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Comparative Study of Washing Treatments for Pastel Drawings

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A treatment procedure for pastel drawings developed at the Portuguese Institute of Museums and Conservation (IMC), consisting of alkaline blotter washing in a humidifying chamber, is compared with three other methods described in the literature. These include immersion in cold water, a combination of water mist and suction table treatment and ultrasonic mist in combination with a dry blotter support. The four treatments were applied to non-aged and aged test samples consisting of three types of paper and four pastel sticks, totalling 120 samples. Effects caused by the afore mentioned treatment methods were evaluated using six semi-quantitative parameters determined by visual observation (i.e. lateral pigment movement, paper staining, pigment loss, decrease in colour intensity, darkening and change of hue), as well as three semi-quantitative parameters obtained through optical microscopy and scanning electron microscopy (pigment agglomeration, compacting and penetration into the paper support). The results showed that the IMC method is not suitable for treating pastel drawings executed on papers with low porosity, but, of all tested methods, it is the most advantageous technique for treating pastel drawings executed on velour paper - the prevalent type of carrier for pastel drawings treated over the years at IMC.
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1. Introduction
Pastel drawings are by nature extraordinarily fragile pieces of art (Moroz 1997). How-
ever, much like other artworks on paper, they may develop local or overall staining
through adverse ambient environments, adverse framing conditions, or accidents. For
pastels that had become disturbingly discoloured, an aqueous treatment may be con-
sidered. However, it is extremely difficult to carry out such an invasive treatment
because the powdery pigments are only lightly adhered to the paper support (Cor-
rigan 1997, Saunders 1999, Weidner and Zachary 1994). The weak adhesion is a
consequence of low concentrations of binding medium used in pastel sticks, which is
also responsible for the velvety nature of pastel drawings or paintings.
Pastel sticks are essentially composed of three components: the pigment, the ex-
tender and the binding agent (Burns 2002, Corrigan 1997, Moroz 1997). Pigments
are generally the same as those used in tempera paints (Moroz 1997) and are mixed
with an extender to add softness; typically kaolin or gypsum (Kosek 1998) can be
Restaurator, Vol. 33, p. 199–219
© De Gruyter Saur 2012
Comparative Study of Washing Treatments for Pastel Drawings
by Vera Lory, Francisca Figueira and António João Cruz
Abstract: A treatment procedure for pastel drawings developed at the Portuguese Institute of Mu-
seums and Conservation (IMC), consisting of alkaline blotter washing in a humidifying chamber, is
compared with three other methods described in the literature. These include immersion in cold
water, a combination of water mist and suction table treatment and ultrasonic mist in combination
with a dry blotter support. The four treatments were applied to non-aged and aged test samples
consisting of three types of paper and four pastel sticks, totalling 120 samples. Effects caused by
the afore mentioned treatment methods were evaluated using six semi-quantitative parameters de-
termined by visual observation (i.e. lateral pigment movement, paper staining, pigment loss, decrease
in colour intensity, darkening and change of hue), as well as three semi-quantitative parameters ob-
tained through optical microscopy and scanning electron microscopy (pigment agglomeration, com-
pacting and penetration into the paper support). The results showed that the IMC method is not
suitable for treating pastel drawings executed on papers with low porosity, but, of all tested methods,
it is the most advantageous technique for treating pastel drawings executed on velour paper – the
prevalent type of carrier for pastel drawings treated over the years at IMC.
Zusammenfassung/résumé at end of article
received: 08.04.2011 revised: 09.03.2012
199
DOI: 10.1515/res-2012-0009
found in pastels, although other white pigments are also used (Townsend 1998). The
most common binder is gum, namely gum tragacanth or gum arabic, but other bind-
ers, such as animal glue, olive or linseed oils may also be employed (Kosek 1998). The
binder concentration must not be too high or too low in order to prevent that the stick
becomes too rigid or crumbly (Daniels 1998).
Aqueous cleaning treatments of pastel drawings were more frequently done in the
past (Plenderleith 1956). Float washing and water immersion are traditional aqueous
treatment methods for pastels (Kosek 1990). The risk of pigment migration or loss
caused by contact with free water inspired the search for alternative treatment meth-
ods. The first and most important development in this respect was the introduction of
the suction table in the 1970s (Weidner 1974); the exerted pressure minimized any
lateral pigment movement. This method was later coupled with a moisture chamber
equipped with an ultrasonic humidifier and an air filter system. Ultrasonic humidifica-
tion is based on the generation of small water droplets that are applied using misting
equipment. This allowed control over the amount of water and the direction of its
movement through the drawings surface. Only a minimum amount of moisture was
necessary to accomplish results that compared well to those achieved by more inva-
sive washing methods such as float washing commonly used for more stable artworks.
