Chemical aspects of the binding media of the Oranjezaal ensemble: an insight into 17th century
Netherlandish materials and methods
Ester S B Ferreira, Jerre van der Horst and Jaap J Boon
Molecular Painting Studies Group
FOM Institute for Atomic and Molecular Physics
Kruislaan 407
1098 SJ Amsterdam
The Netherlands
Tel.: +31 20 6081234
Fax: +31 20 6684106
E-mail: e.ferreira@amolf.nl
Abstract
The binding media of samples from 17th century paintings in the Oranjezaal ensemble were
analysed by online methylation with tetramethylammonium hydroxide using Curie-point
pyrolysis gas chromatography/mass spectrometry (Py–TMAH–GC/MS). Data were processed
using principal component analysis. This ensemble is unique as the conservation treatments are
recorded and the paintings have always been displayed under similar environmental conditions.
The differences in appearance and composition of the binding media of paint samples are found
to be related to the combination of materials used to prepare the paint. The results are consistent
with those from studies of other paintings from the same period and geographical origin with
exception of the lack of conventional evidence for prepolymerization.
Keywords
17th
century,
Netherlandish,
binding
media,
principal
component
tetramethylammonium hydroxide, pyrolysis, gas chromatography/mass spectrometry
analysis,
Introduction
Many factors may alter the composition of an oil paint, namely type and processing of the oil
(Mills and White 1994), age, the pigment, the environment (Schilling et al. 1996, 1999), as well
as the conservation history of the painting. Often many of these factors are unknown, hindering
the interpretation and the comparison between paint samples originating from different paintings.
Rarely can we find a collection of paintings, from different painters, all produced in the same
period, that have been kept in comparable, known environmental conditions and that have
documented conservation histories, so eliminating many of the variables listed above. The
Oranjezaal houses such a collection. It has provided a unique set of samples, ideal for a valid
comparative study of 17th century Netherlandish binding media materials and methods. The
Oranjezaal consists of an ensemble of paintings on large-scale canvases and wooden elements in
the room such as the ceiling and doors. It was created by 12 prominent 17th century Netherlandish
painters, in commemoration of the life of the stadhouder Frederik Hendrik. It includes names
such as Jacob van Campen, Jacob Jordaens, Jan Lievens, Cesar van Everdingen and Salomon de
Braij. All painters were instructed about the composition, were given primed canvases and were
awarded equivalent payment. Only the choice of binding media, pigment, techniques and
pictorial means is characteristic of each artist. This is why the comparison between the chemistry
of the different Oranjezaal paint samples is particularly informative. In most cases, the
differences in chemistry of binding media of different paints can only be explained by the initial
choices of materials and the interaction between the different layers.
Concept
The selected areas include white and dark surfaces of paintings representing most painters
involved in the project. The reasoning behind this resides in the fact that the preparation of these
two paints has, in general, different requirements. The white paints frequently contain lead white,
which catalyses the drying of the oil; however, many brown/black paints have poor drying
properties and usually require a stand oil or the addition of a drier. Moreover, choice of a
different drying oil may be a way of avoiding the yellowing of lighter areas. The comparative
analysis of the binding medium allows an insight into the particular choices of materials of the
different painters potentially helping the attribution of not clearly attributed paintings, for
example. Also as important is the study of the influence of the pigments and other additives in the
current chemistry of the paint film.
17th century Dutch paint media
White et al. (1994) published an elegant overview of the analysis of binding media of a series of
paintings from Rembrandt and other 17th century painters of his circle. Their results suggest that
mainly drying oils, either linseed or walnut oil, were used. The knowledge of pigment properties
was clearly shown in the use of pigments such as smalt, azurite, umber or ochre (good driers)
mixed with poor drying pigments such as bone black. Furthermore, analysis showed that heatbodied oils were used in particular when preparing paint containing poor drying pigments such as
blacks or lakes or to obtain an impasto. The identification of heat-treated oil is based on the ratio
between azelaic acid (A) and suberic acid. These results should, however, be used with care in
light of subsequent research. Schilling et al. (1996, 1997) have shown that light-ageing increases
the levels of dicarboxylic acids other than azelaic acid, and that under both light- and
thermalageing the level of saturated fatty acids decreases significantly. Later work demonstrated
that palmitic acid (P) evaporates preferentially to stearic acid (S) thus altering, at least at the level
of the paint surface, the P:S ratio commonly used for oil identification (Schilling et al. 1999).
Experimental
Methylation with tetramethylammonium
chromatography/mass spectrometry
hydroxide
using
Curie-point
pyrolysis
gas
Prepared samples were analysed by online methylation with tetramethylammonium hydroxide
using Curie-point pyrolysis gas chromatography/mass spectrometry (Py–TMAH–GC/MS) with a
reagent-venting module, a modified method from van den Berg et al. (2001). Details of the
experimental set-up will be published elsewhere.
