"An Astronomer" by Ferdinand Bol: Materials, Colour Change and Conservation
Author(s): MARIKA SPRING, NELLY VON ADERKAS, FLAMINIA RUKAVINA, DAVID PEGGIE
and Ferdinand Bol
Source: National Gallery Technical Bulletin, Vol. 38 (2017), pp. 76-96
Published by: National Gallery Company Limited
Stable URL: https://www.jstor.org/stable/45276232
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An Astronomer by Ferdinand Bol: Materials, Colour Change
and Conservation
MARIKA SPRING, NELLY VON ADERKAS, FLAMINIA RUKAVINA
AND DAVID PEGGIE
Ferdinand Bol's An Astronomer (NG 679) is signed and
servation treatment,4 the subject of this article. The
dated 'fBol*feci(t) / 1652' on the sheet of paper hanging
information they provided considerably extended what
off the edge of the table at the far left (figs 2 and 3).1
was known about the painting from the traditional
By this time Bol was well established in Amsterdam,
imaging techniques of X-radiography and infrared
becoming a citizen of the city in that year, having arrived
reflectography, especially when interpreted in combina-
there from his birthplace Dordrecht in around 1636.
tion with observations on the layer structure, pigment
The work is one of numerous dated works from the
composition and binding media from the limited
period beginning in 1642, just after he had left
number of paint samples.5 This relatively new analytical
Rembrandt's studio, and lasting until 1669, when he
imaging technique, not yet widely available but now
seems to have retired from painting having become
being increasingly used to examine paintings, proved
comfortably wealthy through his second marriage to
to be useful in supporting the cleaning of the painting,
the widow of a rich merchant in that same year.2
especially in understanding the degree of abrasion of
The astronomer, a popular motif in seventeenth-
the original paint in the black beret and cloak, and in
century Dutch painting, appears deep in thought as he
revealing the original modelling indicating the shape
sits with his right hand on his chin, a pose also used by
and form of the drapery, which was concealed beneath
Boi in other works such as his Portrait of a Scholar sitting
old overpaint. The maps were equally valuable in
at a Table (St Petersburg, State Hermitage Museum;
fig . 1), painted around the same time. He wears a black,
fur-trimmed robe and beret, the garments of a scholar,
over a gown painted in orange and red-brown hues
that suggest a changeant fabric. Around his waist is a
thickly painted belt richly decorated with gold embroi-
dery and colourful gems, with a central medallion that
most probably depicts the bust of a Roman emperor.
At the left of the painting are the usual attributes of a
scholar: books, papers, and two globes - a celestial globe
with a terrestrial globe behind it - placed on a damaskcovered table. The tablecloth, the curtain in the top left
corner, the background and the two globes hardly differ
in colour and before the recent cleaning were obscured
by a discoloured varnish creating an almost homogenous brown-green tone across nearly two-thirds of the
painting.
At the time of the conservation treatment the
Gallery was fortunate to have on loan a Bruker Jetstream
M 6 macro X-ray fluorescence (XRF) scanner, through
collaboration with the Delft University of Technology.3
The series of maps of chemical elements produced by
this instrument - associated with specific pigments -
made a significant contribution to the technical
examination carried out in conjunction with the con-
fig. l Ferdinand Bol, Portrait of a Scholar sitting at the Table, about
1650. Oil on canvas, 122 x 98 cm, St Petersburg, State Hermitage
Museum (s T3-767).
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
fig. 2 Ferdinand Bol, An
Astronomer ( NG 679), 1652.
Oil on canvas, 127 x 135 cm,
after conservation treatment.
fig. 3 Ferdinand Bol, An
Astronomer ( NG 679), 1652.
Oil on canvas, 127 x 135 cm,
before conservation treatment.
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
increasing our understanding of the colour changes due
of the folds of the fabric, obscured even further by the
to pigment degradation that have altered the appear-
hazy discoloured varnishes.
ance of this work.
The cleaning of the black areas was therefore
The painting was presented to the National Gallery
approached with caution, facilitated by the information
in 1 8 62 . A bill dating from February of that year records
provided by the macro-XRF maps, interpreted in con-
a restoration by Raffaele Pinti, although this was
junction with the analyses of paint samples (time did
probably only a minor intervention before it was placed
not allow, however, for scanning of the top right corner
on display in March.6 It was surface cleaned in 1876, a
and this is therefore missing in the maps). Conventional
regular necessity at this time in the gallery before meas-
technical imaging techniques were less useful in this
ures were taken to protect paintings from the polluted
case, as the paint was highly absorbing in these areas
London air.7 In 1886, when the painting was described
in the infrared reflectogram, and the X-ray image was
as 'much discoloured', the restorer Dyer was paid for
dominated by broad sweeping strokes across the canvas
'removing cracked varnish, repairing and restorations',
resulting from the application of the preparatory layers
at which time his usual practice would have been to
with a palette knife. Little of the upper paint layers was
apply a varnish based on mastic resin after treatment.8
visible, except in the thickest lead-white-containing
Relining is recorded in the early 1890s, followed by
areas such as the flesh and the white shirt. These broad
surface cleaning and application of another layer of
strokes are even clearer in the XRF map showing the
varnish, after which there was no further significant
distribution of lead, as the stretcher bars do not inter-
treatment until the cleaning and restoration carried
fere with the image (fig . 4).
out by J.C. Deliss, completed in February 1953. The
The paint cross-sections show that the canvas was
discoloured varnishes were removed, but the black paint
prepared with a double ground, comprising a first red-
of the cap and the cloak was found to be worn, at least in
brown layer containing red earth with a little umber,
the areas of the cleaning tests, and much of the repaint
coarse black and red lead or lead white, over which is
covering this abrasion was left in place. Complaints were
grey paint containing lead white, coarse black and a
made just a year later that the surface had again become
little brown umber (fig. 5). 12 This type of preparation,
dirty and cloudy, although this was improved to a
providing a neutral warm pinkish-grey surface for the
certain extent by a dry polish at that time.9
painter to work on, is common in paintings on canvas
When the painting was examined in 2015, before
of the mid-seventeenth century, including numerous
the recent cleaning and restoration carried out by
other works by Bol as well as many by Rembrandt dating
Flaminia Rukavina during her Patrick Lindsay Conser-
from the 1630s onwards. Karin Groen reported the
vation Fellowship in the National Gallery's conservation
presence of this kind of double ground on half of the
department,10 the layers of dammar varnish applied
153 paintings by Rembrandt that she examined, and on
by Deliss were discoloured and rather dull, with an
six of the fourteen paintings by Bol that were included
unsaturated patchy appearance.11 Retouchings covering
in the same study.13
damages had darkened, the most disturbing being those
Although these double grounds were essentially
along the long old tears, one running horizontally
similar, some variations were noted in the exact pigment
through the hand and throat of the sitter, and two
mixtures reported by Groen, including the choice of
others running vertically downwards away from it,
black pigments combined with lead white in the upper
through the fur and the orange robe to the painting's
greyish layer, among which some could be identified as
lower edge. The X-radiograph and the infrared reflecto-
lamp black of very fine particle size, bone black and
gram show the extent of the associated paint loss and it
charcoal black. A little red or yellow pigment seems often
was clear from close examination and from cleaning
to have been added, presumably to give a warmer grey
tests that the repaint extended over original paint, at
tone, and a little brown umber was also found in almost
least to some extent. The worn black areas were less
every case, as advised as a siccative in several of the
straightforward, since not only had Deliss not removed
mixtures for preparation of canvases included in the
all the old repaint, he had applied further glazing in
manuscript of Theodore de Mayerne (162 0-44). 14
'oil-colour' with an oil-and-varnish medium to disguise
Around half of the eight recipes that he had collected
the abrasion. The black hat and cloak had become flat
are for a grey-over-red double ground, most of them
shapes that lacked any modelling defining the structure
suggesting 'bolus' combined with a little umber for the
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
fig. 4 NG 679, lead XRF map (Ph-L) of the whole painting (excluding the top right corner, which was not scanned).
