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).
 Boon, J J, et al. (2002), ICOM-CC 13th Triennial Meeting Preprints, volume I, pp. 401-406.
 Noble, P, et al. (2005) ICOM-CC 14th Triennial Meeting Preprints, volume I, pp. 496-503.
 Keune, K, Boon, J J (2007) Studies in Conservation, 52, pp. 161-176
 Shimadzu, Y, et al. (2008) ICOM-CC 15th Triennial Meeting Preprints, volume II, pp. 626-632.
 Centeno, S A, Mahon, D (2009) The Metropolitan Museum of Art Bulletin, pp. 12-19
 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
, it was possible to indentify this material as zinc
A similar process has been described , 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
fatty acids, formation
and aggregation of
Lead white + barium sulfate
H2O / high RH
The first ground
layer applied onto
the support fills up
the wood grain
detachment of the
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
Landscape with apple trees in blossom and peasant woman (c. 1881-93)
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
Infrared spectroscopy: sample taken from the larger pores
of the wood grain (black line) and reference sample of a
zinc stearate  (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.