Principal component analysis
A FOMpyroMAP multivariate analysis programme (a modified version of the ARTHUR package
from Infometrix Inc (Seattle USA 1978 release)) and Chemometricks (a FOM developed
Matlab® (The Math Inc, MA, USA) toolbox) were used in the principal component analysis
(PCA) of the data (peak areas of the different components). Details of the abbreviations used in
the description of the detected oil components can be found in Table 1.
Preparation of cross sections
Selected samples were embedded in Technovit 2000LC resin. The sample surface was dry
polished with micromesh (successively finer grades up to 12,000).
Scanning electron microscopy with energy-dispersive X-ray analysis
The embedded samples were carbon coated in a CC7650 Polaron carbon coater. Scanning
electron microscopy with energy-dispersive X-ray analysis (SEM–EDX) was performed on a
XL30 SFEG high vacuum electron microscope and EDAX detector. Backscatter electron images
of the samples were taken at 20 kV acceleration voltage, spot setting 3 (beam diameter 2.2 nm,
current density 130 pA). EDX analysis was performed at a spot setting 4 (beam diameter 2.5 nm,
current density 550 pA).
Results
Composition and processing of the oil
The interpretation of the results from Py–TMAH–GC/MS provides information firstly on
material identification. If a drying oil has been used, the A:P ratio should be higher than 1 (Table
2). More accurately, A:S ratio should be used because it is more stable to ageing and there are
fewer possible sources (other than the oil itself ) of stearic acid than palmitic acid. The
identification of the origin of the drying oil is possible using the ratio between components that
do not participate in the drying process, that is, the P:S ratio. A drying oil with P:S less than 2 can
be identified as linseed oil, and P:S greater than 5 as poppyseed oil. Extensive analysis of
multiple oil reference samples has shown that intermediate ratios can be assigned to walnut,
poppyseed oil or mixtures of these with linseed oil (Schilling VOL II Scientific research 775 et
al. 1996). The P:S ratios were here determined using the peak areas. In general, results (Table 2)
suggest that linseed oil is the binding medium in most of the samples. Four of the samples
analysed (19A19, 2A3, 2A8 and 2A9) have a P:S ratio between 2 and 2.5, and one sample
(19A17) has a higher P:S ratio of 4. These results suggest the use of walnut oil or a mixture of
drying oils (in sample 19A17 walnut, poppyseed oil or a mixture of drying oils). With the
exception of 2A8, the samples that are prepared with oils other than pure linseed oil are lead
white pigmented paint samples. This is in accordance with 17th century sources that recommend
the use of walnut or poppyseed oil for paint samples significantly altered by binding medium
yellowing (De Mayerne 1620). Recent work (Keune et al. 2005) suggests that there is a certain
degree of migration of the fatty acids between paint layers during drying, which can alter the P:S
ratios in individual paint layers. Although further research into this subject is required, this new
information must be taken into account when interpreting the data. In some samples, diterpenoid
resin components were also detected but it is not clear if these were part of the original paint
composition or a varnish residue. Secondly, Py–TMAH–GC/MS allows an insight into the
chemical drying process, by studying the degree of oxidation and the range of oxidation products.
In this context, the ratio between the azelaic acid and suberic acid is used as an indication of the
method used to process the oil. These data suggest (Table 2) little use of pre-polymerized oil.
Because, among other reasons, the quantity of detectable dicarboxylic acids other than azelaic
was shown to increase with accelerated ageing (Schilling et al. 1996), the relation between
A:suberic ratio and oil processing method is not clear and these results should be interpreted with
caution.
