Chemical changes in old master paintings II: darkening due to increased transparency as a result
of metal soap formation
Petria Noble
Royal Picture Gallery Mauritshuis
Postbus 536
2501 CM The Hague
The Netherlands
Fax: +31 70 365 3819
E-mail: Noble.P@mauritshuis.nl
Annelies van Loon and Jaap J Boon
Molecular Paintings Research Group
FOM Institute AMOLF
Amsterdam
The Netherlands
E-mail: vanloon@amolf.nl; boon@amolf.nl
Abstract
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 localized darkening due to increased transparency as a result of metal soap
formation in a lead-white-containing imprimatura layer associated with the early wood of the
wood grain in a panel painting. Reduced scattering, as a result of the dissolution of the lead white
particles, explains the darkening observed.
Keywords
localized darkening, increased transparency, degradation on the wood grain, metal soap
formation, lead soaps, lead white
Introduction
In the past five years the phenomenon of metal soap aggregates, whereby metal soaps aggregate,
swell and protrude through to the paint surface causing paint loss, loss of gloss and other visual
disruptions, has spawned numerous publications (Heeren et al. 1999, Plahter 1999, Boon et al.
2002, 2004, Van der Weerd et al. 2002, 2003, Noble et al. 2002, Saunders et al. 2002, Higgitt et
al. 2003, Robinet and Corbeil 2003). To gauge the extent and diversity of the problem, in 2002 a
questionnaire was formulated and sent out to conservators and conservation departments of
museums worldwide.1 The aim of the questionnaire was not only to inform, but also to collect
data. The responses have been tabulated in a spreadsheet, which serves to organize the
information and discern trends. From the data collected, it became clear that there are several
degradative phenomena associated with metal soap formation: efflorescence, aggregates and
increased transparency. This paper focuses on darkening of paint films, as a result of increased
transparency, as yet another aspect of metal soap formation that degrades the appearance of
paintings.
General results of survey 2002–2005
As a direct result of the questionnaire, hundreds of examples of protruding masses and craters
(due to loss of aggregates) were documented in paintings and objects from a broad range of
geographical locations and dates. These include paintings on paper, canvas, panel and copper, as
well as polychrome sculpture, ranging in date from the 15th to the 20th centuries, and including
both treated and untreated works. Of these objects reported, only a small proportion was
analysed, either by us or by other researchers. In each case studied, metal soap aggregates were
identified. Rough bubbly surface textures were often reported as a result of lumps lying just
below the surface paint. In many cases where the protruding lumps cover the entire painting,
cross sectional analyses were able to establish that the aggregates originate in a ground layer. In
surface layers, aggregate formation has thus far only been associated with paint films containing
red lead or lead–tin yellow in old master paintings, and lead/zinc-containing paints in more recent
examples. In many 19th and 20th century paintings, several phenomena were often found to
occur simultaneously in the same picture: not only aggregates, but also semi-crystalline waxy
surface deposits of metal soaps (usually described as bloom or efflorescence). Areas of increased
transparency were also noted in paintings from all periods; in preparatory layers, as well as in
dark and light surface paint layers. These areas, which appear darker in tone than originally
intended, were found to be associated with metal soap formation. In all cases of aggregates
published thus far, only lead or zinc soaps were identified, and in a few cases a combination.
Thus so far metal soap formation has only been associated with oil paint. It is not always possible
to distinguish between lead and zinc originating from an added drier or from lead- or
zinccontaining pigments.
One consistent factor has emerged: exposure to high levels of relative humidity either from
storage, display or treatment. Furthermore, it has become clear that the process can continue for
some time; in several cases the aggregates have broken through later added overpaint (Figure 1).
Clearly, the questionnaire has been able to establish just how widespread the phenomenon of
metal soap aggregate formation is. Given the diversity of the data, the associated phenomenon of
increased transparency/loss of opacity and its consequent darkening was taken up for further
investigation.
Figure 1. Nicolaes Berchem (1620–1683), Allegory of Summer, c. 1680, Mauritshuis, inv. no.