first layer, implying a clay-like red earth rich in alumino-
Coal has been identified in a growing number of
silicates, as was found on The Astronomer. 1 5 One recipe
paintings of the period now that its characteristics when
from a Wallonian primer residing in London instead
encountered as a pigment are better recognised; it is
specifies an initial preparation of 'brown-red or red-
generally coarsely ground, with either highly refracting
brown from England', followed by the instruction to
or deep glossy black particles in which sulphur and
prime with 'a second and last layer of lead white, well-
carbon and oxygen are the main elements detected by
chosen charcoal. Smale coales, and a little umber to
SEM-EDX analysis. While in the De Mayer ne Manuscript
make it dry more quickly'.16 The meaning of the term
and in a few other treatises a black pigment name
'Smale coales' or 'small coals' has not been fully clarified
appears that is specific enough to indicate a coal, it might
in earlier citations of this recipe,17 but it is interesting
also have been classified under the more general term
that it occurs together with 'great coales' in seventeenth-
of black earth.19 Coals, too, vary significantly in their
century accounts related to the coal trade between
composition. The large irregular and inhomogeneous
northern England and Europe, with the former indicat-
black particles in the grey priming on The Astronomer
ing a lower grade of this fuel.18
were found by SEM-EDX to contain predominantly
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
aluminosilicates such as feldspar and quartz, suggestive
of either stone coal (rather than the sulphur-rich coal
more often seen as a pigment) or a black shale (fig. 5).
Black shales are related to coals in that they are sedimentary rocks formed through decay of organic matter
under anoxic conditions. Like coal, they contain, in addi-
tion to silicaceous minerals, reduced free carbon and
often some pyrite (iron sulphide, FeS2), as is the case
here, where in one of the cross-sections some particles
with a characteristic yellow metallic lustre in which
iron and sulphur were detected by SEM can be seen
in the priming (see fig. 28). Certain unusual metallic
elements can be present, most probably associated with
the organic matter, and here a small but significant
quantity of vanadium was detected.20 This, and the
presence of pyrite, is diagnostic of the origin of the
pigment, distinguishing it from charcoal, which can
have a similar particle shape.
Analysis of a sample from the astronomer's robe
during the study carried out in 1992 had revealed
that asphaltum, another geologically modified organic
material, was a component of the black paint.21 This
translucent brownish black is mentioned in the De
Mayer ne manuscript for glazing of shadows, and was
perhaps used in this way here, since ivory black was the
predominant black pigment identified in a sample from
the robe.22 The overpaint seen in the cross-section was
sparsely pigmented with bone black. These pigments,
prepared by charring of animal bones or ivory, both con-
tain mainly calcium phosphate (in addition to carbon),
but could be distinguished using SEM-EDX analysis
by the greater quantity of magnesium in the pigment
prepared from ivory, present in the original paint.23
The Bruker M 6 Jetstream macro X-ray fluorescence
(XRF) scanner is not able to detect elements lighter than
silicon, so it was not possible from the data collected
from the painting before cleaning to discriminate
between the overpaint and the original in the robe using
this technique, on the basis of the difference in magne-
sium content. The phosphorous and calcium XRF maps
(fig. 8), however, confirmed the presence of these
elements from the ivory and bone black pigments in the
area of the robe, as would be expected, and although
the phosphorous map was noisy due to the low sensitiv-
ity of the scanner to this element, the calcium map
fig. 5 NG679, paint cross-section from the grey background at
the right edge, showing large particles of black in the second grey
proved to be useful in understanding the condition of the
paint. The brightest small zones show the filling in
preparatory layer. The silicon, potassium and aluminium EDX
the area of paint loss, presumably containing chalk,
maps indicate that these contain aluminosilicates of various
but the map also reveals the original modelling and
types.
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
structure of the folds of the cloak where it falls over
oil showing a low degree of oxidation that has not 'dried'
the astronomer's right arm, hidden before cleaning
(polymerised) in a conventional manner.26 This has
by the extensive overpaint and layers of discoloured
perhaps been exacerbated even further by the retarding
varnish. At least some of the overpaint is also made
antioxidant effect of asphaltum where it is present, a
visible as brighter touches where it is thickest, across
combination that would be consistent with the vulnera-
the upper part of the arm and shoulder, in a distribution
bility of the black paint during past cleaning, evident
that suggests that it was applied to disguise damage from
from the abraded state of the black cloak.
abrasion in the original black. The manganese and iron
SEM-EDX analyses also indicated that mixed with
from the umber pigment present in both of the ground
the ivory black in the cloak was a very small amount
layers appear strongly in this area of the cloak in the
of finely ground smalt, a blue pigment composed of a
XRF maps of these elements (figs 7 and 9), confirming
potash glass coloured with cobalt. Different grades of
the worn condition of the original paint, and correlating
smalt could be purchased that varied considerably in
with what is known about its state from the photographs
the intensity of colour; not all were intended for painting
taken after the 1952 cleaning, even if only part of the
and they were put to a variety of other uses.27 The
overpaint was removed at that time.
