Blanching of paint and varnish layers in easel paintings: contribution to the understanding of the alteration

Abstract

The blanching of easel paintings can affect the varnish layer and also the paint layer with a blurring effect. The understanding of the physicochemical and optical phenomena involved in the whitening process remains an important challenge for the painting conservation. A set of ca. 50 microsamples from French, Flemish, and Italian blanched oil paintings, from sixteenth to nineteenth century, have been collected for in deep investigations. In parallel, the reproduction of the alteration was achieved by preparing some paint layers according to historical treatises and altering them in a climatic chamber in a humid environment or directly by immersing in ultrapure water. The observation of raw samples with a field-emission gun scanning electron microscope revealed for the first time that the altered layers have an unexpected highly porous structure with a pore size ranging from ca. 40 nm to 2 μm. The formation mechanism of these pores should mostly be physical as the supplementary analyses (Fourier transform infrared spectroscopy, gas chromatography–mass spectrometry) do not reveal any noticeable molecular modification. Considering the tiny size of the pores, the alteration can be explained by the Rayleigh or Mie light scattering.

Full text

INVITED PAPER
Blanching of paint and varnish layers in easel paintings:
contribution to the understanding of the alteration
Anaı
¨s Genty-Vincent
1,2,3
•Myriam Eveno
1
•Witold Nowik
1
•Gilles Bastian
1
•
Elisabeth Ravaud
1
•Isabelle Cabillic
1
•Jacques Uziel
2
•Nade
`ge Lubin-Germain
2
•
Michel Menu
1,4
Received: 7 April 2015 / Accepted: 14 July 2015
Springer-Verlag Berlin Heidelberg 2015
Abstract The blanching of easel paintings can affect the
varnish layer and also the paint layer with a blurring effect.
The understanding of the physicochemical and optical
phenomena involved in the whitening process remains an
important challenge for the painting conservation. A set of
ca. 50 microsamples from French, Flemish, and Italian
blanched oil paintings, from sixteenth to nineteenth cen-
tury, have been collected for in deep investigations. In
parallel, the reproduction of the alteration was achieved by
preparing some paint layers according to historical treatises
and altering them in a climatic chamber in a humid envi-
ronment or directly by immersing in ultrapure water. The
observation of raw samples with a field-emission gun
scanning electron microscope revealed for the first time
that the altered layers have an unexpected highly porous
structure with a pore size ranging from ca. 40 nm to 2 lm.
The formation mechanism of these pores should mostly be
physical as the supplementary analyses (Fourier transform
infrared spectroscopy, gas chromatography–mass
spectrometry) do not reveal any noticeable molecular
modification. Considering the tiny size of the pores, the
alteration can be explained by the Rayleigh or Mie light
scattering.
1 Introduction
Blanching of easel paintings is a recurring alteration that
can affect the varnish layer and also the paint layer,
strongly altering the visual aspect of the paintings. Treat-
ments currently used are not suitably efficient. The
understanding of such an alteration therefore remains an
important challenge for the painting conservation.
This alteration is known to appear on paintings kept in a
humid environment or to be the result of aqueous conser-
vation treatments with a possible heat source, such as (re)-
lining or fixation of loose paint [1–4]. Blanching, however,
is not systematic. The majority of the paintings that have
been relined do not present any sign of blanching. Besides,
altered and unaltered areas can coexist within a same color
range. Considering the complexity of this phenomenon and
despite several studies dated mainly from the 1980 and
1990s, its nature has never been clearly identified [1,2,5–
7]. Several hypotheses have been proposed: a possible
modification of the binder refractive index [2,3,7]oran
apparition of microstructures (microvoids, microcracks,
microcrystallization, microprecipitation, microemul-
sion,…) resulting in multiple light reflection in the layer [4,
6,8,9]. Free fatty acid migration has also been considered
as a probable cause [10]. More recent studies [11,12] have
concluded that blanching could also be the consequence of
a lead soap accumulation on the paint layer surface.
The aim of the present work was to better understand the
physicochemical and optical phenomena involved in the
&Anaı
¨s Genty-Vincent
anais.genty@culture.gouv.fr
1
Centre de Recherche et Restauration des Muse
´es de France
(C2RMF), Palais du Louvre - Porte des Lions,
14 Quai Franc¸ois Mitterrand, 75001 Paris, France
2
Laboratoire de Chimie Biologique (LCB), EA 4505,
Universite
´de Cergy-Pontoise, 5 Mail Gay-Lussac,
Neuville-sur-Oise, 95031 Cergy-Pontoise Cedex, France
3
Fondation des sciences du Patrimoine, Patrima, 33, boulevard
du Port, 95011 Cergy-Pontoise Cedex, France
4
Chimie ParisTech-CNRS, Institut de Recherche Chimie
Paris, UMR8247, PSL Research University, 75005 Paris,
France
123
Appl. Phys. A
DOI 10.1007/s00339-015-9366-y