Table 1. Number, product detected by Py–TMAH–GC/MS, corresponding paint film component
(these components can also be present as acylglyceride esters or metals soaps, the current
methodology does not distinguish the different species and analyses them collectively) and
common name of the different oil components detected by Py–TMAH–GC/MS and analysed by
PCA
Table 2. Different ratios of peak areas of the various paint samples analysed by on-line
derivatization TMAH–Py–GC/MS. (a) P:S (TIC peak area ratio methyl palmitate:methyl
stearate) indication of the nature of the oil; (b) oil identification: L, linseed oil; W, walnut oil; P,
poppyseed oil; M, mixture; (c) A:P (TIC peak area ratio dimethyl azelate:methyl palmitate); (d)
A:S (TIC peak area ratio dimethyl azelate:methyl stearate) both give and indication if a drying or
non-drying oil was used but A:S is more suitable because it is more stable to ageing; (e)
A:suberic indication if the oil was pre-polymerized (TIC peak area ratio dimethyl
azelate:dimethyl suberate. The amount of diacids other that azelate have been described as
increasing with light-ageing (Schilling et al., 1996, 1997)); n.y.a., not yet analysed
Principal component analysis
The main components of the binding media, as identified by the current methodology, include
short-chain saturated fatty acids, diacids, methoxysubstituted diacids (all cleavage products from
oxidation of the unsaturated fatty acids), long-chain saturated fatty acids as well as some
unsaturated and/or oxidized C18 fatty acids (Figure 1). This is in agreement with what had been
previously observed in oil paint films (van den Berg et al. 2002). Diacids are susceptible to
unwanted alkylation when reacting with TMAH (van den Berg et al. 2001); therefore total peak
area for azelaic acid methyl ester was estimated by adding the peak areas of methylation and permethylation products. Other components not commonly reported include methoxy substituted
short-chain saturated fatty acids. The presence of short-chain fatty acids in all samples and monounsaturated C18 fatty acid in a few is unexpected in 350-year-old paint
Figure 1. TIC of samples of (a) 26A13 (a lead white sample with a negative PC1 score) and (b)
18A7 (a bone black containing sample with a positive PC1 score) after Py–TMAH–GC/MS
film. The short-chain fatty acids could be present as glycerol esters, part of the network, or
trapped as metal soaps. The analysis of selected samples using a milder methylation reaction and
on-column injection has confirmed the presence of mono-unsaturated C18 fatty acid (results not
shown). Although the mentioned components are detected in most samples, they are present in
different relative amounts, reflecting different oil preparation and/or drying processes. The most
systematic way of comparing the chemistry of these samples is PCA, which was applied to the
peak areas of the different oil components, mentioned in the previous paragraph. Again, it should
be mentioned that the unique characteristics of these samples make this approach valid because
most external sources of variance are known or identical for all samples. PCA shows that the first
two principal components (PCs) describe 46 per cent of the variance within all the samples.
These will be the only PCs discussed in the context of this publication; the corresponding
loadings for all features are shown in Figure 2. The first principal component (PC1) discriminates
between longchain fatty acids (saturated (24, 27, 29, 31, 32), monounsaturated (26) and partly
oxidized chains (28, 30)) (PC1 +) and diacids (10, 12, 15, 18, 20) (PC1 –), final products of the
oxidative drying process, thus describing the degree of oxidation of the binding media. PC2
shows that short-chain diacids (4, 6, 8) have different loadings from long-chain ones (20, 22, 25),
suggesting that the formation of these oxidation products has different mechanisms. The loadings
of all features for the two PCs are described in Figure 2.
Discussion
PC1 and PC2 describe part of the variance between all analysed samples (Figure 3). There are
differences between the chemical composition of the binding media of the lead white (●) and the
brown or black samples (●) reflected in the low overlap of their scores. It becomes clear in Figure
3 that white samples have in general a higher oxidation level than the dark samples. This is not
surprising because lead white is an auxiliary dryer and many dark pigments have poor drying or
antioxidant properties. The features with negative PC1 and positive PC2 loadings (for example
diacids such as azelaic (15), suberic (12) and sebacic (18) acids, as well as other oxidation
products (Figure 2)) describe the main components of samples with negative PC1 and positive
PC2 scores, namely mainly the lead white samples (Figure 3). In general black/brown samples are
better described by features with positive PC1 loadings (for example long-chain fatty acids such
as palmitic (24), oleic (26) and stearic (27) acids, other saturated fatty acids). There is, however, a
higher variance between these samples, which
Figure 2. Loadings of the features (for full description see Table 1) for PC1
Figure 3. Scores of the analysed objects for the first PCA. Lead white ( ●) and the brown or black
samples ( ●)
is not surprising because the variability of compositions is higher than in the lead white samples.
Complete interpretation of the chemical profile and detailed composition of the binding medium
requires full characterization (identification of pigment and other additives) of all the samples.
These results will be published in detail elsewhere. However, in the following paragraphs some
examples are given.
Case studies: identification of pigments and additives
The samples selected have very different PC1 and PC2 scores and should represent the variance
described by these two PCs. The selected samples were embedded, the layer build-up, when
available, studied and the pigment identified by microscopy and SEM–EDX. The selection
criteria included different binding media component distribution reflected on the first and second
PC scores.
1. Black paint sample 2A8 (PC1 score 2.48; PC2 score 3.42); painting 2: Apollo op de
Zonnewagen voorafgegaan door de Aurora (west ceiling) The sample embedded was not
complete and a description of the build-up is not available. Py–TMAH–GC/MS shows a drying
oil (P:S ratio suggests walnut oil or a mixture of oils was used). The azelaic acid dimethyl ester
peak is very intense and the remaining diacids peaks are relatively low, suggesting the oil had not
been pre-polymerized. This sample has a low PC1 score, indicating the highly oxidized, welldried paint film (Figure 3). To help the interpretation of this result, the sample was analysed by
SEM–EDX. It was determined that the pigment is finely ground bone black (Ca, P) mixed with
finely distributed umber (Fe, Mn) and traces of lead particles. Lead was also detected where no
discrete lead particles were found, suggesting that it was dispersed in the medium as a drier. Bone
black has poor drying properties (van Loon et al. 2005) and walnut or a mixture of drying oils
have slower drying properties than pure linseed oil; the addition of umber and a lead drier
wouldhave helped the drying of the oil and explain the high azelaic acid content.