depth of colour depends on both the cobalt content
Ivory black was commonly used by Rembrandt
and the particle size (smaller particle size giving a paler
and his contemporaries and, along with other black
colour), so distinctions between grades could be based
pigments, was well known for being a poor drier. Many
on one or other, or both of these factors. It has been
seventeenth-century treatises mention strategies to
suggested that in certain contexts the smalt was not
ensure faster drying of oil paints when using problem-
added for its colour but as a bulking agent or a drier in
atic pigments, with blacks, red lakes and indigo being
oil paint (as with manganese, cobalt is known to catalyse
those most often cited. These included numerous recipes
the drying of oil), and it would be natural to attribute
for pre-polymerisation of the oil by heating or boiling
its presence in the poorly drying black paint to its
it with materials that could function as a siccative, as
potential to function as a siccative. Where smalt has
well as recommendations for driers to be included in
been found in very dark or pure black passages in other
the paint mixture on the palette either as an alternative
seventeenth-century paintings, for example in numer-
or in addition.24 A study investigating the use of driers
ous works by Rembrandt, it has usually been interpreted
in blacks in the paintings in the Oranjezaal (Huis ten
in this way.28 An important aspect of the value of macro-
Bosch, The Hague) - all dating from the middle of the
XRF scanning lies in its ability to enhance interpretation
seventeenth century - revealed that umber was a
of what can be deduced from a limited number of paint
component of the black paint in 14 of the 17 works
samples by revealing the distribution of a material
examined, suggesting that it was common practice to
across the painting. In the cobalt XRF map (fig. 10) it
incorporate a small amount of this pigment (manganese
can be seen that the smalt is not spread evenly across
being a well-known siccative for oil). In Bol's Astronomer ,
the black cloak (as would be expected for a drier) but is
although a small amount of umber was used in both the
present in greater amounts in the lighter areas of the
red and grey preparatory layers, it was not included in
modelling, where it seems to have been added to give
the black paint of the cloak. SEM-EDX mapping of a
the paint a slightly lighter greyish tone in the highlights
cross-section from the cloak instead found some lead
on the loose folds of the wide velvety black sleeves of
dispersed throughout the black layer. This, and the lead
the cloak. Before conservation treatment the structure
soaps (carboxylates) detected by FTIR, suggest that Bol
of the cloak was almost hidden by the overpaint, and
was relying on the use of a boiled oil prepared from a
although the condition of this area meant that in this
recipe that included a lead compound. GC-MS analysis
cleaning (as in the last one) it was decided that much of
confirmed that the binder was heat-bodied linseed oil. In
it should be left in place, much was gained by a careful
the Oranjezaal study, even though at least one drier and
thinning especially in the areas of the lighter folds,
more commonly two in combination were present, there
guided to a great extent by the cobalt and calcium
still seemed to be wide variations in the degree of oxida-
XRF maps.
tive drying of the black paints.25 The low azelate to
Autoradiography of paintings by Rembrandt has
palmitate ratio seen in the GC-MS analyses of the black
similarly hinted that smalt in dark passages was not
in the painting by Bol might similarly suggest a drying
always being used only as a drier, through revealing
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
variations in its distribution across a painting that relate
to modelling or to the composition. This was most easily
seen in the images locating arsenic, an element that is
almost always present in the pigment and that is associ-
ated with the cobalt ore used in its manufacture. The
pale greyish or even brownish appearance of the smalt
seen in paint samples from the same paintings has been
taken to imply that it was its effect on optical or physical
properties of the paint that was appreciated.29 Smalt
is an inherently unstable pigment, however, being a
potash glass, and to fully evaluate its role in the paint
in each case it is useful to distinguish between that
which has degraded and that which is a low grade that
was always weak in colour. When smalt deteriorates,
potassium from the potash glass leaches from the
particles into the binding medium, a process encouraged
by humid conditions. Potassium soaps are formed by
reaction with free fatty acids in the oil, which can then
migrate towards the surface of the paint where further
interactions occur with air-borne pollutants and with
fig. 6 NG 679, detail showing the sitter's sleeve.
other mobile fractions of the paint such as lead soaps,
resulting in a crust made up of complex potassium-
based salts that gives a blanched appearance most
disfiguring in the darkest paints.30 The colour in smalt
is due to tetrahedrally coordinated cobalt ions in the
glass structure, stabilised by potassium ions, so as
potassium is lost the equilibrium shifts towards an
octahedral conformation resulting in loss of colour.31
Well-preserved smalt contains 12-15 weight per cent
K20, while that which has degraded often contains
levels as low as only 2-3 weight per cent K20, making
quantitative SEM-EDX analysis a useful tool for determining whether the smalt has indeed changed and was
therefore originally more blue. 32
Quantitative elemental analysis of the smalt
particles in the cloak was a challenge as they are only a
few microns in size, but the results indicated that the
potassium levels were very low, at around 0.3 weight
per cent K90. They are therefore certainly deteriorated,
as might be expected given that the tiny particles have a
large surface area for reaction with the oil. The cobalt
content can give an indication of the original intensity
of colour, here measured as around 2 weight per cent
fig. 7 NG 679, manganese XRF map (Mn-K) of the detail shown
in FIG. 6.
CoO. Generally around 3-5 wt % CoO is found in
seventeenth-century smalt,33 but even a small amount
of cobalt has a considerable tinting effect on the glass,
although here this is offset by it having been ground to a
very fine powder, making the pigment lighter than one
with the same glass composition but larger particle size.
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
fig. 8 NG 679, calcium XRF map (Ca-K) of the detail shown
fig. io NG 679, cobalt XRF map (Co-K) of the detail shown
in FIG. 6.
in FIG. 6.
Nevertheless, it can be seen in the cobalt XRF map that
it was added to the ivory black pigment to create subtle
greyish highlights and it must therefore have possessed a
certain degree of colour that influenced the appearance
of the paint beyond increasing translucency or altering
the handling properties (fig. 10).
The cobalt XRF map of the full area of the painting
that was scanned shows how extensive BoFs use of
smalt was in other areas (fig . 26, p. 88), its deterioration
no doubt accounting at least partially for the many
passages of rather flat and now brownish paint. At least
some smalt is present on the celestial globe, where it
correlates with darker more greyish patches relating
to the figures depicted for the constellations, most
obviously in the figure of Bootes (the Ploughman),34 in
the drapery looped around his shoulders and body, as
well as in the area on which he stands (figs 11 and 12).
The calcium XRF map (fig. 13), deriving from either
yellow lake or ivory black, does not coincide with the
fig. 9 NG 679, iron XRF map (Fe-K) of the detail shown in fig. 6.
same locations as the smalt, but instead corresponds
with the more yellow-brown tones immediately either
side of the vertical loop on the stand and in the larger
area of shadow across the left of the globe. The only
other elements detected by macro XRF scanning in the
areas of smalt in the globe were those associated with
it, such as potassium and nickel,35 suggesting it was
not mixed with other pigments and a blue colour was
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
intended. Deterioration of the smalt-containing paint to
a brownish grey has made the features on the globe so
similar in tone to the overall yellow-brown colour that
they are now almost invisible.