whitening process. A set of ca. 50 paint microsamples
collected from the sixteenth to the nineteenth century
French, Flemish, and Italian blanched oil paintings, as well
as mock-up samples, have been studied. This work is based
on a multiscale approach. Structural modifications have
been highlighted at a macroscale by 3D digital microscopy
and at a micro-/nanoscale by field-emission gun scanning
electron microscopy (FEG-SEM). Investigations at the
molecular scale have been performed by Fourier transform
infrared spectroscopy (FTIR) and gas chromatography–
mass spectrometry (GC–MS).
2 Experimental
2.1 Samples
A set of ca. 40 paint microsamples have been collected
from 12 French, Flemish, and Italian blanched oil paint-
ings, ranging from the sixteenth to the eighteenth centuries.
Another set of ca. 10 microsamples have been taken from
two paintings, one dating from the second part of the
seventeenth century and the other dating from the nine-
teenth century in order to study the blanching of varnish
layers (Table 1).
In parallel, a series of mock-ups were made according to
historical treatises [13–17] and our first results obtained
from the analysis of ancient paintings. The effects of three
parameters were tested: the binder preparation, the nature
of the pigments, and the presence of an extender (i.e.,
chalk–calcium carbonate). Two different recipes were used
to prepare a lead medium. The first one is inspired from the
Formula for the second lead medium, probable technique
of Leonardo da Vinci, given by Maroger [13] and from a
recipe described in 1633 in Folio 28 of Turquet de May-
erne’s treatise [14]. This recipe has already been repro-
duced by Cotte et al. [15,16]. The second one is based on
the recipe of black oil given by Yvel [17] for the oil/
litharge ratio and on Maroger [13], who recommends the
addition of water to get a lighter medium with a better
consistency.
Both binders were made from walnut oil (HMB-BDA,
France), water, and PbO (Merck-Eurolab-Prolabo, France)
in the mass proportions 4-4-1 (recipe 1) and 10-10-1
(recipe 2). PbO was first ground with oil. This mix was
heated for 40 min at 60 C. After adding water, it was
heated at 100 C for 180 min. Binders were then ground
with pigments with or without chalk and applied on
microscope glass slides. The relative quantities of binder,
pigments, and chalk used for the preparation of the painting
mock-ups are given in Fig. 1. As blanching is localized
principally on green, brown, blue, and dark areas [1,2,9,
18], the following four pigments have been used: raw
umber (Kremer, 40612, Germany), green earth (Sennelier,
213, France), black ivory (Kremer, 12000, Germany), and
azurite (Kremer, 10200, Germany).
Two slides were prepared for each composition. After
2 months of drying, one slide was put in a climatic
chamber (Suntest XXL?, Atlas) with alternately humid
and dry conditions. The xenon arc lamp used with a win-
dow glass filter was set at 50 W/m
2
in the 300–400 nm
range. The following cycle has been done three times to
simulate a relining (aqueous conservation treatments with
heat source). Step 1: 50 C, 40 % RH (relative humidity);
Step 2: 40 C, 60 % RH; Step 3: 40 C, 80 % RH; Step 4:
40 C, 60 % RH; 30 h per step. The obtained mock-ups are
presented in Fig. 1.
To simulate water damage, the reproduction of the
alteration for varnish was achieved by immersing varnish
layers applied on microscope slides in ultrapure water from
0 to 31 days. Natural resins (mastic and dammar) as well as
synthetic resins (Paraloid B72, Laropal A81, Regalrez
1094, MS2A) were tested. More information about these
resins can be found in [19,20]. The evolution of alteration
was followed visually for 31 days, and samples were taken
and observed by FEG-SEM after 1.5, 10, 15, and 31 days.
2.2 Methods
2.2.1 3D digital video microscopy and optical microscopy
Samples were first observed with a 3D digital video
microscope, Hirox KH 8700, coupled with a revolver zoom
lens MXG-2500REZ (magnification from 935 up to
92500). The 3D images allow us to determine the topog-
raphy but also to distinguish precisely, at high magnifica-
tion, the altered and unaltered areas within a sample. The
samples have also been observed under UV-light illumi-
nation to highlight the presence of varnish with an optical
microscope (Nikon Labophot-2) coupled with a Nikon DS-
Ri1 camera.
2.2.2 Field-emission gun scanning electron microscopy
(FEG-SEM)
Field-emission gun scanning electron microscopy was
performed on a JSM-7800F with the PC-SEM version
5.1.0.1 software (JEOL). Samples were observed without
any preparation and without coating. Secondary electron
(SE) images were collected at 1 kV with a probe current of
ca. 18–20 pA and a working distance of ca. 6–7 mm.
2.2.3 Fourier transform infrared spectroscopy (FTIR)
Mock-up samples were analyzed by Fourier transform
infrared spectroscopy in a diamond cell. The analyses were
A. Genty-Vincent et al.
123