The cleaning is based on diffusion mechanisms (Weidner 1993, Weidner and Zachary
1994). However, the suction table washing treatment was later perceived more criti-
cally because of its tendency to draw pigments from the surface of the paper or textile
support into its interior (Kosek 1990). Blotter washing combined with ultrasonic mist
in a humidification chamber constitutes yet another alternative to float washing which
was recommended for the treatment of friable and water-sensitive works (Albro et al.
1990, Keyes 1994). The use of this technique was more controllable and less cum-
bersome than the Weidner/Zachary suction table system. However, the traditional
immersion method continued to be recommended even after the appearance of these
alternative treatment methods (Moroz 1997). Summarizing, it seems that all methods
can be problematic, as it has been shown in a comparative study of three pastel treat-
ment methods including immersion, humidification and suction (Daniels 1998).
The main alterations of pastel media resulting from aqueous treatment that are re-
ported in the literature are lateral dislocation of pigments, their penetration into the sup-
port, their dispersion in the water bath; further colour changes and the agglomeration
or compacting of pigments (Kosek 1990, Weidner and Zachary 1994, Daniels 1998).
Treatments including ultrasonic mist seem to cause fewer adverse effects (Daniels
1998), although pigment agglomeration and compacting may occur.
At the Instituto dos Museus e da Conservação (IMC, the Portuguese Institute of
Museums and Conservation), a pastel washing treatment based on ultrasonic misting
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Comparative Study of Washing Treatments for Pastel Drawings
has been in use since 1992, although the method was published much later (Figueira
et al. 2005). It was successfully used in the treatment of pastel paintings on velour
paper [1] by the 19th-century Portuguese artist José Malhoa. These works had de-
veloped foxing stains shortly after being removed from their original glass enclosure
(Figueira and Fontes 1998) and were to be permanently rehoused in this original
enclosure. For this reason, it was thought important to clean the stained pastels by
aqueous treatment and raise the pH of their paper supports as it is known that air-tight
enclosures enhance the degradation of acidic papers (Havermans 1999). Although the
aqueous treatment method was given positive evaluations in some unpublished stud-
ies at IMC using naked eye and stereomicroscope observation of test samples, other
studies have shown that alterations can occur in the pastel surface morphology, which
may not be detected with these methods (Daniels 1998). This present comparative
study of several washing treatments is centred on more objective, precise and sensi-
tive evaluation approaches.
The pastel drawing samples were treated with three methods employed in the
previous study by Daniels (1998) and the IMC method. Alterations of the media that
occurred as a consequence of the treatments were monitored and analysed through
visual observation, photography, stereomicroscopy, optical microscopy and scanning
electron microscopy (SEM). The study design closely followed the one by Daniels
(1988) in regard to the use of sample preparation, treatment choices and comparison
of their results. However, some aspects of sample composition and treatment methods
differ from Daniels’ experiment. Also, we complemented Daniels qualitative evalua-
tion by a semi-quantitative evaluation.
2. Experimental
2.1 Preparation of test drawings
120 samples were prepared using four different pastel sticks on three different types
of paper commonly used for pastel drawings. The papers were chosen for their differ-
ent textures: a modern velour paper, the most porous and most textured of all paper
samples; an Arches Aquarelle paper, the least porous and moderately textured; and a
black Canson drawing paper, of moderate porosity and smooth texture. Some of the
papers’ properties, according to information provided by the manufacturers or deter-
mined in our laboratory, are reported in Table 1. Although the composition of modern
velour paper differs substantially from its 19th-century precursor, its textural surface
characteristics were felt to match those of the early velour papers more closely than
the filter paper used by Daniels.
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Vera Lory, Francisca Figueira and António João Cruz
The black, white, blue and red pastel sticks were selected from two different
brands. They are identified in Table 2, which lists some of their properties accord-
ing to the information provided by manufacturers or experimentally determined in
Table 1: Properties of the papers used
Material description Arches Aquarelle paper Canson Mi-Teintes
black paper
Modern pastel
velour paper
fibre composition 100% cotton 50% cotton
synthetic flocked fibres
adhered to a lignin
containing paper.
sizing gelatine gelatine
thickness 0.54 mm 0.27 mm 0.44 mm
texture mild textured low textured highly textured
water absorbency* no changes after 45
minutes
the drop was absorbed
after 20 minutes
the drop was absorbed
immediately
* the test was carried out with a water drop of approximately 6 mm in diameter.
