Technical Research Bulletin
Technical Research Bulletin
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VOLUME 2 2008
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Publications
VOLUME 2 2008
11/10/2008 13:24:12
The photo-ageing behaviour of selected
watercolour paints under anoxic conditions
Capucine Korenberg
Summary The photo-ageing behaviour of the artists’ pigments chrome yellow, alizarin crimson, orpiment,
realgar, Prussian blue and Antwerp blue was examined under anoxic conditions. Watercolour paints were
applied on Whatman filter paper and photo-aged in air, airtight enclosures and anoxic enclosures using ultraviolet-filtered light for more than 30 Mlux.hours. It was observed that anoxic conditions had no effect on the
fading process for the chrome yellow and realgar paint samples. In contrast, the alizarin crimson and orpiment
paint samples faded considerably less under anoxic conditions than in air. Airtight enclosures were also found
to reduce fading for these samples, although not to the same extent as anoxic enclosures. This was probably
due to oxygen depletion inside the enclosures. When photo-aged under anoxic conditions, the Prussian blue
and Antwerp blue paint samples faded considerably more than in air. The same trend was observed for the
samples aged in airtight enclosures. In addition, all the samples of Antwerp blue paint faded significantly
more than those of Prussian blue.
INTRODUCTION
It has long been known that some materials, such as parchment or skin, degrade faster in the presence of oxygen; as a
result there are many culturally important objects displayed
in showcases filled with inert gases, such as the original
documents of the Constitution of India in the Parliament
Library in New Delhi or the Royal Mummies in the Egyptian
Museum, Cairo [1]. The use of oxygen-free environments to
reduce the photo-ageing of organic colorants has also been
investigated. The Russell and Abney report, published in
1888, studied the fading of 39 watercolour pigments in vacuo,
i.e. without air or moisture. A visual assessment concluded
that, apart from violet carmine, purple carmine, Prussian
blue, purple madder, sepia and vermilion, the pigments did
not fade significantly when exposed to sunlight [2]. Arney
et al. measured the rates of fading of 17 organic colorant
systems (oil paints, felt-tip markers, dyes on paper, dyes
on wool and watercolours) exposed to light from fluorescent lamps at different concentrations of oxygen in dry air
and reported a beneficial influence of low-oxygen environments, except in the cases of vermilion and Prussian blue oil
paints, which faded more rapidly under reduced oxygen [3].
The results of these studies are valuable, but further work
is needed to assess more thoroughly whether anoxic condi-
tions are beneficial to the display of pigmented surfaces [4].
Because the fading of the pigments is often assessed visually,
for example in the Russell and Abney report, it is difficult
to quantify and predict the effect of anoxic conditions on
photo-ageing, and it would be useful to repeat these experiments in a more systematic way. In particular, experiments
should be conducted using ultraviolet-filtered light, as
current practice in most museums is to exclude ultraviolet
(UV) light in displays of objects with pigmented surfaces.
The study reported here is a preliminary investigation into
the effect of UV-filtered light on paints under anoxic conditions and focuses on six watercolour paints. Chrome deep
(a deep shade of chrome yellow), alizarin crimson and
Prussian blue were selected as they are known to darken
or fade on exposure to light in air and are readily available.
The presence of an extender has been reported to have a
considerable effect on the lightfastness of Prussian blue in
air [5], so Antwerp blue, which is a mixture of Prussian blue
and an extender, was also investigated to examine the effect
of an extender in Prussian blue under anoxic conditions.
Finally, realgar and orpiment were included in the study as
their behaviours under anoxic conditions have never been
assessed. These pigments are of considerable interest to the
British Museum since they are present on many objects in
the collection, such as ancient Egyptian coffins and papyri.
49
CAPUCINE KORENBERG
PIGMENTS UNDER STUDY
A short description of the pigments under study is given
below, including the information available in the published
literature on their photo-ageing properties.