Table 1 Complete references of the 13 studied paintings including the sampled layers and their colors
Painter Title Date Museum Inventory number Size (mm
2
) Altered layer Color
D’Oggiono, Marco
(pupil of Leonardo da
Vinci)
(ca. 1467–1524)
La ce
`ne 1506 Muse
´e national de la
renaissance, Ecouen
INV781 2600 95490 Paint and
varnish
Publish, orange, brown,
dark
Anonymous Descente de croix 1600–1650 Muse
´e des Beaux-Arts,
Carcassonne
890.9.145 930 91190 Paint Brown, dark, blue
Sandrart, Joachim 1 von
(1606–1688)
Sainte famille dans un
paysage
1606–1688 Muse
´e des Beaux-Arts,
Rennes
801.1.27 1292 91365 Paint Green
Van der Meulen, Adams
Frans (1632–1690)
Sie
`ge de Courtrai Ca. 1667 Muse
´e National du
chateau de Versailles
et du Trianon
MV5846/INV
1477/LP 2836
2300 93260 Paint Green
Van der Bent, Johannes
(ca. 1650–1690)
Paysages, figures et
animaux
1650–1690 Muse
´e des Beaux-Arts,
Rennes
794.1.3 950 91250 Paint Light and dark brown,
dark
Anonymous L’Aurore 1650–1700 Muse
´e du Louvre, Paris INV8690 1880 (diameter) Paint Flesh color, orange,
light and dark brown,
green purplish, black
Van Schrieck, Otto Marseus
(1619–1678)
Chardons, e
´cureuils,
reptiles et insectes
Ca. 1660–1678 Muse
´e des Beaux-Arts,
Quimper
873.1.367 1355 91020 Paint Dark blue
Cotelle, Jean (the younger)
(1645–1708)
Vue de la fontaine de
I’Encelade avec
Jupiter foudroyant
1650–1700 Muse
´e National du
cha
ˆteau de Versailles
et du Trianon
MV735 2015 91375 Paint and
varnish
Green
Cotelle, Jean (the younger)
(1642–1708)
Vue des cinquante-deux
jets de Trianon avec
Mars et Ve
´nus devant
Apollon et Vulcain qui
va les faire prisonnier
avec un filet
1688 Muse
´e National du
cha
ˆteau de Versailles
et du Trianon
MV777 2030 92290 Paint Green
Desportes, Alexandre
Franc¸ois (1661–1743)
Chiens et gibier mort 1726 Muse
´e de la chasse et de
la nature, Paris,
INV3934 1100 91360 Paint Green
Nattier, Jean Marc
(workshop of)
(1685–1766)
Portrait de Louise-
Marie de France, dite
Madame Louise
Ca. 1750 Muse
´e National du
cha
ˆteau de Versailles
et du Trianon
MV 4442 1345 91046 Paint Green, brown
Chardin, Jean Baptiste
Sime
´on (1699–1779)
Les Attributs des arts 1765 Muse
´e du Louvre, Paris INV3199 910 91450 Paint Dark brown
Crignier, Louis (1790–1824) Jeanne d’arc en prison 1824 Muse
´e des Beaux-Arts,
Amiens
MP re
´col.90.2.83 1164 9885 Varnish –
Blanching of paint and varnish layers in easel paintings: contribution to the understanding…
123

