oleate, lead palmitate, lead stearate and lead oleate. Features that are characteristics of specific compounds were observed. Soaps obtained from different fatty acids with the same metal ion show differences, as do soaps obtained with the same fatty acid but with different metal ions. Identification is key to understanding how and why metal soaps form on actual objects, and this may lead to preventive measures.
In an on-going study of the materials and techniques of twentieth-century Canadian painters, similar conservation issues in oil paintings by various artists have been noted. These include delamination and lifting paint, zinc soap protrusions and surface efflorescence or accretions. Examples of these phenomena are presented. Delamination in an oil painting from 1956 was found to be related to an underlayer with a high concentration of zinc fatty acid salts (zinc soaps). In two paintings that date from 1936 and 1937, zinc soaps have aggregated and formed protrusions that have broken through the paint surface. The protrusions were analysed using a combination of SEM-EDX, GCMS and FTIR. The FTIR spectra were compared to those of synthetic zinc palmitate, stearate, azelate and oleate. The combined GCMS and FTIR results indicate that the protrusions contain primarily zinc palmitate and stearate. Peak splitting in the FTIR spectrum, which is not observed in synthetic zinc palmitate, stearate or binary palmitate-stearate salts, is likely due to structural distortion. The final example describes a disfiguring surface accretion on a 1952–1954 painting caused by the reaction of zinc with a low molecular weight carboxylic acid (2-hydroxypropanoic or lactic acid).
This work introduces the complementary use of μ-Raman and μ-Fourier transform infrared (IR) spectroscopy for the detection of specific carbon chains and cations for the identification of metal carboxylates within oil paint microsamples. Metal carboxylates (metal soaps) form naturally when free fatty acids react with metal cations and may also be found as additives or degradation products. Twenty-two metal carboxylates were synthesised, and their spectra assembled in a reference database. Metal salts of cations commonly present in oil paintings were used, including lead, zinc, calcium, cadmium, copper and manganese. The fatty acids selected were the saturated acids palmitic (C16 : 0) and stearic (C18 : 0) and the polyunsaturated oleic acid (C18 : 1). Azelaic acid (C9 diacid), a product resulting from autoxidation of polyunsaturated acids, was also included. Metal carboxylates were characterised by Raman and IR spectroscopy, and their structures were confirmed by X-ray diffraction. Raman and IR spectroscopy proved to be complementary techniques for a full identification of the metal carboxylates in complex aged paint. Raman enables the differentiation of the carbon chain length in the C–C stretching region (1120–1040 cm�-1), and IR distinguishes the metal cation in the COO-� stretching absorption region (1650–1380 cm�-1). Principal component analysis was applied to the spectra in order to facilitate a fast and accurate method to discriminate between the different metal carboxylates and to aide in their identification. Finally, spectra from case studies were successfully projected in the principal component analysis models built, enabling a higher confidence level for the identification of copper palmitate and copper azelate in two 19th-century Portuguese oil paintings.
The presence of zinc oxide oil paint and the condition problems observed in a group of paintings from the collection of the Hirshhorn Museum and Sculpture Garden prompted analytical examination of the museum's mid-20th century holdings. Results reveal a link between upper layer deterioration and underlying zinc oxide paint layers, and suggest that certain visible signs of deterioration may signal the presence of more serious and widespread condition problems. The popularity of zinc oxide house paint among mid-century artists creates a higher probability of this type of deterioration in works from this period.