Chrome deep
Chrome deep is a variety of chrome yellow (lead chromate, PbCrO4). Lead chromate can exist in two crystalline forms, monoclinic and orthorhombic, and its colour
is related to crystal habit and particle size. Chrome yellow
has been reported to darken and become brown or greenish
when exposed to light [6], and it has been observed that
crystal size affects the lightfastness of lead chromate, with
large crystals being more stable than small crystals [7]. The
darkening of chrome yellow upon exposure to light is not
well understood, but it has been claimed that the colour
change could be due to its decomposition into a lead oxide
and a chromium compound [7]. Alternatively, it has been
suggested that the darkening of zinc yellow (zinc potassium chromate hydrate) could be due to an alteration in
the oxidation state of chromium or a pH-induced change
[8], and it is possible that similar mechanisms cause the
darkening of chrome yellow.
Alizarin crimson
Alizarin crimson is a synthetic pigment containing the
colorant alizarin, 1,2-dihydroxyanthraquinone. Alizarin is
the principal colorant in madder lakes, where it occurs in
conjunction with other anthraquinones, the ratio depending
on the species of madder used and the method of preparation. The moderate lightfastness of alizarin crimson and
alizarin lakes is well documented [9–11]. When exposed to
light from a Xenon arc lamp alizarin has been reported to
be converted to a yellow compound, which in turn becomes
colourless on further exposure [12].
Orpiment
Orpiment is an arsenic sulphide (As2S3) and has been
reported to become paler and colourless on exposure to
light as it converts to arsenic oxide (arsenolite, As2O3) [13].
Lee and Leach have shown that the rate of fading of orpiment depends on humidity as well as light level [14]. The
presence of UV light accelerates the rate of fading and high
relative humidities have detrimental effects.
Realgar
Realgar (α-AsS) has a deep orange-red colour and has been
observed to turn bright yellow upon exposure to light [13].
50
This fading of realgar is reported to be extremely rapid [15],
and was believed for many years to be due to the conversion of realgar to orpiment upon light exposure. However,
Douglass et al. have shown that the photo-degradation
product of realgar is pararealgar, a polymorph of realgar,
and that realgar transforms to pararealgar only when irradiated with light with wavelengths between 500 and 670 nm
[16]. The transformation from realgar to pararealgar also
occurs in the presence of heat: samples of realgar paints
left in the dark at approximately 38°C for 26 days became
bright yellow and pararealgar was detected on their surface
[17]. In accelerated photo-ageing tests, pararealgar eventually converts to arsenic oxide [17].
Prussian blue and Antwerp blue
Prussian blue is a complex chemical compound containing
iron ions in two different oxidation states, Fe(II) and Fe(III).
Because of its chemical structure, the behaviour of Prussian
blue under reducing or oxidizing conditions is complicated
(see for example [18]), and only an overview of its behaviour
is given here. When Prussian blue is chemically reduced, it
transforms to Berlin white, which has a similar structure to
Prussian blue but with all the iron sites occupied by Fe(II)
ions. The reduction of Prussian blue to Berlin white can also
be photo-induced. When Prussian blue is oxidized, it transforms to Prussian yellow, which has the same structure as
Prussian blue but with all the iron sites occupied by Fe(III)
ions. Pure Prussian blue is inherently stable to light and the
yellowish-grey appearance of aged Prussian blue oil paints
has been attributed to oxidation, rather than light fading
[5]. However, the presence of impurities, extenders or additives in Prussian blue paint affects its lightfastness and the
behaviour of Antwerp blue (Prussian blue containing an
inert extender) was assessed in the present study to examine
the effect of an extender under anoxic conditions. A notable
feature of Prussian blue pigments or dyes that have faded on
exposure to light is that they sometimes regain their colour
when they are subsequently stored in the dark, a phenomenon known as ‘phototropy’. Nevertheless, this recovery
process is not completely reversible; for further discussion
of the degree of colour recovery of Prussian blue in the dark
see [18].