performed with a PerkinElmer FTIR-Spectrum 2000 using
a deuterated triglycine sulfate (DTGS) detector and a
cesium iodide (CsI) beam splitter. The spectra were col-
lected in the 4000–400 cm
-1
range with a spectral reso-
lution of 4 cm
-1
(64 scans).
2.2.4 Gas chromatography–mass spectrometry (GC–MS)
Mock-up samples (paint or varnish layers) as well as
samples from the painting of Louis Crignier (1790–1824),
Jeanne d’arc en prison (varnish), were analyzed by gas
chromatograpy–mass spectrometry after appropriated
derivatization. GC–MS system equipped with a quadripole
mass spectrometer detector (Shimadzu GCMS-QP2010)
was employed for analyses. Chromatographic separation
was performed after splitless injection on CP-Sil 8CB 30 m
capillary column of 0.25 mm internal diameter with
0.25 lm film thickness.
The injector temperature was set at 310 C, transfer
line 320 C, and temperature programming started
from 80 C isothermal for 2 min, then heating rate
7C/minto150C, then another heating rate of 4 C/
minupto340C, and finished by 5 min isothermal
segment. Helium was the carrier gas working in linear
velocity regime at 36.3 cm/s. Mass spectrometry relied
on ionization by electron impact of 70 eV in the
source maintained at 200 C. The mass range was
50–950 m/z.
Varnish samples of about 50–200 lg were silylated with
50 lL BSTFA ?1 % TMCS (Supelco, Bellefonte, PA,
USA) at 75 C for 30 min and then evaporated with N
2
.
The residue was solubilized in CH
2
Cl
2
in proportion of
10lL for 100 lg of solid sample. In parallel, the same
samples were methylated with Meth-Prep II (Grace,
Deerfield, IL, USA) and toluene (1:1, v/v) at 75 C for
30 min using the same sample quantities and reaction
mixture in the same proportion as for CH
2
Cl
2
in former
derivatization procedure. Paint layer samples were only
methylated in proportion of 150 lL of reagents for ca.
100 lg of solid sample. An aliquot of 1 lL of solutions
containing derivatized compounds was injected to the GC–
MS system.
Fig. 1 Photograph in visible light (details) of the mock-ups prepared
from two recipes with four pigments (raw umber,green earth,ivory
black, and azurite) with or without calcium carbonate. Half of the
microscope slides were placed in a climatic chamber. The mock-ups
A and B present an important alteration
A. Genty-Vincent et al.
123

3 Results and discussion
3.1 Blanching of varnish layers
This part of the study is based on samples from two
paintings: Louis Crignier (1790–1824), Jeanne d’Arc en
prison and Jean Cotelle (1642–1708), Vue de la fontaine de
l’Encelade avec Jupiter foudroyant (Table 1). The results
obtained on both paintings are comparable, and only the
analysis performed on samples from the first one will be
presented. The blanching of the varnish layer on this
painting originates from a water damage and is principally
localized on the right side and bottom of the painting due to
water flow because of water retention in the frame
(Fig. 2a). The visual appearance is strongly altered in these
areas, and the paint composition is not visible through the
blanched varnish.
The examination of the sample, positioned on its edge,
with a 3D digital videomicroscope revealed the following
stratigraphy (Fig. 2b): a white ground layer (1), a dark
brown paint layer (2), and three layers at the top (3–5)
which are varnish layers according to the observation done
under UV-light illumination (Fig. 2c). Whereas layer 5 is
oxidized but still translucent, layer 3 has become white and
opaque.
Analyses have been performed by FTIR spectroscopy
and by GC–MS to determine the nature of the varnish and
if the whitening is linked to a chemical transformation. The
varnish has been identified by FTIR spectroscopy as a
natural triterpenic resin thanks to the presence of the fol-
lowing characteristic peaks: ca. 3410 cm
-1
(-OH
stretching band); 2947–2953 and 2877 cm
-1
(methylic
(-CH
3
and -CH
2
) stretching band); 1711 cm
-1
(C=O
stretching band); 1456 and 1385 cm
-1
(C–H bending
band); and 1259 cm
-1
(C–O stretching band) (Fig. 3a).
The methylated samples analyzed by GC–MS reveal the
presence of derivatives of oleanoate skeleton characterized
by abundant ions 189 and 203 m/z (Fig. 3b) [21]. The
presence of these compounds as well as the comparison
with standard mastic sample allows identifying this resin as
mastic, although the precise compound identification was
not done. However, from this results, the most important
information is that no significant differences have been
brought to light between altered and unaltered samples
(Fig. 3). The same conclusion can be drawn from the
analyses of mock-up samples.
Altered and unaltered samples have been then studied
with a field-emission gun scanning electron microscope
(FEG-SEM). An innovative and successful approach
requiring no sample preparation has been developed to
Fig. 2 a Photograph in visible light, Louis Crignier, Jeanne d’Arc en
prison, 1164 9885 mm
2
C2RMF/A. Maigret; Parts on the right
and on the bottom are altered. Aaltered sample; Bunaltered sample;
Cmuch altered sample; bEdge of the sample A by 3D digital
videomicroscope. 1white ground; 2brown layer; 3–5varnish layers.
Scale bar 50 lm; cSample A under visible (left) and UV (right) light
with an optical microscope. Scale bar 100 lm
Blanching of paint and varnish layers in easel paintings: contribution to the understanding…
123