Table 2: Suppliers and composition of pastels used
Material
description Black pastel White pastel Blue pastel Red pastel
brand Winsor & Newton Winsor & Newton Winsor & Newton Talens-Rembrandt
commercial name charcoal grey 142 zinc white 748 cobalt blue hue
179 (tint 4)
permanent red
deep 371,5
pigment iron oxide zinc oxide
phthalocyanine
blue and synthetic
ultramarine blue
pyrrole red and
pyrrole rubine
pigment chemical
composition
magnetite and
hematite zinc oxide
copper phthalo-
cyanine and
sodium aluminium
silicate with
sulphur
compounds
diketopyrrolo
pyrrole and diketo-
pyrrolo pyrrole
extender
barium sulphate
and calcium
carbonate
barium sulphate
and calcium
carbonate
barium sulphate
and calcium
carbonate
kaolin,
barium sulphate
and calcium
carbonate
binder gum tragacanth gum tragacanth gum tragacanth
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Comparative Study of Washing Treatments for Pastel Drawings
this study. Some of the pastels were chosen based on the known tendency of certain
pigments to be adversely affected by exposure to water: iron oxide, for example, can
turn darker (Weidner and Zachary 1994) and easily creates clumps (Daniels 1998);
zinc oxide can change into zinc carbonate (Eastaugh et al. 2005); and synthetic ultra-
marine blue may turn darker and may compact (Daniels 1998).
The manufacturers’ information on the pastels’ composition was confirmed by micro
X-ray fluorescence spectrometry, micro Fourier transform infrared spectroscopy and
micro Raman spectroscopy, which identified both the colour providing pigments as
well as those used as extenders. The papers were cut into pieces of 10 cm by 10 cm
and the pastel sticks were applied manually, exerting the same pressure, repeating
the same drawing motif. Only one colour was applied to each paper sample, which
yielded 12 samples for each combination of paper and pastel. The paper samples to-
talled 120 specimens including both the artificially aged and non-aged samples that
underwent 4 different treatments and their respective reference samples.
Half of the samples (60) were submitted to the four washing methods without
further treatment, as in Daniels study. The other half of the test drawings (60) was
submitted to a natural “fixation” process to enhance the pigment-paper bond which
was followed by artificial ageing. The fixation process was carried out by first exposing
the test drawings to an atmosphere of 80–90% RH for a period of 10–15 minutes; sub-
sequently, the samples were placed directly onto a smooth screen printing polyester
mesh (T120) face down, topped by a polyester non woven fabric, a dry blotter and a
wooden board (Fig. 1). No further weight was placed on top. The blotter was replaced
after 20 minutes and then was replaced daily for the duration of one week. This
two-step process, which will be referred to as “fixation process” in the following, in-
creases the contact of the pigment particles with the support and causes a better adhe-
sion. The use of steam as a fixative by artists has been reported in literature (Daniels
1998). These sixty drawings were then subjected to artificial ageing for 24 days at
65% RH and 80ºC, according to ISO 9706:1994(F), in a FITOCLIMA 150 FDTU
chamber.
Fig. 1: Device used for the drying of the test drawings.
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Vera Lory, Francisca Figueira and António João Cruz
2.2 Treatment of the samples
Ninety-six test drawings, both non-aged and artificially aged, were treated using the
following four methods: 1) immersion in distilled water for one hour with no agita-
tion; 2) suction and water spraying the surface using a suction table; 3) exposure
to ultrasonic mist for 1 or 4 hours over a dry blotter. All three methods were ex-
ecuted according to Daniels experiments and samples were afterwards allowed to air
dry (Daniels 1998). The IMC method has some similarities with the ultrasonic mist
method used by Daniels, but differs in the following: the drawings are in contact with
a blotter (pH 8) soaked with a dilute solution of calcium hydroxide instead of a dry
blotter, and the drying process proceeds under slight weight (Figueira et al. 2005).
The three methods used by Daniels were slightly altered: 1) suction table treat-
ment was increased to four hours because it was felt that treatment efficiency would
more closely resemble the immersion method, where papers samples are in contact
with water much longer (Daniels 2002); 2) for the same reason, the ultrasonic mist
method was applied only for a four hour period instead of one and four hours; 3) the
test drawings were flattened using the “fixation process” (Fig. 1) after air drying, in
order to make them more comparable to the IMC method. The washing procedures
are described in the following, the IMC method being explained in more detail:
2.2.1 IMC washing method
The IMC washing method includes five steps:
Step 1: a dry blotter is soaked in a calcium hydroxide solution (pH 9) for 20 min-•
utes. Meanwhile, the test drawings are placed in a humidification chamber and
exposed to ultrasonic mist in order to relaxe them. The humidification chamber is
equipped with closable openings to enable handling during humidification.