EXPERIMENTAL
Paints
Realgar and orpiment were purchased as dry pigments from
Kremer and mixed with a solution of gum arabic and water
in the laboratory. The remaining pigments were purchased
as ready-made watercolour paints from Winsor & Newton.
It should be noted that the pigments used in the present
study are all modern and that historic pigments, which are
THE PHOTO-AGEING BEHAVIOUR OF SELECTED WATERCOLOUR PAINTS UNDER ANOXIC CONDITIONS
made by different processes and contain manufacturing
residues or contaminants, may show a different behaviour [5]. All the paints and dry pigments under study were
analysed by Raman spectroscopy prior to the experiments
to check their composition. A Jobin Yvon Infinity spectrometer with green (532 nm) and near infrared (785 nm)
lasers, with maximum powers of 2.4 mW and 4 mW at
the sample respectively, was used. Collection times varied
between 20 and 100 seconds, with at least five repetitions
used to produce each spectrum. The near infrared (IR) laser
was used to analyse the realgar paints as it does not cause
realgar to convert to pararealgar [15]; for the other paints,
the green laser was used.
The spectra obtained for all the paints matched reference
spectra of the corresponding pigments. Some pararealgar
was detected in the realgar pigment; this probably formed
upon exposure to light during the manufacturing process.
The Antwerp blue and Prussian blue paints were also
found to contain gypsum (CaSO4·2H2O) as an extender. As
expected, the peaks corresponding to gypsum were much
stronger for Antwerp blue than Prussian blue, indicating a
greater amount of gypsum in Antwerp blue than in Prussian blue. The Prussian blue and Antwerp blue watercolour
paints were also analysed using a Brucker ArtTAX X-ray
fluorescence spectrometer (voltage 50 kV and current
0.80 mA) to determine whether they were of the so-called
‘soluble’ Prussian blue variety, KFe[Fe(CN)6]·xH2O or of
the ‘insoluble’ variety, Fe4[Fe(CN)6]3·xH2O [18]. The presence of potassium in the Prussian blue and Antwerp blue
samples suggests that the pigments are of the ‘soluble’
variety.
Paint samples
The watercolour paints were applied on Whatman filter
paper as pigment-rich layers. Whatman filter paper was
selected because it had been shown not to discolour upon
photo-ageing [19]. To enclose samples in an anoxic environment, pouches of Mitsubishi EscalTM gas barrier film were
made using a Crosweld heat sealer. Escal film is a laminar
transparent film made of an inner layer of polythene, a
vacuum-deposited ceramic on a polyvinyl alcohol substrate
and an outer layer of polypropene. It is highly impermeable
to oxygen and moisture. The samples were inserted into the
Escal pouch together with Mitsubishi RPK oxygen scavengers. Oxygen level indicators, the colour of which changes
from blue to pink when the oxygen content drops to less
than 0.1%, were included in the pouches containing oxygen
scavengers. It was ensured that the oxygen indicators were
pink in the enclosure containing the anoxic samples before
starting the photo-ageing tests; it usually took several hours
for the indicators to become pink.
Three samples of each paint were prepared. A set of
samples of each paint was photo-aged without being
enclosed in a pouch; these samples are referred to as the ‘air
samples’ in the text. A second set of samples was photo-aged
in an Escal pouch with RPK oxygen scavengers (referred to
as ‘RPK samples’ in the text) and the last set of samples was
photo-aged in a sealed Escal pouch with no oxygen scavengers (referred to as ‘Escal samples’ in the text).
ACCELERATED PHOTO-AGEING TESTS
The samples were irradiated in a purpose-built lightbox
fitted with eight Philips 18W/930 TL-D 90 deluxe fluorescent lamps. These lamps are the same as those used in many
of the showcases in the galleries at the British Museum. The
UV radiation was filtered using a polycarbonate sheet and
the UV level measured inside the lightbox was in the range
1–2 μW.lumen–1. The light level was measured continuously using a Hanwell Luxbug data logger. The samples
were photo-aged for approximately 30 Mlux.hours, which,
according to the reciprocity principle, corresponds to a light
exposure of approximately 100 years at 80 lux for 10 hours
a day.