ensure the non-modification of the internal structure of the
sample. It has been proved that the embedding in resin can
change this structure (resin penetration, polishing). [22]This
approach enables to reveal for the first time that the altered
layers have an unexpected highly porous (spongious) struc-
ture (Fig. 4). There is a high correlation between the local-
ization of the porosity in the stratigraphy of varnish layers
and the whitening. Indeed, in our sample, the pores are only
present in layer 3 that has become white and opaque (Fig. 4a)
but not in layers 4 and 5. A pore size ranging from ca. 100 nm
to 1 lm was noticed (Fig. 4b, c). Moreover, no porosity was
observed in the unaltered samples.
Mock-up samples prepared by immersing natural var-
nish (mastic and dammar) and synthetic varnish (Paraloid
B72, Laropal A81, Regalrez 1094, MS2A) in ultrapure
water at room temperature were studied. Under these
conditions, both types of natural varnish have changed but
with different alteration kinetics. The blanching of the
dammar varnish appears more rapidly. Nevertheless, after
31 days, the blanching is more visible for the mastic than
for the dammar varnish. The follow-up of this alteration
reveals that the translucent varnish does not become
directly white. It highlights a bluish effect in the first days.
Photographs of the mastic varnish in visible reflected and
transmitted light at T0 and after 1.5, 15, and 31 days are
reported in Fig. 5. For the unaltered varnish (T0), the white
light is transmitted through the layer. When the layer is
light blue (T15), the transmitted light has a low intensity
and is rather yellow/orange. However, the light is almost
not transmitted by the white layer (T31) which appears
more opaque. Some premature cracks are visible due to a
poor adhesion between the glass slide and the varnish layer.
Fig. 3 Varnish samples from Louis Crignier, Jeanne d’Arc en prison.
aFTIR spectrum of an unaltered sample B (black), an altered sample
A(green), and a much altered sample C (red); bGC–MS
chromatogram of an unaltered sample B (black) and a much altered
sample C (pink) with several peaks of oleanoate analogues (triangle)
Fig. 4 Varnish sample from Louis Crignier, Jeanne d’Arc en prison. FEG-SEM images at 1 kV; scale bar 1lm. aEdge of the sample, the
presence of porosity in the blanched layer 3; bSurface of the altered layer 3; cDetail of the surface of layer 3
A. Genty-Vincent et al.
123