Step 2: the moistened test drawings are temporarily removed from the chamber •
in order to place the wet blotter inside. When lifted from the calcium hydroxide
solution, the blotter had a pH of 8. The samples are then placed on top of the blot-
ter. If needed, the test drawings are further moistened by spraying water on their
back side.
Step 3: samples and blotters are exposed to mist in an atmosphere of 95% RH for •
four hours (with real drawings the blotters are changed when discoloured).
Step 4: the test drawings are left overnight inside the humidity chamber on clean •
wet blotters (pH 8) without further humidification. The openings of the humifica-
tion chamber are left half opened.
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Comparative Study of Washing Treatments for Pastel Drawings
Step 5: the test drawings are dried under controlled conditions: they are placed •
face down onto a T120 polyester mesh with no interleaving material, covered with
a polyester non-woven material and a dry blotter and weighed down lightly with a
wooden board with no additional pressure (Fig.1). The blotter is changed frequently
until the back of the test drawings feels dry. The boards, measuring 50 cm × 70 cm
each, are then weighed down with an additional 15 kg. The blotters are changed
daily for at least one week.
2.2.2 Ultrasonic misting
Inside a humidity chamber, a relative humidity of 95% is created using a Honeywell
ultrasonic humidifier. The test drawings are placed on dry blotter paper inside the
chamber. They are removed after four hours and are allowed to air dry. They are then
flattened according to Step 5 of the “fixation process”.
2.2.3 Immersion washing
The test drawings are immersed in distilled water for one hour, with no agitation and
are allowed to air dry. They are then flattened according to Step 5 of the “fixation
process”.
2.2.4 Suction table treatment combined with water spraying
This treatment is performed on a Mayline suction table operating at a pressure of
40 mbar. The test drawings, supported by blotter paper, are placed on the perforated sur-
face of the suction table, then suction is turned on and water is sprayed using a Dahlia
spray at a distance of 60 cm until the surface is glossy wet. Spraying continues when the
water has penetrated the paper and the paper surface has lost its wet gloss. The proc-
ess is repeated intermittently for four hours, then the test drawings are allowed to air
dry. Finally, the drawings are flattened according to Step 5 of the “fixation process”.
2.3 Treatment evaluation methods
The treated samples were compared to non-treated reference samples. The following
parameters were observed:
Lateral pigment dislocation: pigment particles migrated from their original location •
and settled in a different place, only slightly adhering to the support without pen-
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Vera Lory, Francisca Figueira and António João Cruz
etrating the paper fibres; apart from these pigment particles, no other staining of
the paper could be observed.
Paper staining: coloured compounds of pastels (ex. a dye) spread out laterally; the •
paper was stained, but pigment particles were not observed.
Pigment loss: pigment was lifted and dispersed in water.•
Pigment agglomeration: pastel layer formed clumps.•
Pigment compacting: pastel layer was flattened.•
Pigment particles migrated from elevated parts of the support surface and settled •
in its recesses.
Loss of colour intensity as result of pigment dislocation, loss or sinking.•
Darkening of colour.•
Alteration of hue; alteration of colour tonality. •
Samples were assessed according to each parameter by visual observation with the
naked eye or aided by the stereomicroscope or optical microscope; in some cases a
scanning electron microscope (SEM) was employed. A scale from 0 to 5 was used that
would allow semi-quantitative evaluation:
0 = not detectable
1 = very mild
2 = mild
3 = moderate
4 = severe
5 = very severe.
To support visual observation, drawings were documented with a digital camera
(Nikon Coolpix 8700). Observation under the microscope involved two setups: the
test drawings were observed under a stereo microscope (Leica MZ6 with a Leica cam-
era DC 200, at 40x magnification) and under an optical microscope (Leitz-Dialux 20
with a Leica camera DC 500, at 110x magnification), which required small samples
to be taken from the test drawings. Samples were also examined by means of an scan-
ning electron microscope (SEM) type JEOL JSM-T330A (20 kV, 0.1 mA) at 150x and
500x magnifications. For SEM imaging the samples were coated with gold. Colour
changes were evaluated only through visual observation, because pastel particles only
partially cover the paper surface, and the colour of the paper influenced the instru-
mental measurement, giving incorrect values for the pigment colour.