1091, canvas, 94 cm × 88 cm. Overall (inset), and detail from (red) drapery of standing figure
(left of centre) where aggregates have broken through two layers of relatively recent overpaint
Loss of opacity/increased transparency
Increased transparency has been observed and noted in numerous publications for a long time.2
There are countless pictures where increased transparency of leadwhite- containing paint films
allows underdrawing, underpainting or artists’ alterations to become more visible than originally
intended (most pentimenti involve the use of lead white). Although in some cases underlying
layers were meant to be visible, in paintings such as Adriaen Coorte’s Still Life with Asparagus
(Rijksmuseum, Amsterdam inv. no. A-2099) from 1697, there is no doubt the underlying dark
background that has become disturbingly visible is due to the lead-white-containing paint of the
asparagus having become more translucent.3 Increased transparency has been generally attributed
to the increase in refractive index of the oil-binding medium as the paint ages; however, as a
result of more recent studies, metal soap formation whereby mineral lead white has been shown
to slowly convert into lead soaps is now considered to play a major role. Increased transparency
was already observed in cross sections of the affected ground layers of Rembrandt’s Anatomy
Lesson of Dr Nicolaes Tulp from 1632 (Mauritshuis inv. no. 146), where all the small lead white
particles surrounding large ones were shown to have dissolved (Keune et al. 2002). Increased
transparency was also demonstrated in intermediate and surface paint layers, which has resulted
in strong visual distortion of the original light/dark contrasts. In a 17th century landscape by
Roelandt Savery (Mauritshuis inv. no. 157) – Orpheus Enchanting the Animals with his Music
dated 1627 – saponified lead soap masses were demonstrated by using scanning electron
microscopy (SEM–EDX)4 and specular reflection Fourier transform infrared imaging (FTIR)5
throughout the (now) extremely dark brown paint layer that covers a large portion of the picture
(Figure 2). As a result of darkening of this layer, the animals, birds and tiny details depicted on
top of this layer are now barely visible to the naked eye.6 It has also been found that this loss of
opacity occurs selectively. Already in 1999, selective ‘darkening’ of the ground in the unpainted
canvasses of the American 19th century painter Frederic Church was observed (Zucker 1999).
Figure 2. Roelandt Savery (1576/78–1639), Orpheus Enchanting the Animals with his Music,
1627, Mauritshuis, inv. no. 157, panel, 62 cm × 131.5 cm. Overall and light microscope images
of a cross section from distant landscape (upper left), with corresponding FTIR images. The
saponified brown layer is the intermediate layer below the (green) top layer of the landscape.
The white bands in the FTIR maps represent areas of high intensity
Where such grounds have been deliberately left uncovered by the artist, or the overlying paint is
thin, or lost, disturbing effects result. In Carel Fabritius (circle of ) Man in a Helmet (Groningen
Museum, inv. no. 1931-103), the lead-whitecontaining ground/imprimatura that only fills the
interstices of the wood panel appears to have become transparent (Figure 3). Although chemical
analysis was not possible in this case, the presence of large conglomerates of Dutch
stackprocessed lead white observed with the binocular microscope implies that the
Figure 3. Carel Fabritius (circle of ), Man in a Helmet, Groningen Museum, inv. no. 1931- 103,
panel, 38.5 cm × 31 cm. Overall (inset) and detail from the right cheek, near moustache. Here the
darkened ground that only fills the interstices of the wood confers a disturbing stripiness
priming layer is composed mostly of lead white. Like the ground in the Anatomy Lesson of Dr
Nicolaes Tulp, it would seem that all the small lead white particles have saponified leaving only
the large coarse particles in a transparent matrix. Even to the naked eye, the darkened ground
confers a disturbing stripiness in the thinly painted parts of the picture. It is also notable that
protruding aggregates measuring 100–500 μm in diameter have erupted through the surface paint
(Figure 3). Loss of opacity is also observed associated with the wood grain on the radial surface
of panel paintings, creating disturbingly dark stippled lines that have often been interpreted as
abrasion. Investigation of a panel painting by Aert van der Neer dated mid-1650s, River
Landscape (Mauritshuis inv. no. 912), has shown that the paint layers on top of the porous early
wood on the radial surface have become selectively more transparent, appearing darker in
relation to the nearby intact paint on the late wood (Figures 4 and 5).7,8 Observations with the
binocular microscope (to magnification ×50) reveal that this is associated with darkening of the
underlying lead-white-containing imprimatura layer that either shimmers through the very thin
sky paint or is exposed as a result of loss of the sky layer. This painting is the subject of more indepth study, the results of which are presented below.