The strongest cobalt signal is in a small patch to the
right of the sitter's shoulder (figs 10 and 26) - another
area of the composition that is now rather indistinct,
but perhaps relating to some fabric on the back of the
sitter's chair - and in the tablecloth, where variations in
the amount present according to the modelling reveal
how much has been lost of the original structure of the
fig. il NG679, detail showing the globe.
light and shade across the folds (figs 14-18). A sample
from the deepest greenish-grey shadow immediately to
the left of the outer contour of the astronomer's cloak
confirmed that smalt is the main component of the paint
(fig. 19), recognised only through SEM-EDX analyses
because only vestiges of the original colour were perceptible as a greyish hue in a few particles. The backscattered
electron SEM image revealed the characteristic jagged
shape of the smalt particles, and EDX mapping indicated, as expected for the degraded pigment, that they
are rather low in potassium, with only a few containing
enough to register in the map. A considerable quantity
of calcium-containing particles were also present in
the paint, and although a few could be identified as
ivory black (calcium phosphate), it is mainly present
fig. 12 NG679, cobalt XRF map (Co-K) of the detail shown in
FIG. 11.
as calcium carbonate. This can only be interpreted as the
substrate of a translucent yellow lake, although HPLC
analysis was not able to detect any dyestuff, perhaps
because it is another unstable pigment with a tendency
to fade.36 A small amount of a red lake on an alumina
substrate was also found, together with some yellow
earth (as indicated by the iron EDX map) finely dispersed
throughout the paint. A second sample, from the thicker
greyish-yellow on the very corner of the table, appeared
a much brighter yellow in the paint cross-section (fig.
20) and contained a similar mixture but without the
ivory black and red lake that was presumably added to
give a darker hue to the paint in the shadow.
The smalt in the tablecloth has a similar cobalt
content of about 2-3 wt % to that in the black cloak, but
fig. 13 NG 679, calcium XRF map (Ca-K) of the detail shown in
FIG. 11.
is of significantly larger particle size and would therefore
originally have been a stronger blue, indicating that
Bol used a different higher grade of the pigment. Again,
the low levels of potassium in the pigment make it
certain that it has altered and has lost its original inten-
sity of colour, so that this mixture of a blue pigment
with yellow in the form of a bright yellow earth and a
deeper yellow lake must have been intended to be green,
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
the yellow earth being the only stable pigment and
therefore responsible for the residual yellowish tone of
the area.37 The calcium XRF map is difficult to interpret,
as it shows the distribution of both the yellow lake and
bone black, but there appears to be more in darker areas,
where the cobalt signal is slightly weaker. Very little
calcium seems to be present in the lightest fold running
from the corner of the table, but here the signal is
fig. 14 NG 679, detail showing the tablecloth.
perhaps being blocked by the thicker yellow earth-rich
paint on top, evident in the iron XRF map (fig . 17). The
iron map gives a suggestion of the pattern and texture
of the cloth, especially at the corner, although, as elsewhere in the painting, it includes a strong contribution
from the ground layers, particularly in the right hand
corner of the detail.
More unexpectedly, the XRF scanning detected
some copper, located in small discrete touches that are
fig. 15 NG 679, calcium XRF map (Ca-K) of the detail shown in
FIG. 14.
also part of the pattern, and in parallel strokes down the
fold hanging from the corner, sometimes corresponding
to what seems to be deep brown patches of paint (fig.
18). No copper pigments were present in the two cross-
sections from the tablecloth, but in the sample taken
for analysis of the dyestuff some calcium soaps and
also possibly some copper soaps were detected by FTIR
fig. 16 NG 679, cobalt XRF map (Co-K) of the detail shown in fig.
14.
fig. 19 NG 679, cross-section of a paint sample from a deep
shadow at the right of the tablecloth.
fig. 17 NG 679, iron XRF map (Fe-K) of the detail shown in fig. 1 4.
fig. 20 NG 679, cross-section of a paint sample from a highlight
fig. is NG 679, copper XRF map (Cu-K) of the detail shown in
on the fold in the cloth where it hangs from the corner of the
FIG. 14.
table.
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
that a similar vermilion-containing mixture was used
for other orange-red areas in the books, globes and in
flesh paint (especially the cheeks and lips), as well the
small red jewels on the braid on the hat (fig. 28).
However, this pigment is concentrated mainly in the
astronomer's orange-red robe, which has deep reddish
shadows and bright orange-red highlights applied in
fig. 21 NG 679, cross-section of a paint sample from the deepest
the same distinctive zigzagging strokes seen in the
shadow of the curtain at the right edge of the painting
curtain. The mercury map also reveals the only pentiment in the painting, where the vermilion-containing
brushstrokes can be seen to extend underneath part of
microscopy.38 The latter must have been formed by a
the astronomer's left sleeve, showing that its bottom
reaction of a copper-containing pigment with fatty acids
edge was originally at a steeper angle, and it has been
irl the oil, and would therefore be an indication of the
shifted downwards, a change which is also just percepti-
use of verdigris rather than azurite, since it is more
ble in the infrared reflectogram.
likely to have interacted with the binder.
The appearance of the curtain in the upper left
corner has likewise suffered due to the deterioration of
Analysis of a sample from a reddish-orange highlight near the top of the man's thigh, where it runs over
the dark red, identified the pigments in the mixture with
both smalt and lake pigments, although the complexity
more certainty as yellow earth, vermilion and yellow
and variety of the mixtures employed makes it more
lake on a chalk substrate, with a little lead white and
difficult to speculate about its original appearance. The
ivory black (see fig. 29). The deep red and more
only paint sample analysed, from the left edge just
translucent paint beneath the orange highlight in the
above the globes in an area of deep shadow that is now
cross-section contains red lake, more easily seen by the
dark grey-brown, showed two layers in cross-section, the
pink appearance of the particles when viewed under
upper one lighter and perhaps faded; they are similar
ultraviolet illumination. The dyestuff in the pigment was
in composition, containing smalt, a lake on a calcium-
identified as Mexican cochineal,40 while the substrate
containing substrate, yellow earth and ivory black, as
includes both aluminium and calcium (with the latter
well as a small amount of vermilion and a little lead
as calcium sulphate), and a considerable quantity of
white (fig. 21). The XRF maps of the curtain (figs
potash alum (potassium aluminium sulphate), which is
23-25) bring out distinctions in the distribution of the
perhaps present because it was added in excess and not
pigments, visualising details that are now almost invisi-
fully neutralised during the making of the pigment.41
ble except under bright lighting conditions. The iron
Some of the calcium is present as calcium carbonate,
XRF map (fig. 24) shows the presence of earth pig-
implying (given the results from other areas of the
ments throughout the area, the analysis of the sample
painting) the presence of some yellow lake to shift the
suggesting a yellow rather than red variety, used in
colour towards a more orange-red hue. The calcium XRF
greater quantity in the lighter folds to the right. The
map confirms the high levels present in the whole of the
calcium XRF map, as in the tablecloth, shows the distri-
robe (see fig. 30), especially in the darker areas, where
bution of both ivory black (used here in the shadows)
both the red lake (with some calcium in the substrate)
and yellow lake (fig. 23). It must be the latter that is
and yellow lake contribute. The deep reds have a slightly
present in the lively zigzagging thin strokes of paint
dull hue that suggests a certain degree of fading; the
that suggest a pattern on the cloth, since they also
robe as a whole is rather similar in tone to the broad
appear in the mercury XRF map indicating a mixture
band of the fur lining of the cloak, visible where its
with vermilion in this orange-red paint (fig. 25). An
edge is turned back along the figure's right side, a lack
impression, at least, is given of a more colourful curtain
of contrast that may not have been intended but that
intended to appear more luxurious, like those in other
has developed over time.