All test samples were observed in a stereomicroscope, while SEM analysis was only
carried out on velour paper test drawings due to limitations in time. Under the stereo-
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Comparative Study of Washing Treatments for Pastel Drawings
microscope, pigment compacting or agglomeration could not be observed on velour
papers, and no pigment penetration in Arches Aquarelle and Canson papers was no-
ticed because of the different paper surface morphologies. The porous surface of ve-
lour paper and its structure renders pigment penetration easily visible, but blurs mor-
phological changes of the pigment layer; Arches Aquarelle and Canson papers on the
other hand have less porous surfaces which make the penetration of particles hard to
observe, while morphological changes are clearly detectable.
3. Results and discussion
3.1 General assessment
The ultrasonic mist technique and the IMC method led to the formation of water
droplets on the surface of Arches Aquarelle and Canson black papers, which is due to
the low water absorbency of these test papers (Fig. 2); consequently those treatments
were discontinued, as would happen in a real-life situation, and no further data was
obtained for those cases. The problem was probably caused by the paper’s low perme-
ability (Table 1) which leads to the recommendation that these two treatments should
not be used for papers with low water absorbency.
Fig. 2: Drops formed during ultrasonic mist treatment on Arches Aquarelle paper.
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Vera Lory, Francisca Figueira and António João Cruz
Fig. 3: Changes detected visually in non-aged samples;
parameters: P1 = lateral pigment movement; P2 = paper staining; P3 = pigment loss; P4 = loss of
colour intensity; P5 = darkening; P6 = changes in hue;
each parameter was determined from 4 test drawings (IMC and ultrasonic mist methods) or 12 test
drawings (immersion and suction table methods).
Fig. 4: Changes detected visually in aged samples; parameters: see Fig. 3.
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Comparative Study of Washing Treatments for Pastel Drawings
In general, changes were more visible in non-aged than in aged drawings. Those
differences were more pronounced in the properties related to the mechanical behav-
iour of pigment particles and their bonding with the paper support (lateral pigment
movement, pigment loss, pigment penetration and paper staining) and less pronounced
in the properties related to colour (hue changes and loss of colour intensity). The loss
of colour intensity may be due to pigment penetration, pigment loss or lightening of
colour (Figs 3 and 4). The fact that changes are less visible in aged drawings could
be partially explained by decreased solubility of gums upon aging, a well known phe-
nomenon that is caused by an irreversible loss of water, aggregation of polymer chains
and cross-linking between gum molecules, a reaction that can be accelerated by some
polyvalent metal constituents of pigments (Daniels 1995). However, it is unlikely that
this is the main cause for the general decrease of changes with aging, since this phe-
nomenon was also observed in the pastel drawings made with a red pigment which,
according to the manufacturer, contains no binder. Thus, an alternative explanation
might be increased pigment caking, which can occur upon ageing (Daniels 1998).
In general, immersion and suction table treatments caused more visible changes
in drawings (Table 3), including pigment movement, paper staining (Fig. 5), pigment
penetration, pigment loss and hue changes (Figs 3 and 4). This phenomenon was
observed in all sets of papers including velour paper (Table 3) – the only paper treated
with all four methods.
The changes caused by the IMC and the ultrasonic mist methods were less visible
and mainly related to loss of colour intensity and darkening. These two effects depend
little on the treatment method, being similar for all four treatments (Figs 3 and 4).
IMC and the ultrasonic mist treatment were not performed on samples made of
Arches Aquarelle and Canson black drawing papers; in these cases, the immersion
Fig. 5: Paper staining in the non-aged Arches Aquarelle drawing after immersion treatment.
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Vera Lory, Francisca Figueira and António João Cruz
method caused less visible changes when compared to the suction table method, both
in non-aged and aged drawings.
3.2 Pigment movement
Lateral pigment movement, caused by contact with free water, was clearly visible
with the naked eye in samples treated with immersion and suction table methods
(Figs 3 and 4). It was more visible after immersion treatment in non-aged samples
(Fig. 3), as might be predicted considering the contact of the pigment particles with
free water. Aging, however, significantly altered this situation in the case of the im-
mersion treatment (Fig. 4) but was not significant in the case of suction table treat-
ment. Apparently, increased pigment caking and decreased binder solubility caused
by aging (Daniels 1995, 1998) are sufficient to reduce pigment migration caused by
contact with free water, yet it does not diminish the effects of the pressure caused by
suction.