Figure 4. Aert van der Neer (1604–1677), River Landscape, c. 1650, Mauritshuis inv. no. 912,
panel, 44.8 cm × 63 cm. Overall (inset) and detail from centre right showing disturbing stippled
lines (cleaned state)
Figure 5. Microscope detail from (rose-coloured) sky showing localized darkened paint
associated with the uneven structure of the oak panel
In several cross sections from the sky, three layers are present: a chalk ground (up to 60 μm
thick); a warm light brown imprimatura (10–20 μm); and a single thin pinkish-grey sky layer
(approximately 10 μm) consisting of lead white with the addition of a little vermilion, umber and
glass (also carbon black in the greyer areas of the sky). In two cross sections from the dark lines,
the imprimatura, which is essentially composed of large and small particles of lead white and
large rounded particles of a relatively transparent red–brown umber (EDX: Fe, Mn, Si, Al, (Mg,
K)), has become transparent in places throughout the layer. Furthermore, small reddish umber
particles are observed in these transparent areas in close proximity to the more Mn-rich brown
umber particles. It is notable that in the samples from affected areas a thick chalk ground is
present, whereas in unaffected areas the chalk layer is thin or barely present. This difference in
the thickness of the ground is also apparent at the sides (end grain) of the panel when viewed
with the binocular microscope. We conclude that where the wood is denser (the late wood),
hardly any ground is present and where the wood is porous (the early wood), the ground is thick
because it penetrated the cells opened by the planing and sanding of the panel. When the
underlying wood of these affected areas on the surface is viewed in the transverse section of the
panel (end grain), the exposed large open pores of the vertical parenchyma cells in the early wood
are huge in comparison with the smaller pores in the late wood (Figure 6). The porosity of these
early wood areas can be clearly discerned on the radial surface on the back of the panel. We
conclude that it is primarily these areas that become filled with ground during preparation of a
radial surface.
Figure 6. Detail of transverse section (end grain) of the oak panel support showing porous early
wood and dense late wood
Imaging FTIR analysis confirmed metal soap formation in the transparent regions of the
imprimatura by strong absorption in the 1510–1520 cm–1 region that is characteristic for lead
carboxylates. In Figures 7 and 8, the SEM backscattered electron images (BSE) of a cross section
from non-darkened intact sky area and a cross section from an area in the sky that has darkened
are displayed. The difference in the morphology and distribution of the lead white is striking. In
the unaffected imprimatura layer (Figure 7) we see a homogeneous distribution of fine and
coarse particles typical of 17th century Dutch stack-processed lead white with a lack of
compactness suggestive of a medium-rich layer. In comparison, in the imprimatura layer in a
cross section from the darkened area (Figure 8), all the fine lead white particles have reacted
away leaving only the large coarse particles. This corresponds with the observation of large lead
white particles in a transparent brownish matrix in these areas on the surface of the painting, as
well as with areas of increased transparency in other paintings currently being studied. The grey
halos around the larger lead white particles, visible in the backscatter image owing to the
diminishing density of lead in these regions, indicate the progressive transformation of lead white
into lead soaps. Furthermore, these areas show a cloudy morphology where only the contours are
reminiscent of their original granular structure. It is also notable that there is a complete absence
of lead white particles in the lower part of the layer. The chalk in the ground layer does not
appear as uniform and compact as we normally observe with chalk grounds; in the BSE image it
can be seen that different types of micro-fossils are present in the chalk (EDX: Ca) and, although
it is otherwise unaffected, appears unusually porous. The EDX data also indicate small variations
in the concentrations of iron and manganese in the umber particles, which seems to correspond to
the colour differences observed. What appears to be partial/surface dissolution of a large umber
particle was also observed in another cross section from a darkened area.
Discussion
The reduction in opacity (that is, increase in transparency) in the painting can be explained by the
destruction of mineral matter owing to the formation of metal soaps replacing particulate lead
white, whereby the smaller lead white particles first react away leaving only the larger particles
that are also clearly in the process of transforming into lead soaps. That the calcium carbonate
ground is unaffected is because of the basic conditions required for this process (Boon et al.
2002).9 Differences in the thickness of the ground in relation to the panel’s wood structure
correspond with different degrees of lead soap formation in the paint layer. We propose that the
fatty acids required for the soap formation are partly supplied by the thick chalk ground that acted
as a sponge absorbing oil triglycerides from the paint layers above when the painting was made.
This is
Figure 7. SEM BSE image of cross section from unaffected intact (grey) sky
Figure 8. SEM BSE image of cross section from darkened area in (grey) sky (compare with
Figure 7). The sky layer is missing from this sample
borne out by the complete absence of lead white particles observed in the lower part of the
imprimatura layer (Figure 8). As a result of transformation of the mineral lead white into lead
soaps, light penetrates deeper into the paint layer, resulting in a darker appearance, whereas in the
intact areas the strong light scattering ability of the lead white particles causes these areas to
appear opaque. A colour change may also take place owing to interaction of the lead soaps with
the partly dissolved umber particles. Partial dissolution of umber facilitated by the basic
conditions is also held responsible for the differences in tone in the umber particles that range
from red to brown.10 The thickness of the oil-soaked chalk ground is considered to be a decisive
factor in the final appearance of the painting. A slight darkening of the chalk layer will,
furthermore, affect the overall appearance, because the ground, and possibly the panel, will shine
through the transparent upper layer(s). The conversion of the imprimatura layer to a soap-rich
layer also results in the layer taking up a larger volume than it originally had; the swelling of the
layer explains why the overlying sky paint layer has flaked off in much of the darkened areas.