paintings by Bol, such as Rembrandt and his Wife Saskia
The remaining areas of The Astronomer were painted
(Royal Collection) 39 or Lady with a Fan (National Gallery,
with a similar, limited palette, which could be partially
London; NG 5656).
identified from the XRF maps. The sitter's flesh tones (in
The mercury map of the whole painting indicates
his face and hands) include a mixture of lead white and
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
fig. 22 NG 679, detail showing the curtain at the top left of the
fig. 24 NG 679, iron XRF map (Fe-K) of the detail shown in fig. 22
painting.
fig. 23 NG 679, calcium XRF map (Ca-K) of the detail shown in
fig. 25 NG679, mercury XRF map (Hg-K) of the detail shown
FIG. 22
in FIG. 22
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
fig. 26 NG 679, cobalt XRF map (Co-K), full mosaic.
vermilion, although his left hand also contains an
seen in the XRF maps, however, since the tin K-alpha
iron-based pigment, presumably red earth. The irises
line overlaps with that of calcium, and while this
of his brown eyes were painted using an earth-based
difficulty could be overcome by mapping of the tin
pigment, as can be seen from the bright semi-circular
L-alpha line, the sensitivity of the detector in the high
shapes that they make in the iron map (see fig . 3 1). The
energy region is low.
fur of the figure's sleeve was painted in short, stippled
In the background, differences in paint composition
strokes using a mixture that seems to have included
are evident in the XRF maps between the back wall
ivory black and vermilion. The same combination of
and the architectural column in the middle, a distinction
pigments was used in the wood of the globes. The elabo-
that was hardly visible before cleaning but that became
rate gold-embroidered belt includes earths and yellow
more evident once the discoloured varnish had been
lakes, as does the braid on his hat, which is decorated
removed. The slightly lighter grey-brown of the archi-
with gems made with small, bright dots of vermilion.
tecture is stronger in the mercury map, and must
The highlights probably made in lead-tin yellow are not
therefore include some vermilion, while the deeper
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
fig. 27 NG 679, potassium XRF map (K-K), full mosaic.
grey-brown of the back wall includes more calcium,
brown and yellow-brown hues to be visible, but another
most probably indicating a greater amount of ivory
issue that had to be addressed during the retouching
black in the paint. No sample was available from this
and varnishing after cleaning was the blanching at
area to confirm the precise mixture that was used, but a
the surface seen in numerous areas of the painting,
sample from the architecture at the right edge identified
especially evident in the lower part of the tablecloth, the
lead white, ivory black, some vermilion, yellow earth,
globes and the back wall to the right of the curtains.
umber and a little red lake, as well as some of the blue
In the tablecloth and the globes, where the paint is rich
iron phosphate pigment vivianite, a complex mixture
in smalt, this is almost certainly caused by the crusts
that includes several iron-containing pigments that
that form as a result of migration of potassium from
are not distinguished by the XRF scanning.
the pigment, a phenomenon mentioned above. Yellow
Removal of the discoloured varnish made a signifi-
lakes and bone or ivory black are also associated with
cant improvement to the legibility of the composition
blanching, and they too may have contributed here to
by allowing the subtle variations in the nuanced grey-
this problem in both the tablecloth and the globes,
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
fig. 28 NG 679, mercury XRF map (Hg-L), full mosaic.
but more especially in the back wall where the paint does
not seem to include smalt.42 In some of the crosssections a thin layer is present at the surface in which
lead, potassium and sulphur were detected by SEM-EDX,
probably as the lead potassium sulphate that has been
identified in the crusts that have been analysed during
studies of this degradation phenomenon for all three
of these pigments.43 It may account for the very patchy
fig. 29 Cross-section of a paint sample from a highlight in the
figure's red robe where it runs over a deep shadow
distribution across the tablecloth in the potassium XRF
map, which seemed to be stronger than was expected,
given the low levels of potassium detected by EDX in the
smalt particles themselves deeper down in the paint in
a cross-section, and which does not seem to be related
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
fig. 30 NG 679, calcium XRF map (Ca-K), full mosaic.
to variations in the paint mixtures according to the
which played an important role in improving the
modelling (fig . 2 7). The other blanched areas also seem
confidence in the interpretations. The samples, however,
to correlate with the potassium map, perhaps again
gave only a limited view of the extensive colour changes
correlating with a potassium-containing crust at the
that have occurred in this work due to Bol's liberal use
surface.
of the unstable pigments smalt and yellow lake in
The case study presented here makes evident the
almost every part of the composition. The contrasts
value of macro-XRF scanning in addressing questions
and distinctions between the colours have been lost, but
arising during conservation treatment, especially in this
the XRF mapping results give an impression of the rich
painting, in which the materials and technique that
and varied fabrics that Bol intended to depict, including
the artist has used meant that conventional imaging
the elaborate patterned curtain that was perhaps made
techniques were not useful. The observations that could
of velvet, the changeant orange and red robe of the figure
be made were augmented to a great extent by the detailed
that was perhaps a shot silk and the damask-patterned
analyses of the paint samples carried out alongside,
tablecloth that is now a prominent dull yellow at the
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
fig. 31 NG 679, iron XRF map (Fe-K), full mosaic.
front of the picture space. The luxury of the astrono-
Acknowledgements
mer's surroundings that this new knowledge of the
The authors are extremely grateful to Delft University of
original appearance evokes gives his melancholic pose a
Technology for loan of the Bruker M6 Jetstream macro
new dimension - one in which he contemplates not only
X-ray fluorescence (XRF) scanner, especially Dr Joris Dik,
the trappings of his profession, but also of his wealth.
Antoni van Leeuwenhoek Chair, Materials in Art and
Archaeology, Department of Materials Science and
Dr Annelies van Loon, Paintings Research Scientist at
the Rijksmuseum, Amsterdam.
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
Notes
cases new cross-sections were prepared from existing
unmounted paint fragments.
1 N. MacLaren, revised by C. Brown, The National Gallery
Catalogues: The Dutch School, London 1991, pp. 29-31.
2 MacLaren 1991 (cited in note 1). For a general biography
of Bol, see A. Blankert, Ferdinand Bol (1616-1680):
Rembrandt's Pupil, Doornspijk 1982.