Pigment movement was not detected visually in the case of the IMC and the ultra-
sonic mist methods, but under the microscope, these treatments had clearly caused
pigment migration, manifested as pigment agglomeration, compacting and penetra-
tion into the paper substrate (Fig. 6).
According to these observations, all four methods generally caused comparable
degrees of pigment movement and, in the majority of cases, those effects can be de-
scribed as between mild and moderate (values between 2 and 3 in Table 4). Never-
Table 3: Average of changes detected by visual observation (expressed in a scale ranging from 0 = no
change to 5 = very severe).
Treatment
method applied
Average of changes
all papers velour paper
non-aged drawings aged drawings non-aged drawings aged drawings
IMC 0.4* 0.2* 0.4* 0.2*
ultrasonic 0.5* 0.2* 0.5* 0.2*
immersion 1.9* 0.4* 2.1* 0.7*
suction 1.2* 1.0* 0.6* 0.6*
* values are the average of six parameters; for immersion and suction methods, each parameter was
calculated from 12 test drawings when all papers are considered, in all other cases, each parameter
was calculated from 4 test drawings.
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Comparative Study of Washing Treatments for Pastel Drawings
theless, the IMC method consistently caused the lowest changes, independent of ar-
tificial ageing or the analytical method used (Table 4). In contrast to results obtained
by visual observation, observations under the microscope suggested that the immer-
sion method caused fewer detectable changes when compared to the ultrasonic mist
method, especially in the case of the aged samples. However, these results are only
Fig. 6: Velour paper with black pigment (non-aged);
a: reference sample, b: sample of the test drawing treated with ultrasonic mist, showing pigment
compacting, agglomeration and penetration into support (SEM images).
Table 4: Average of changes, related to pigment movement, observed under the microscope (ex-
pressed in a scale ranging from 0 = no change to 5 = very severe).
Method
Average of changes
stereomicroscope SEM
non-aged drawings aged drawings non-aged drawings aged drawings
IMC 2.1* 2.0* 2.0* 1.4*
ultrasonic 3.2* 2.7* 3.1* 2.3*
immersion 2.6* 2.3* 3.4* 1.9*
suction 3.4* 2.9* 2.8* 2.2*
*values are the average of three parameters, except in the case of drawings treated by IMC and
ultrasonic methods for which only two parameters were determined with the stereo-microscope;
each parameter was calculated from 4 test drawings except in case of pigment compacting that was
determined with the stereomicroscope for drawings treated by immersion and suction methods and
for which 8 test drawings were used.
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Vera Lory, Francisca Figueira and António João Cruz
statistical, as can be seen in Figs 7 and 8. Macroscopic and microscopic observations
yielded different assessments of the effect of immersion treatment and ultrasonic mist
treatment, which can probably be explained by differences in size and texture of the
agglomerates formed by the two treatments. No direct relationship between the ex-
tent of changes detected by visual observations and the agglomerate features detected
under the microscope could be determined.
Considering each of the parameters evaluated under the microscope separately, the
changes caused by the IMC method in the aged drawings are described, at most, as
mild (values less than or equal to 2), while some changes caused by the other meth-
ods, namely the immersion and the suction table methods, exceed moderate (values
greater than 3) (Fig. 8). Moreover, the IMC method causes the least changes accord-
ing to the majority of parameters (Figs 7 and 8), both in non-aged and aged samples.
Considering that Daniels (1998) noted that mist treated samples did not display ag-
Fig. 7: Changes detected in non-aged drawings under the microscope;
parameters: P7 and P10 = pigment agglomeration, P8 and P11 = pigment compacting, P9 and P12
= pigment penetration: parameters P7 to P9 were determined with a stereomicroscope, parameters
P10 to P12 by means of SEM; each parameter was determined from 4 test drawings, except param-
eter P8 for immersion and suction methods, where 8 test drawings were used for each.
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Comparative Study of Washing Treatments for Pastel Drawings
gregates while still wet and that the major difference between the ultrasonic mist and
the IMC methods lies in the drying process, the good results obtained by the IMC
method could be related to its drying process. In the IMC process, the samples are
placed between two surfaces that exert pressure during the drying process. That pres-
sure may be sufficient to counter the movement of particles, leading to agglomeration,
which occur during air drying, driven by interfacial energetic factors (Daniels 1998).
The downward forces exhorted by the IMC drying process, during which drawings
are placed face down, may have diminished pigment penetration in comparison to
drawings which were left to air dry.