Concerning the synthetic varnishes, no visible whitening
has been observed under the same alteration conditions,
except for the Laropal A81 where a light haze has been
detected after 15 days. The synthetic varnishes do not
develop any porosity in the conditions of our experiments.
The samples were studied by FEG-SEM to bring to light
the presence or absence of porosity. Observations per-
formed at the same magnification on mastic varnish at three
steps of the alteration are reported in Fig. 6. A significant
increase in the porosity size is observable between T1.5
and T31. A pore size distribution corresponding to these
three samples is presented in Fig. 7: T1.5 in black, T5.5 in
green, and T31 in red. A pore size of ca. 20 to 300 nm with
a maximum in the range 40 to 50 nm has been noticed for
the sample T1.5 (blue in reflected light) (Figs. 6a, 7) near
the surface. The sample T5.5 (between blue and white in
reflected light) has a pore size ranging from ca. 25 nm to
1lm (Figs. 6b, 7) in almost the full thickness. Finally, the
opaque white sample totally altered, T31, is composed of a
combination of small and interconnected large pores of ca.
25 nm–2 lm (Figs. 6c, 7). The average porosity size of the
sample T5.5 is comparable to the one observed on Louis
Crignier’s painting (Fig. 4). The protocol used to reproduce
the alteration, namely immersing in ultrapure water at
room temperature, is therefore valid.
The visualization of these porosities is a major advance
for the understanding of the alteration. Indeed, the refractive
index difference between the varnish (n=1.53–1.55) and
the pores probably filled with air (n=1) leads to a strong
light scattering in the layers. The porosities can be assimi-
lated to spherical particles, and the scattering Rayleigh and
Mie theories can therefore explain the visual appearance of
the altered varnishes. The Rayleigh theory [23,24] is used for
particle sizes much smaller than the wavelength of light
(radius of maximum ca. 25 nm). The intensity of the scat-
tered radiation, I, is proportional to k
-4
, so that the shorter
wavelength (blue) will be 16 times more scattered than the
longer wavelength (red). Moreover, as the blue radiations are
scattered by particles, only the red radiations will be trans-
mitted. Consequently, for a varnish with pores of mainly
40–50 nm like sample T1.5 (Fig. 6a), blue radiations are
more scattered than red ones, and the sample appears blue in
reflection and red in transmission. It perfectly fits the
observed color in reflected and transmitted light (Fig. 5).
Fig. 5 Photographs taken in reflected (on a black background) and in transmitted light at different times of sample evolution
Fig. 6 Mastic varnish samples from mock-ups. FEG-SEM images at the same magnification at 1 kV; scale bar 1lm. aT=1.5 days; b)
T=5.5 days; cT=31 days
Blanching of paint and varnish layers in easel paintings: contribution to the understanding…
123

When the particle size is greater than 50 nm, the Mie
theory [24,25] should be used. Contrary to the Rayleigh
scattering, the Mie scattering is not wavelength-dependent.
It induces a scattering of all wavelengths and therefore a
white color. In sample T31 (Fig. 6c), the pores are large
and in high concentration. It induces an important scat-
tering, and the light is almost not transmitted through the
layers. As the size of the pores increases with the
immersing time, the sample will follow Rayleigh theory at
the beginning of the pore formation until a particle size of
ca. 50 nm (blue color) and then Mie theory (white color).
However, there is an important size distribution, and pores
of more or less than 50 nm can coexist during the growing
process, explaining why some samples appear light blue
(both Rayleigh and Mie scatterings).
When the pore size increases, the porosity concentration
becomes relatively high, and it induces an interconnection
of the pores. Consequently, the percentage of solid matter
will be too low to ensure the layer cohesion, leading to a
possible cleavage between the varnish and the paint layers.
This case has already been observed on the painting of
Louis Crignier in the much altered areas. The results
obtained for the varnish are summarized in Fig. 8.
3.2 Blanching of paint layers
About 40 paint microsamples were studied to ensure a
significant sampling (Table 1). They were taken in altered
and unaltered areas mainly from green, brown, and dark
colors, except for L’Aurore, a particularly altered painting,
where orange, purplish, and flesh colors were, for instance,
sampled [26] (Table 1).
All samples were studied by FEG-SEM. The results
obtained from two samples originating from the painting
Portrait de Louise-Marie de France, dite Madame Louise,
workshop of Jean-Marc Nattier (1685–1766) are presented
in Fig. 9. As blanching does not appear homogeneous,
altered, less altered, and unaltered areas can coexist within
a same color range. The comparison of an altered sample
(sample 1—Fig. 9b) with a partially altered sample (sam-
ple 2—Fig. 9c) enables to identified the marker of this
alteration. The presence of porosity has been observed in
the altered sample (Fig. 9d) but not in the unaltered areas
of the second sample (Fig. 9e). Due to the presence of
pigments, the internal structure of a paint layer differs from
that of a varnish layer. Pigment particles impose some
constraints to the formation of pores which are less
spherical. In all samples, a porosity range from about
100 nm to 2 lm was noticed.
For the mock-ups, it can be observed that the nature of
the pigments has an influence on the appearance of the
alteration. Indeed, only mock-ups prepared with green
earth and raw umber have changed (Fig. 1). No significant
difference was detected on azurite and ivory black samples.
These results are in agreement with the observations done
on ancient paintings during this study and described in
Fig. 7 Pore size distribution for the samples T1.5 (in black), T5.5 (in green), and T31 (in red)
A. Genty-Vincent et al.
123