COLOUR MEASUREMENTS
The colour of the samples was monitored using a Minolta
CM-2600d spectrophotometer, which was set to measure
an area with a diameter of 8 mm. A template was used to
position the spectrophotometer at the same spot on the
surface of the sample each time a measurement was taken.
Each measurement was repeated four times and the average
value was recorded; where the samples were contained in a
pouch the measurements were taken through the Escal. The
results are expressed as CIE (Commission Internationale de
l’Eclairage) L*, a* and b* coordinates under the standard
illuminant D65 and using the 10° supplementary standard
observer [20]. In this system L* indicates lightness and
a* and b* are the chromaticity coordinates; +a* is the red
direction, –a* the green direction, +b* the yellow direction
and –b* the blue direction. The overall colour change, ΔE00,
was calculated using the CIE 2000 formula [21]. To give an
idea of the scale of change monitored, it has been reported
that a ΔE00 of approximately 1.5 corresponds to a colour
change that is visually perceptible [22].
PRELIMINARY TESTS
The effect of light on the Escal film was investigated to
establish whether it discoloured significantly. Two pieces
of Escal film were stapled onto Whatman filter paper and
photo-aged for 16 Mlux.hours. The Escal film samples did
not change colour noticeably: at the end of the experiment,
colour changes of 1.0 and 0.7 ΔE00 units were measured for
the two samples.
51
CAPUCINE KORENBERG
RESULTS AND DISCUSSION
Alizarin crimson
Chrome deep
The RPK alizarin crimson sample faded considerably
less than the air sample, as shown in Figure 2. While the
air sample became visibly paler with light exposure, the
colour change for the RPK sample was not perceptible. The
sample kept in an Escal pouch with no oxygen scavengers
faded significantly less than the air sample, an effect that
could be caused by oxygen depletion inside the pouch. The
aged samples were analysed using Raman spectroscopy
and their spectra were similar to the spectra of the unaged paints, even for the sample that had faded in air. As
for the aged chrome deep paints, the concentrations of the
compounds produced on exposure to light were possibly
too low to be detected, or the compounds were not Raman
active. The air samples did not show evidence of becoming
slightly more orange, as reported by Johnston-Feller et al.
[12]; this could be because UV-filtered light was used in
the present study.
Upon exposure to light, all the chrome deep samples darkened to a similar extent, as illustrated in Figure 1. The
change in colour was characterized by a decrease both in
L* and b*. As stated previously, chrome yellow pigments
have been reported to darken when exposed to light and the
decrease in L* observed here is in agreement with this. That
all the samples changed colour to a similar extent shows
that the absence of oxygen does not prevent the darkening
process, but also that anoxic conditions do not have an
adverse effect on the colourfastness of chrome deep. This
agrees with a 1951 study, which reported that the darkening
of chrome yellow on exposure to light occurs in the absence
of both oxygen and moisture [6; pp. 190–191]. The paints
were analysed at the end of the experiment using Raman
spectroscopy and the spectra obtained were similar to the
spectrum of the paint before ageing. It is possible that the
concentrations of the compounds produced on exposure to
light were too low to be detected or that these compounds
were not Raman active. Given the small amount of paint
present on a sample, identifying these compounds would
have required the use of more sophisticated analytical tools
such as X-ray photoelectron spectroscopy or chromatography. As the aim of the present study was to determine
whether an anoxic environment could reduce the effects
of photo-ageing, the identity of these compounds was not
investigated further.