3.3 Paper staining and pigment loss
Paper staining and pigment loss were not observed visually in the test drawings treat-
ed with IMC and ultrasonic mist methods; partly this was expected in cases where
no lateral movement was detected macroscopically (Figs 3 and 4). Pigment loss was
neither observed in aged nor non-aged samples treated with the suction table method,
which can be explained by the inward forces exerted by suction. Pigment loss was
Fig. 8: Changes detected in aged drawings under the microscope; parameters: see Fig. 7.
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Vera Lory, Francisca Figueira and António João Cruz
only detected in immersion treated samples which were directly in contact with free
water; the loss was much more significant in the non-aged samples. Paper staining
was found in suction treated samples almost independently of whether they had been
artificially aged or not. Paper staining caused by immersion treatment depends sig-
nificantly on the aging status of the samples, being described as moderate in the case
of the non-aged samples but not detected in the case of the aged samples (Figs 3
and 4).
3.4 Colour changes
All four treatment methods caused visually perceivable colour changes (Figs 3 and 4).
Darkening is the most pronounced effect, especially in non-aged drawings, and is little
influenced by the treatment method. In the non-aged samples the effect was less than
mild while in the aged drawings it was less than very mild. In both cases the best
method, according to this parameter, is the immersion treatment. Loss of colour in-
tensity was caused by all treatments, but it was very mild (less or equal to 1) regard-
less if samples were aged or not aged. The IMC method is the treatment that led to
the least amount of changes. This may be explained by the reduced pigment penetra-
tion during the IMC drying process.
Regarding hue changes, which was the least visible colour change, the IMC and
the ultrasonic mist methods did not cause any detectable alterations in any sample
(Figs 3 and 4). As these two methods did not cause pigment movement (macroscopi-
cally detectable), paper staining or pigment loss, it is possible that hue changes could
be related to changes in the surface topography, as already suggested by Daniels (Dan-
iels 1998).
4. Conclusion
The IMC method (and also the ultrasonic mist method) proved inadequate for treat-
ment of pastel drawings executed on supports with low water absorbency, such as
Arches Aquarelle and Canson papers. However, for drawings on velour papers which
have high water absorbency due to their open surface structure, the IMC and the
ultrasonic mist treatments caused the least changes according to visual observation. It
should be noted that both methods imply the use of ultrasonic mist. On a microscopic
scale, the extent of change resulting from the IMC method seems to be less than the
one caused by the ultrasonic mist method with the drying process being the main
difference between these methods. The IMC drying process seems to reduce some
adverse effects. Consequently, the IMC method can be recommended as a washing
214
Comparative Study of Washing Treatments for Pastel Drawings
treatment for pastel drawings executed on velour paper the prevalent support of
pastel drawings treated over the years at IMC. The results presented in this study
confirm that no aqueous washing treatment leaves the pastel layer unaltered, which
has to be considered when discussing treatment options for pastels.
Acknowledgements
The authors are grateful to Nuno Leal for the SEM analyses, which were made at the
Centro de Investigação em Ciência e Engenharia Geológica, Faculdade de Ciências e
Tecnologia, Universidade Nova de Lisboa, Portugal (CICEGe/FCT-UNL).
Note
[1] Velour paper is a term given to papers with a velvet-like finish; they are produced
by first coating the paper’s surface with an adhesive and subsequently flocking the
adhesive coating with fine fibrous particles of rayon, cotton, or wool; velour papers
are capable to hold a lot of pigment.