Naturally, this phenomenon also has to do with the use of thin paint layers that (now) allow the
underlying im primatura layer to play a much larger role in the final appearance ofthe picture
than originally intended. Increased transparency is generally ascribed to a rise of the refractive
index (RI) of the oil-binding medium with age (1.48 for fresh linseed oil to 1.57 for a very mature
paint film). For pigments with a high RI, such as lead white (RI: 1.9–2.1), this rise is not enough
to explain the increased transparency observed in some paint films, and as Townsend (1993)
suggests, it must be the RI of the pigment that alters. Although the refractive index of lead soaps
has not been established, a value between that of the oil and lead white is assumed (Robinet and
Corbeil 2003). The conversion of mineral lead white to a lead-soap-rich mass, as in the case of
the imprimatura layer in the Aert van der Neer painting, will significantly lower the refractive
index of the pigment and, as a consequence, the difference between the oil medium and the
pigment, leading to an increase in transparency.
Conclusions
From insight gained about metal soap formation, and responses from the questionnaire, the
conversion of particulate mineral lead white to amorphous organic complexes of metal soaps is
now considered to play a major role in increased transparency, and the consequent darkening of
lead-white-containing paints. This chemical change, which results in the decreased ability of lead
white to scatter light, explains the reduction in hiding power. Furthermore, an example of
selective darkening due to localized lead soap formation in the imprimatura layer determined by
the uneven structure of the wood grain in panel paintings is presented.
Acknowledgements
Our gratitude goes to all those who took the time to fill in the questionnaire. Support from
colleagues in the Mauritshuis, especially Jørgen Wadum and Peter van der Ploeg, is gratefully
acknowledged. We also thank Sabrina Meloni and Alice Mohan who provided samples from the
Savery and the Aert van de Neer. The research is part of the De Mayerne Program MOLMAP
(2002-2006) and research program 49 of FOM, which are both funded by the Netherlands
Organisation for Scientific Research (NWO, The Hague).
Notes
1 The questionnaire can be downloaded from http://www.amolf.nl.
2 Already towards the end of the seventeenth century the Dutch painter Gerard de Lairesse
observed the phenomenon of increased transparency. It is explicitly mentioned in his Groot
Schilderboeck published in 1707 (van Eikema Hommes 1998: 117).
3 In addition to increased transparency, tiny craters filled with a whitish substance
(corresponding to the phenomenon of metal soap aggregates) were observed (van Eikema
Hommes 1998: 118, Wallert 1999: 94).
4 SEM–EDX analysis was performed on a FEI SFEG XL30 high vacuum electron microscope
with EDAX detector at 20 kV (FOM-AMOLF, Amsterdam).
5 Specular reflection FTIR imaging was done on paint cross sections using the Biorad FTS
Stingray 6000 system (FOM-AMOLF, Amsterdam).
6 Treated by Sabrina Meloni (SRAL intern 2002–2003) and extensively documented in Meloni,
S, 2003, ‘Orpheus betovert dieren van Roelandt Savery’, unpublished final thesis, Limburg
Conservation Institute (SRAL), Maastricht.
7 The localized darkening in this picture was first observed during the treatment of the picture by
Alice Mohan (Maîtrise de Sciences et Techniques (MST), University Paris I Panthéon-Sorbonne)
during her three month internship in the Mauritshuis 2002. The same phenomenon, though less
disturbing, is present in Aert van der Neer’s River Landscape at Sunset, ca. 1650 (Mauritshuis,
inv. no. 913).
8 Dendrochronological research (Professor Dr Peter Klein, 2002, University of Hamburg)
demonstrated that the oak from both van der Neer panels is of Baltic origin, the most recent
heartwood rings dating from 1633 (River Landscape), and from 1619 (River Landscape at
Sunset).
9 Acidic emissions or extracts from the wood do not seem to play a significant role in the
darkening process as initially thought, because the thick chalk ground present in the darkened
areas would act as a buffer to neutralize acidic compounds.
10 The De Mayerne manuscript warns that umber-containing paints can darken and become dull.
Mention is made of a process known as bleeding (van Eikema Hommes 1998: 114–115, 129).
According to Hess (1979) ‘bleeding’ refers to the diffusion of a colouring matter […] and also to
the discolouration resulting from such diffusion.
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