Realgar
There was no significant effect from enclosing realgar
samples in Escal pouches with or without oxygen scavengers
compared to the air samples, see Figure 3. All the realgar
samples changed from bright orange to deep yellow after an
exposure of less than 1 Mlux.hour. The colour change was
characterized by a sharp drop in a* and sharp increases in
b* and L*; L* then decreased slowly and stabilized. When
figure 1. Colour change for the air, Escal and RPK samples of chrome deep paint
52
THE PHOTO-AGEING BEHAVIOUR OF SELECTED WATERCOLOUR PAINTS UNDER ANOXIC CONDITIONS
the aged samples were analysed at the end of the experiment using Raman spectroscopy with the near IR laser,
pararealgar was detected across the entire surface and no
realgar was found, indicating that anoxic conditions do not
prevent the photo-induced transformation of realgar into
pararealgar. This finding is consistent with the study by Lee
and Leach, which reported that consolidating realgar with
isinglass (a proteinaceous extract from the swim bladders of
tropical fish) to limit oxidation does not inhibit fading [17].
As discussed earlier, pararealgar has been found to convert
to arsenic oxide and this reaction was expected not to take
place under anoxic conditions. However, the air samples
figure 2. Colour change for the air, Escal and RPK samples of alizarin crimson paint
figure 3. Colour change for the air, Escal and RPK samples of realgar paint
53
CAPUCINE KORENBERG
did not appear to fade once realgar had transformed to pararealgar so it was not possible to observe whether this reaction was hindered under anoxic conditions. Very little has
been published on the conversion of pararealgar to arsenic
oxide and it may be that this is a very slow process.
Orpiment
As shown in Figure 4, the colour change of the orpiment
paint at the end of the experiment was considerably smaller
for the anoxic sample compared to the air sample. The air
sample became increasingly paler in colour when exposed
to light. This was associated with an increase in L* and a
decrease in both a* and b*. The fading of the air sample is
consistent with the transformation of orpiment to arsenic
oxide, which is colourless. Enclosing the orpiment paint in
an anoxic environment prevents fading to a large extent.
It was also noted that the Escal sample faded much less
than the air sample. The aged samples were analysed using
Raman spectroscopy after the tests and only orpiment was
detected. However, this does not necessarily mean that
arsenic oxide was not present, as arsenic oxide does not give
a strong Raman signal [15].
Prussian blue and Antwerp blue
The photo-ageing behaviour of the Prussian blue and
Antwerp blue paints in oxygen-free conditions was very
different from the other pigments studied. As can be seen
in Figures 5 and 6, fading was greatest for the RPK samples,
a result that is in agreement with the findings of Rowe, who
reported that silk textiles dyed with Prussian blue that were
placed in an anoxic enclosure and left by a south-facing
window for three weeks, faded considerably [23]. The fading
of the paint samples was associated with a large increase in
L* and a*, and a moderate decrease in b*. The significant
fading under anoxic conditions is probably due to the fact
that the re-oxidation of Berlin white does not take place in
the absence of oxygen and the reduction of Prussian blue is,
therefore, the prevalent process.
The paints aged under anoxic conditions were analysed
using Raman spectroscopy with green laser light. The paints
were analysed as soon as they were removed from the Escal
pouches so that the recovery process had not taken place.
The un-aged paints had strong peaks at 2091 and 2155 cm–1
in their Raman spectra, which correspond to the Fe(III)–
C≡N–Fe(II) stretch in Prussian blue. For the faded paints,
peaks were detected at 2115 and 2144 cm–1. The shift from
2155 to 2144 cm–1 following photo-ageing can be attributed to the formation of Fe(II)–C≡N–Fe(II) bonds in Berlin
white. There was no standard reference Raman spectrum
available for Berlin white, but the reduction of iron in hexacyanoferrate complexes has been found to lower the C≡N
stretching frequency, see, for example [24].
After an exposure of approximately 7 Mlux.hours, it
was observed that the ΔE00 values for the Prussian blue and
Antwerp blue air samples slowly decreased. This behaviour
was not expected, as, although Prussian blue is phototropic,
it is not known to regain its colour upon further exposure to
light. It is possible that under the experimental conditions
figure 4. Colour change for the air, Escal and RPK samples of orpiment paint
54
THE PHOTO-AGEING BEHAVIOUR OF SELECTED WATERCOLOUR PAINTS UNDER ANOXIC CONDITIONS
used in this study the Berlin white that formed upon light
exposure re-oxidized to Prussian blue to a certain extent.