3 The Bruker M6 letstream was the first commercially available macro X-ray fluorescence (XRF) scanner. This mobile
system, developed by Bruker Nano GmbH in close collaboration with Antwerp University and Delft University of
Technology, allows paintings to be scanned while secured
on an easel in front of the scanner, with no contact being
made with the paint surface. See M. Alfeld, J.V. Pedroso, M.
van Eikema Hommes, G. Van der Snickt, G. Tauber, J.
Blaas, M. Haschke, K. Erler, J. Dik and K. Janssens, A
mobile instrument for in situ scanning macro-XRF investigation of historical paintings', Journal of Analytical
Atomic Spectrometry, 28, 2013, pp. 760-7.
4 The Bruker M6 Jetstream macro-XRF scanner was
equipped with a 30 W Rhodium target micro-focus X-ray
tube attached to a motorised stage, with a silicon drift
detector. The beam was defined by a polycapillary optic,
which allowed a variable beam size. Scans were
carried out at 50 kV and a current of 600 'iA, with a 50
ms dwell time and 1000 pm step size. By slowly moving
the measuring head on the x-y stage, the painting is
scanned line by line, pixel by pixel. An area of 80 x 60 cm
is scanned in one session, typically lasting several hours.
Paintings of larger dimensions need to be scanned in
sections and the separate XRF images assembled into a
mosaic. The size of the picture necessitated nine scans in
total, which were stitched together using the image processing software NIP2. By recording the emitted X-ray
fluorescence radiation, the chemical elements present
in the paint (associated with specific pigments) can be
identified. With the Bruker M6, only elements heavier
than silicon can be detected. Since the technique not only
shows the chemical elemental distribution at the paint
surface, but also below, it can visualise changes in composition and provide information about the materials used.
As with X-ray and infrared imaging, the entire paint
layer stratigraphy contributes to the acquired XRF maps.
Surface layers may absorb part of the fluorescence radiation emitted by deeper layers. In particular, elements with
low energy radiation (such as phosphorus or potassium)
are more strongly absorbed than those with high energy
radiation (such as mercury [Hg-L] or lead [Pb-L]). A good
understanding of the layer build-up is therefore essential
for interpretation of the XRF data.
5 Four samples were taken by Joyce Plesters in 1969 and a
further ten samples in 1992 by Ashok Roy, in each case as
part of research on the technique of paintings from the
circle of Rembrandt. Analysis of the binding medium was
carried out in 1992 by Raymond White using Gas Chromatography; the results were published in R. White and
J. Kirby, 'Rembrandt and his Circle: Seventeenth-Century
6 See N ational G allery Conservation Dossier for NG 6 7 9 .
7 D. Saunders, 'Pollution and the National Gallery', National
Gallery Technical Bulletin, 21, 2000, pp. 77-94.
8 See R. White and J. Kirby, A survey of nineteenth- and
early twentieth-century varnish compositions found on a
selection of paintings in the National Gallery collection',
National Gallery Technical Bulletin, 22, 2001, pp. 64-84,
where analyses of the varnish on two paintings treated by
Dyer in 188 7 indicated that it was composed of a mixture
of mastic and dammar resins.
9 The conservation history is recorded in the National
Gallery Conservation Dossier for this painting.
10 A more detailed account of the cleaning and restoration is
given in F. Rukavina, M. Spring, N. von Aderkas, D. Peggie,
'Technical Analysis "and Conservation of Ferdinand Bol's
An Astronomer (1652) in London', in Ferdinand Bol and
Govert Flinck. New Research, ed. S.S. Dickey, Zwolle 2017,
pp. 160-7.
11 Deliss described applying two layers of watercolour
retouching in damaged areas, with dammar varnish
between them, followed by some further toning in 'oil
colour' in the blacks and a final spray coat of dammar; see
National Gallery Conservation Dossier for this painting.
1 2 The first red-brown layer is composed of a red earth which
was found by SEM-EDX analysis to be rich in aluminium,
present as aluminosilicates and some potassium feldspar
(Al, Si and K). The earth pigment includes a little titanium.
A very small amount of umber was confirmed through the
detection of manganese, and a little lead pigment was
detected, but it was difficult to determine whether it
was present as red lead or lead white, and in one sample
some coarse black pigment was present. The second
brownish-grey layer contains lead white, a little umber
and an inhomogeneous coarse black pigment (see note
20 for more detailed analysis of this black pigment).
13 K. Groen, 'Grounds in Rembrandt's Workshop and in
paintings by his contemporaries', in E. van de Wetering,
with contributions by K. Groen, P. Klein, J. van der Veen,
M. de Winkel, A Corpus of Rembrandt Paintings: IV, The Self
Portraits, Dordrecht 2005, pp. 318-34, and associated
Tables, pp. 660-77. See especially Table VII, pp. 675-7,
which includes information on the ground layers in 14
paintings by Bol dating from 1640 to 1667. Six of these
have double grounds of the type found on NG 679.
This type of double ground was not only used in the
Netherlands; it was also common elsewhere in Europe.
14 Groen 2005 (cited in note 12); M.J.N. Stols-Witlox,
Historical recipes for preparatory layers for oil paintings
in manuals, manuscripts and handbooks in North
West Europe, 1550-1900: analysis and reconstructions,
PhD thesis, Amsterdam School of Historical Studies,
2014, available at http://dare.uva.nl/searchPmetis.
record.id=430263 (accessed 23 August 2017).
15 See note 12 for details of SEM-EDX analysis of the red
ground layer.
1 6 une seconde & derniere couche de Banc de plomb, de Charbon
Dutch Paint Media Re-examined', National Gallery
de braise bien choisy. Smale coales, & un peu de terre dombre
Technical Bulletin, 15, 1994, pp. 64-78. Further samples
were taken of the varnishes, overpaint and original paint
for GC-MS and FTIR analyses during the 2015 conservation treatment. For the pigments and layer structure, the
existing samples were re-examined and further analyses
were carried out by SEM-EDX during the treatment
and afterwards to aid interpretation of the XRF scanning results. No new samples were taken, but in some
pour faire plus vistement seicher'; see J.A van de Graaf, Het De
Mayerne manuscript als bron voor de schildertechniek van
de barok, Utrecht 1958, p. 138. By this time artists could
have purchased ready primed canvases from the primeur ;
see Stols-Witlox 2014 (cited in note 14), pp. 219-21.
17 Groen 2005 (cited in note 13); Stols-Witlox 2014 (cited
in note 14).
18 J.U. Nef, The Rise of the British Coal Industry, vol. 2,
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Marika Spring, Nelly von Aderkas, Flaminia Rukavina and David Peggie
Abingdon 1932 (1st edn), second impression 1966.