previous articles [1,2,9,18]. Additionally, binder prepa-
ration has an impact. Samples prepared with recipe 1 are
not altered or less altered in a humid environment (climatic
chamber) compared to those prepared with recipe 2. The
difference between both recipes is the quantity of drier
(litharge): 11.1 % in recipe 1 and 4.8 % in recipe 2.
Moreover, the addition of chalk as an extender also facil-
itates the emergence of this alteration.
FEG-SEM images from sample B (green earth, recipe 2,
CaCO3, climatic chamber, Fig. 1) are presented in Fig. 10.
The alteration of this sample is important but still super-
ficial. The bottom of the stratigraphy is still dark green
Fig. 8 Correspondence
between the visual appearance
of the layer, the pore size, and
the involved optical
phenomenon
Fig. 9 a Photograph in visible light, workshop of Jean-Marc Nattier,
Portrait de Louise-Marie de France, dite Madame Louise,
1345 91046 mm
2
, with the localization of the altered (1) and
unaltered (2) samples. C2RMF/A; Maigret; bAltered sample 1
under visible light, scale bar 100 lm; cless altered sample 2 under
visible light, scale bar 500 lm; dFEG-SEM image of the altered
sample 1 at 1 kV, scale bar 1lm; eFEG-SEM image of the unaltered
sample at 1 kV. Scale bar 1lm
Fig. 10 a Edge of the sample B (mock-up) under visible light (HIROX). Scale bar 200 lm; bFEG-SEM image at 1 kV. Scale bar 10 lm.
cZoom on the altered part. Scale bar 10 lm
Blanching of paint and varnish layers in easel paintings: contribution to the understanding…
123

(Fig. 10a), and the pigment particles are well agglomerated
in the binder (Fig. 10b). At the top, the layer becomes
lighter and presents some porosity from 500 nm to 4 lm
(Fig. 10b, c). The appearance of the altered layer of this
mock-up completely corresponds with the one observed on
ancient paintings.
Analyses performed by IRTF and GC–MS on all sam-
ples do not reveal so far any significant difference between
altered and unaltered samples. The blanching seems, like
for the varnish layers, rather due to light scattering. As the
pores have a size of more than 50 nm, the blanching can be
explained by the Mie scattering of light.
4 Conclusion
The visualization of submicronic to micronic pores (from
40 nm to 2 lm) is a major advance toward the under-
standing of this alteration of paint and varnish layers. This
structural modification was detected thanks to an approach
requiring no sample preparation. When using a more tra-
ditional method (i.e., embedding in resin and polishing a
sample cross section), the resin fills the pores rendering the
proper observation impossible. Considering the presence of
this porosity in the altered layers, the blanching or the
bluish effect observed on ancient paintings and mock-ups
can be explained by the Mie or Rayleigh scattering,
depending on the pore size. The follow-up of the alteration
for varnish layers has revealed that this phenomenon is a
dynamic process. It is important to notice that this study
has not revealed any microcracks, free fatty acid migration,
and pigment alteration, reasons formerly proposed by dif-
ferent authors to explain the blanching.
The phenomenon that contributes to the apparition of
this porosity remains so far unclear. However, the influence
of different parameters such as quantity of drier (litharge)
in the binder, nature of the pigments, and the presence of
calcium carbonate was noticed for the blanching of paint
layers. The mock-ups revealed also that the supply of
humidity and heat is not sufficient to obtain a blanching of
the paint layer, the chemical composition of the painting
has to be also considered.
The understanding of this phenomenon is a challenging
issue, still under investigation at the C2RMF. It could later
provide appropriate guidelines for durable conservation
treatments that will efficiently and safely attenuate or limit
the blanching of paintings.
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