Materials and suppliers
120 T Screen, fabric: polyester printing screen,
mesh count diameter (mesh/cm μm) – 120-34PW(T)
Anyida Hardware Mesh Co.,Ltd, No.106, Hongqi Street, Anping, Hebei, P. R.China
053600, http://www.wiremesh-cloth.com
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Zusammenfassung
Nassbehandlungstechniken für Pastellzeichnungen – eine vergleichende Untersuchung
Am Instituto dos Museus e da Conservação (IMC) wird routinemäßig eine Methode zur Nassbehand-
lung von Pastellzeichnungen angewandt, bei der das entsprechende Objekt einem „Blotter-Washing“
mit einer alkalischen Lösung in einer Feuchtkammer ausgesetzt wird. Die Möglichkeiten und Gren-
zen dieser Methode werden mit drei weiteren Techniken verglichen, die in der Literatur beschrieben
sind. Dabei handelt es sich um Badbehandlungen in kaltem Wasser, das Durchziehen von wässri-
gen Aerosolen auf dem Unterdrucktisch und das Aufbringen von wässrigen Aerosolen über einen
trockenen Löschkarton. Die genannten vier Techniken wurden auf nichtgealterten und künstlich
gealterten Proben, die jeweils mit vier ausgewählten Pastellstiften auf Papier hergestellt wurden,
durchgeführt. Die Testreihen umfassten insgesamt 120 Proben. Die Auswirkungen der jeweiligen
Behandlung wurden zunächst visuell beurteilt, vor allem in Bezug auf die laterale Verschiebung von
Pigmentteilchen, Pigmentverluste, Farbveränderungen, insbesondere Verdunklung der Farbschicht,
aber auch Verluste in der Farbintensität. Mithilfe lichtmikroskopischer und REM-Untersuchungen
wurden Auswirkungen in Bezug auf die Pigmentagglomeration und -verdichtung sowie das Ein-
sinken von Pigmentteilchen in die Papiermatrix bestimmt. Die Resultate haben gezeigt, dass die
am Institut übliche Behandlungstechnik ungeeignet ist für die Anwendung bei Pastellzeichnungen
auf Papier mit einer geringen Porosität. Allerdings ist dieses Verfahren im Vergleich zu den anderen
untersuchten das geeignetste für die Behandlung von Pastellzeichnungen auf so genannten Velour-
217
Vera Lory, Francisca Figueira and António João Cruz
papieren – der vorherrschende Typ von Pastellzeichnungen, die in den vergangenen Jahren am In-
stituto dos Museus e da Conservação (IMC) behandelt wurden.
Résumé
Etude comparative de traitements de lavage pour les dessins au pastel
A l’Institut Portugais des Musées et de la Conservation (IMC) on utilise d’habitude une méthode de
lavage des dessins au pastel au cours de laquelle l’objet en question est exposé à un « Blotter-Washing »
avec une solution alcaline dans une chambre humide. Les possibilités et limites de cette méthode
seront comparées à trois autres méthodes décrites dans la littérature, telles que l’immersion dans
l’eau froide, la succion d’aérosols aqueux sur une table à basse pression et la vaporisation d’aérosols
aqueux sur un papier buvard sec. Les quatre techniques nommées ci-dessus ont été appliquées à
des échantillons vieillis et non vieillis artificiellement qui avaient été réalisés sur le papier par quatre
crayons sélectionnés de pastel. L’ensemble des séries de tests a été effectué sur 120 échantillons. Les
effets des traitements respectifs ont d’abord été évalués par une observation à l’oeil nu de six para-
mètres (comme le déplacement latéral du pigment, les taches sur le papier, la disparition du pigment,
la perte d’intensité de la couleur, l’obscurcissement et les altérations de la couleur). Ensuite à l’aide
d’analyses au microscope optique et à la microscopie électronique à balayage on a pu déterminer les
répercussions en ce qui concerne l’agglomération des pigments et leur compression ainsi que leur
pénétration dans le support en papier. Les résultats ont démontré que la méthode utilisée habituelle-
ment à l’Institut IMC n’était pas appropriée dans le cas de dessins au pastel sur papier présentant une
porosité minime. Par contre cette méthode est par rapport aux autres la plus efficace pour le traite-
ment de dessins au pastel sur ce qu’on appelle des papiers velours – c’est surtout ce type de dessins
au pastel qui a été traité pendant des années à l’Institut des Musées et de la Conservation (IMC).
Authors and contact
Vera Lory completed her BA studies in general conservation-restoration and her MA in paper con-
servation, both at the Universidade Nova de Lisboa in 2010. The present study was her dissertation
project, which was undertaken during her final-year internship at the paper conservation studio of
IMC.
Rua Júlio Diniz, 1 – 4º Esq.
2685-216 Portela LRS
Portugal
Mail: veralory@gmail.com
218
Comparative Study of Washing Treatments for Pastel Drawings
Francisca Figueira received her BA in paper conservation in 1985 and has worked since then at
the paper conservation studio of IMC at the works of art on paper section. She has recently been
involved in morphological characterization of discolouration processes.
Rua das Janelas Verdes, 37
1249-018 Lisboa
Portugal
Mail: franciscafigueira@yahoo.com
António João Cruz holds a PhD in chemistry (1993), but since then has developed his activity in
research and teaching in the field of the materials of the works of art and its scientific analysis. He
also became interested in the problems and the history of conservation and restoration. Currently he
is professor at the Polytechnic Institute of Tomar.
Escola Superior de Tecnologia de Tomar
Instituto Politécnico de Tomar
Estrada da Serra
2300-313 Tomar
Portugal
Mail: ajcruz@ipt.pt
219
Vera Lory, Francisca Figueira and António João Cruz
ResearchGate has not been able to resolve any citations for this publication.
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