Such a recovery process has never been reported previously
and additional work would be needed to investigate this
phenomenon further.
As shown in Figure 7, the fading of the Antwerp blue
sample was much more pronounced than that of the Prus-
sian blue sample under anoxic conditions, a trend that was
also observed in air, although to a lesser extent. This is in
agreement with the results presented by Kirby and Saunders
for the photo-ageing of Prussian blue paints in air [5].
The exact reasons underlying this difference are not fully
understood, but they could be related to increased interreflectance of light in the paint layer [12].
figure 5. Colour change for the air, Escal and RPK samples of Prussian blue paint
figure 6. Colour change for the air, Escal and RPK samples of Antwerp blue paint
55
CAPUCINE KORENBERG
figure 7. Comparison of the lightfastness of Prussian blue and Antwerp blue paint samples aged under anoxic conditions
After light exposure the samples were left in the dark
for one month in the presence of oxygen and the colour
was then measured again. All six samples regained their
colour but, except for the air samples, the recovery was not
complete, see Table 1. Rowe also reported that textiles dyed
with Prussian blue and photo-aged under anoxic conditions
did not recover their colour completely [23].
table 1. ΔE00 values for the photo-aged Prussian blue and Antwerp
blue paint samples after one month in the dark in air
Sample
Exposed in air
Exposed in a sealed Escal envelope
Exposed in a sealed Escal envelope with RPK
Prussian
blue
0.2
1.7
1.8
Antwerp
blue
0.6
2.1
3.4
In contrast, the absence of oxygen had no adverse or beneficial effect on the fading of the chrome deep and realgar
paints. Finally, in agreement with previous studies, Prussian blue and Antwerp blue paints photo-aged under anoxic
conditions faded considerably. Antwerp blue was observed
to fade significantly more than Prussian blue. Also, upon
subsequent exposure to air in the dark for a month, the RPK
and Escal samples of Prussian blue and Antwerp blue did
not recover their colour completely. These results confirm
that artefacts containing Prussian blue pigments should not
be displayed under anoxic conditions.
ACKNOWLEDGEMENTS
The author would like to thank Susan Bradley and David Saunders for
helpful discussion and comments.
CONCLUSIONS
MATERIALS AND SUPPLIERS
The alizarin crimson paint did not fade perceptibly under
anoxic conditions and the orpiment paint faded only slightly.
In addition, it was found that enclosing these same paints
in Escal pouches without oxygen scavengers significantly
decreased the degree of fading, although not to the same
extent as when using oxygen scavengers. This intermediate
behaviour was probably due to oxygen depletion inside
the Escal pouches. The use of anoxic or airtight enclosures
could, therefore, be beneficial for the display of objects that
contain such paints and perhaps also other pigments based
on anthraquinones such as madder, kermes and cochineal.
56
•
•
•
•
Kremer pigments: AP Fitzpatrick Fine Art Materials, 142
Cambridge Heath Road, London E1 5QJ, UK.
Luxbug datalogger: Hanwell Instruments Limited, 12–13 Mead
Business Centre, Mead Lane, Hertford SG13 7BJ, UK.
Mitsubishi EscalTM gas barrier film, RPK oxygen scavengers and
oxygen level indicators: Conservation by Design, Timecare Works,
5 Singer Way, Kempston, Bedford MK42 7AW, UK.
Winsor & Newton watercolour paints and gum arabic:
L Cornelissen & Son, 105 Great Russell Street, London WC1B 3RY,
UK.
THE PHOTO-AGEING BEHAVIOUR OF SELECTED WATERCOLOUR PAINTS UNDER ANOXIC CONDITIONS
AUTHOR
Capucine Korenberg ([email protected]) is a
scientist in the Department of Conservation and Scientific Research
at the British Museum.
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