Appendix F, An estimate of Scots Coale; 1670, State
papers, No. 252, General Register House, Edinburgh,
p. 409, gives tables of 'Great Coales' and 'Caching Smale
Coales' from different locations, including the relative
prices that they would fetch in Scotland and in Holland.
A footnote explains that most of the coal is exported to
Holland, with some to 'Ostend', 'a few Small Coales for
France and some Great Coales for London'.
19 M. Spring, R. Grout and R. White, "'Black Earths": A
Study of Unusual Black and Dark Grey Pigments used
by Artists in the Sixteenth Century', National Gallery
Technical Bulletin, 24, 2003, pp. 96-114.
20 Potassium feldspar (K, Al, Si detected) and other aluminosilicates were detected by SEM-EDX analysis within the
particles (see fig. 5), indicative of stone coal, a type of
of the oxidative drying process, thus the low azelate level
relative to palmitate suggests a lower degree of oxidation
of the binding medium compared to a well 'dried' film.
2 7 Pale grades were used as a laundry blue to function as an
optical whitener or to tint paper. See M. Spring, C. Higgitt,
and D. Saunders, 'Investigation of Pigment-Medium Interaction Processes in Oil Paint containing Degraded Smalt',
National Gallery Technical Bulletin, 26, 2005, pp. 56-70;
and M. Spring, V. Kugler and S. Bean, 'Quantitative energy
dispersive X-ray analysis of the blue pigment smalt in
the variable pressure scanning electron microscope',
Historical Technology, Materials and Conservation, SEM &
Microanalysis, eds N. Meeks, C. Cartwright, A. Meek and
A. Mongiatti, London 2012, pp. 114-22.
28 Roy and Kirby 2006 (cited in note 23), pp. 45-7.
29 P. Meyers, M.W. Ainsworth and K. Groen, 'Chapter 2:
black shale, or of another type of black shale. Spot
Pigments and other painting materials', in Art and
analysis sometimes detected a little titanium and/or
vanadium in addition, which again can be found in black
shales (including those with an asphalt content, as well as
stone coals). In the rest of the layer some quartz was also
present, and small yellowish particles identified as pyrite
Autoradiography: Insights into the Genesis of Paintings
by Rembrandt, Van Dyck, and Vermeer, New York 1982,
pp. 101-4, especially p. 102.
30 Spring, Higgitt and Saunders 2005, (cited in note 27).
31 L. Robinet, M. Spring, S. Pagès-Camagna, D. Vantelon and
N. Trcera, 'Investigation of the Discoloration of Smalt
Pigment in Historic Paintings by Micro-X-ray Absorption
Spectroscopy at the Co K-Edge', Analytical Chemistry, 83,
2011, pp. 5145-52.
32 See M. Spring, 'Fading, darkening, browning, blanching:
a review of our current understanding of colour change
and its consequences in old master paintings', in Colour
Change in Paintings, eds R. Clarricoates, H. Dowding and
A. Gent, London 2016, pp. 1-14.
by SEM-EDX (FeS2), both of which must be associated
with the black pigment. Pyrite is a common component
and is associated with formation under anaerobic reducing conditions. See Y.-J Hu, Y.-M Zhang, S.-X. Bao and T.
Liu, 'Effects of the mineral phase and valence of vanadium
on vanadium extraction from stone coal', International
Journal of Minerals, Metallurgy and Materials, 19 (10),
2012, pp. 893-8; and H.L.O. Huyck, 'When is a metalliferous black shale not a black shale?', in Metalliferous Black
Shales and Related Ore Deposits, U.S. Geological Survey
Circular 1058, Denver 1989, pp. 42-56.
2 1 White and Kirby 1994 (cited in note 5 ) .
22 Van de Graaf 1958 (cited in note 16), p. 146.
2 3 Ivory and bone black are distinguished by the presence of
magnesium, which is found in greater amounts in ivory
black. A. van Loon and J.J. Boon, 'Characterization of the
deterioration of bone black in the 1 7th century Oranjezaal
paintings using electron-microscopic and micro-spectroscopic imaging techniques', Spectrochimica Acta Part B, 59,
2004, pp. 1601-9. A. Roy and J. Kirby, 'Rembrandt's
Palette', Art in the Making : Rembrandt, eds D. Bomford, J.
Kirby, A. Roy, A. Rüger and R. White, London 2006, p. 44.
24 See E.S.B. Ferreira, L. Speleers and J.J. Boon, ' 'Le noir
ne seiche jamais sans addition", methods to ensure the
33 The particle size was measured in the backscattered
SEM image. Standardless quantitative EDX analysis was
carried out in an EVO-MA 10 variable pressure SEM using
an Oxford Instruments X-Max 80 mm2 EDX detector
and INCA software. For the methodology and accuracy of
the quantitative measurements, tested using Corning
Museum of Glass reference standards, and for a discussion
of trends in the cobalt content of smalt in different
periods, see Spring, Kugler and Bean 2012 (cited in note
27). The values measured were, as weight % oxide (normalised): 0.6 % Na20, 0.9 % A1203, 69.8 % Si02, 0.2 %
K20, 1.8 % CaO, 2.2 % FeO, 2.1 % CoO, 0.5 % NiO, 4.1 %
As203, 1.5 % Bi203. These are an average of the measurements taken from every particle in the paint sample from
the black cloak, but the small particle size means that the
drying of black paints in the Seventeenth-Century Oranje-
results are significantly affected by the matrix. Neverthe-
zaal ensemble', 5th International Symposium : Painting
less, it is still possible to draw broad conclusions from
them. The low potassium content indicates that the smalt
Techniques ; History, Materials and Studio Practice, Rijksmu-
seum, Amsterdam, 18-20 December 2013, ed. A. Wallert,
Amsterdam 2016, pp. 123-32. Ferreira et al. provide a
survey of seventeenth-century primary sources such
as the De Mayerne Manuscript on painting with black
pigments in oil, including methods for improving drying
properties.
25 In their study of the blacks in the seventeenth-century
paintings in the Oranjezaal, Ferreira et al. found that they
all included driers, but still exhibited a wide range of
drying effects compared with the relatively consistently
well-dried white passages. See Ferreira, Speleers and Boon
2016 (cited in note 24).
26 The fatty acid ratios (palmitate/stearate = 1.4, azelate/
palmitate = 0.4, azelate/suberate = 3.5) determined by
GC-MS analysis of a sample of black paint from the figure's robe, removed from within a cleaning test, indicate
that the paint medium was a heat-bodied linseed oil. The
diacids such as azelate and suberate are the final products
is degraded.
34 See MacLaren 1991 (cited in note 1). For a betterpreserved depiction of globes in seventeenth-century
paintings, see Thomas de Keyser's Portrait of Constantijn
Huygens and his (?) Clerk, 1627, National Gallery, London
(NG 212).