2. Black paint sample 18A7 (PC1 score 3.16; PC2 score 0.99); painting 18: Frederik als krijgsman
die het water beheerst The binding medium analysis suggests a poorly dried sample, with a very
low A:P ratio. Octadecenoic acid was detected (Figure 1b). The presence of unsaturation in a
350-year-old paint film is surprising and suggests that this paint/pigment(s) film has antioxidant
properties. Observation of a crosssection of one sample from the same area indicates that the dark
paint is actually composed of two paint layers. These would not have been distinguished when
selecting particles for Py–TMAH–GC/MS analysis under the microscope and the combined
analysis is likely. The top layer is mainly composed of large bone black pigments particles; the
low density of the BSE image indicates a low content in heavy elements. However, a little lead
and a few umber particles could be detected. In the second layer, a different pigment mixture was
found, including a finer grade of bone black, a few discrete lead tin yellow particles and purely
organic pigment particles. No significant amount of umber was detected in this layer. This layer
may contain the poorly dried binding media described by the Py–TMAH–GC/MS.
3. White paint samples (18A6 and 18A8) (PC1 scores 1.09 and 0.73; PC2 scores 0.58 and 0.28,
respectively); same painting as (2) The samples were collected to relate the binding medium
composition to the physical properties. Sample 18A6 was taken from a thick impasto whereas
18A8 was taken from a smooth paint area. The binding medium composition, as far as the oil
component distribution is concerned, is almost identical: a well-dried linseed oil film. These two
samples have close PC1 scores (Figure 3), suggesting that the oil has the same origin and
chemistry. However, the appearance of the two white paints was different. SEM–EDX analysis of
two small samples revealed that although the pigment was in both cases lead white, the size
distribution of the particles was quite different (Figure 4). 18A8 is composed of finely ground
pigment particles (0.1–1.5 μm) of homogenous or at least continuous distribution. 18A6, on the
other hand, seems to be
Figure 4. BSE image of samples 18A6 (magnification: (a) 1000×; (b) 4000×) and 18A8
(magnification: (c) 1000×; (d) 4000×)
composed of a mixture of finely ground (0.1–1.5 μm) and large particles (10–25 μm). The
variation of particle size to modify paint properties is also visible in other samples from the same
painting (Eikema-Hommes et al. forthcoming). Groen (1997) has proposed that the paint
rheology is highly dependent on pigment particle size. White et al. (1994) mention, in the context
of a study of the binding media of Rembrandt’s circle, that ‘bodying’ paint with pigment is one
way of creating impasto. Our observations support these statements. The use of pre-polymerized
oil in the preparation of an impasto was not supported by our data.
Conclusion
Linseed oil was the main binding medium in the analysed samples from this group of paintings
showing different degrees of oxidation and levels of drying. Little evidence of pre-polymerization
of the oil was found but rather the use of driers as additives to paints containing pigments with
poor drying or antioxidant properties. Although all the paintings studied have a common history,
still a significant variance in binding medium composition was found. This can mainly be
assigned to the nature and interaction of the medium, with pigments and other additives. There
are differences in the way lead white and dark pigments influence the final composition of the
binding medium as is reflected in the different PC1 and PC2 scores that describe them (Figure 3).
The binding media properties here described mostly agree with published observations of binding
media from the same period and geographical origin (White et al. 1994, Mills and White 1988,
1989). The only exception being that contrary to what was observed in previous work (Mills and
White 1982), no evidence for the use of pre-polymerization of the oil was found in the studied
samples. The degree of light-ageing was reported to affect the amount of suberic acid (as well as
other diacids apart from azelaic acid (Schilling et al. 1996)), implying that the A:suberic ratio is
affected by the history of the painting. Therefore the interpretation of this ratio should be
cautious, especially when the history of the paintings is unknown.
Acknowledgements
We thank Annelies van Loon (FOM-AMOLF) for the help with SEM–EDX analysis, Maartje
Stols-Witlox (SRAL/ICN) for information on 17th century sources, and Lidwien Speleers (FOMAMOLF) and Gisela van der Doelen (ICN) who began the project ‘Comparative Studies of
Paintings in the Oranjezaal’. We also acknowledge the remaining members of the Molecular
Painting Studies Group for their comments on this manuscript. This project is part of the De
Mayerne Programme, which is supported by the Dutch Foundation for research (NWO, The
Hague) and is part of FOM programme 49 supported by FOM, Utrecht.
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