35 Nickel is a common constituent of smalt, deriving from
the cobalt ore used in its manufacture. See Spring, Higgitt
and Saunders 2005 (cited in note 2 7).
36 D. Saunders and J. Kirby, 'Light-induced Colour Changes
in Red and Yellow Lake Pigments', National Gallery Techni-
cal Bulletin, 15, 1994, pp. 79-97.
37 See note 33 for methodology and for the results from
the smalt in the black cloak. The results of quantitative
analysis of the smalt in a paint sample from the tablecloth
were, measured as weight % oxide (normalised): 0.2 %
Na20, 2.2 % A1203, 78.8 %Si02, 6.9 %K20, 1.8 %CaO, 2.6
% FeO, 2.3 % CoO, 0.6 % NiO, 3.5 % As203, 1.1 % Bi203.
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An Astronomer by Ferdinand Bol: Materials, Colour Change and Conservation
38 The FTIR spectrum obtained from a sample of paint
removed from the tablecloth showed two sharp absorbance bands, centred around 1576 cm1 and 1541 cm1.
These are most probably related to the carbonyl stretching
modes of metal carboxylates. Calcium stearate has two
sharp bands associated with the carbonyl stretch centred
around 1575 cm1 and 1540 cm1, for example, and is
perhaps therefore the main soap present, although there
may also be some contribution from copper soaps (copper
stearate has a single carbonyl stretching band centred
around 1586 cm1). In paint such as this, however, which
contains several pigments that can form soaps, it is
difficult to assign FTIR bands with certainty. The mixture
includes smalt, which reacts with oil to form potassium
soaps, yellow lake on a calcium carbonate substrate,
which would be capable of reacting to give calcium
soaps and a smaller amount of an unidentified coppercontaining pigment (seen only by macro-XRF scanning)
likely to be either verdigris or azurite, the former in
particular almost always reacting to form copper soaps.
39 R. de Sancha, 'Rembrandt and Bol in The Royal Collection.
A close examination of Rembrandt's Portrait of Agatha
Bas and Ferdinand Bol's Rembrandt and his Wife Saskia ',
in Rembrandt Now: Technical Practice, Conservation and
Research', forthcoming London 2018.
40 Mexican cochineal was identified using HPLC by the
presence of carminic acid.
41 For association of potassium with red lake pigments see
J. Kirby, M. Spring and C. Higgitt, 'The Technology of Red
Lake Pigment Manufacture: Study of the Dyestuff Substrate', National Gallery Technical Bulletin, 26, 2005,
pp. 71-87. P. Noble, A. van Loon, G. Van der Snickt,
K. Janssens, M. Alfeld and J. Dik, 'Development of new
imaging techniques for the study and interpretation of
late Rembrandt paintings', in Preprints, ICOM-CC 17th
Triennial Conference, 15-19 September 2014, Melbourne,
Australia, ed. J. Bridgland, Paris 2014, art 1310 (8pp). For
variations in the amount of potassium in red lakes among
Dutch seventeenth-century paintings see: E.M. Gifford
and L. Deming Glinsman, 'Collective style and personal
manner. Materials and techniques of high-life genre
painting', in A.E. Waiboer, with A.K. Wheelock Jr. and
B. Ducos, with contributions by P. Bakker, 0. Buvelot, E.M.
Gifford, L. Deming Glinsman, E. Schavemaker, E.J. Sluijter
and M.E. Wieseman, Vermeer and the Masters of Genre
Painting ; Inspiration and Rivalry, exh. cat., Musée du
Louvre, Paris, National Gallery of Ireland, Dublin and
National Gallery of Art, Washington DC, 22 October
2017-21 January 2018, New Haven, London and Dublin
2017, pp. 65-83 and 270-4.
42 For blanching associated with yellow lakes, see M. Spring,
'Pigments and Color Change in the Paintings of Aelbert
Cuyp', in Aelbert Cuyp, ed., A.K. Wheelock Jr, exh. cat.,
Washington DC, London and Amsterdam, 2001-2, Washington 2001, pp. 64-73; M. Spring and L. Keith, Aelbert
Cuyp's Large Dort : Colour change and conservation',
National Gallery Technical Bulletin, 30, 2009, pp. 71-85;
and Spring 2016 (cited in note 32). For blanching on the
surface of bone black paint films see A. van Loon, 'Chapter
3: The whitening of bone black in oil paint films', Color
Changes and Chemical Reactivity in Seventeenth-Century
Oil Paintings, PhD Thesis, FOM Institute for Atomic and
Molecular Physics (AMOLF), Amsterdam 2008.
43 See Spring, Higgitt and Saunders 2005 (cited in note 27)
and A. van Loon, P. Noble and J.J. Boon, 'White Hazes
and Surface Crusts in Rembrandt's Homer and Related
Paintings', in Preprints, ICOM-CC 16th Triennial Conference, 19-23 September 2011, Lisbon, ed. J. Bridgland,
Almada 2011, paper 1316 (10 pp).
NATIONAL GALLERY TECHNICAL BULLETIN VOLUME 38 | 95
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LOS ANGELES, CALIFORNIA
Photographie credits
All images © The National Gallery, London, unless credited otherwise
© Los Angeles County Museum of Art, California: fig. 6, p. 9.
below.
MADRID
AMSTERDAM
Museo Nacional del Prado, Madrid
© Rijksmuseum, Amsterdam: fig. 3, p. 58; fig. 4, p. 58; fig. 5, p. 58.
BAYONNE
© Photo MNP / Scala, Florence: fig. 1, p. 56.
NAPLES
Museo Nazionale di Capodimonte, Napoli
Musée Bonnat-Helleu, Bayonne
© RMN-Grand Palais - René-Gabriel Ojéda: fig. 24, p. 45.
BERGUES
© Musée du Mont-de-Piété, Ville de Bergues: fig. 16, p. 40.
FRANKFURT AM MAIN
© Photo Scala, Florence - courtesy of the Ministero Beni e Att.
Culturali: fig. 7, p. 9.
ROTTERDAM
Museum Boijmans Van Beuningen, Rotterdam (Koenigs Collection):
fig. 11, p. 37.
Städel Museum
© Städel Museum - U. Edelmann / ARTOTHEK: fig. 15, p. 40: fig. 35,
p. 52.
ST PETERSBURG
With permission from The State Hermitage Museum, St Petersburg:
Photograph © The State Hermitage Museum. Photo by Yuri
LONDON
Molodkovets: fig. 1, p. 76: Photograph © The State Hermitage
The British Museum, London
Museum. Photo by Vladimir Terebenin: fig. 29, p. 72.
©The Trustees of The British Museum: fig. 18, p. 64; fig. 23, p. 45: fig.
• 25, p. 46: fig. 28, p. 46.
Whereabouts unknown
Fig. 6, p. 59.
96 I NATIONAL GALLERY TECHNICAL BULLETIN VOLUME 38
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