Material analyses of ‘Christ with singing and music-making Angels’, a late 15th-C panel painting...

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Material analyses of ‘Christ with singing and music-making Angels’, a late15th-C panel painting attributed to Hans Memling and assistants: Part I.non-invasive in situ investigations†

Geert Van der Snickt,*a Costanza Miliani,bc Koen Janssens,a Brunetto G. Brunetti,b Aldo Romani,bc

Francesca Rosi,bc Philippe Walter,d Jacques Castaing,d Wout De Nolf,a Lizet Klaassen,e Ineke Labarquee

and Regine Wittermannf

Received 25th February 2011, Accepted 6th September 2011

DOI: 10.1039/c1ja10073d

In cultural heritage science, compositional data is traditionally obtained from works of art through the

analysis of samples by means of various bench-top instruments (scanning electron microscope, Raman

spectrometer, etc.). Alternatively, the object can be transported to a laboratory where it may be

examined, usually by spectroscopic methods working in reflection mode. However, this paper describes

how a complementary set of mobile and portable instruments was deployed in situ to gain

a comprehensive view on the materials and related ageing compounds of an (almost) unmovable 15th-C

polyptych, prior to and in preparation of the extraction of a limited number of samples. In line with the

methodological approach discussed, PXRF was first employed as an efficient screening tool. The

ensuing elemental data was supplemented by more specific information on both organic as inorganic

materials supplied by reflection near- and mid-FTIR spectroscopy and fluorimetry. In completion,

a limited number of diffraction patterns were collected with a mobile XRD instrument in order to

identify the constituent crystalline phases in pigments, grounding materials and degradation products.

In this way, it could be demonstrated how a rich array of colours was obtained by means of a limited

palette of pigments: lead white, lead tin yellow, azurite, natural ultramarine, bone black, vermillion,

madder lake, and a green copper-organo complex were detected and situated on the panels.

Remarkably, next to chalk also gypsum was found in the ground layer(s) of this Western European

easel painting. The relatively large surface of the background was covered with gold leaf; the analyses

seem to point towards the labour-intensive water gilding technique. The versatility of this combination

of analytical techniques was further illustrated by the accurate characterisation of degradation

products affecting the readability and conservation of the painting: the overall presence of a calcium

oxalate-based film of variable thickness was established. Nevertheless, further analysis of cross-

sectioned samples was considered desirable in order to study the stratigraphy, to gain direct access to

altered and sub-imposed layers and to allow highly detailed analysis of micrometric degradation

products by state-of-the art techniques (i.e. synchrotron radiation).

aDepartment of Chemistry, University of Antwerp, Universiteitsplein1, B-2610 Wilrijk, Belgium. E-mail: [email protected]; Fax:+32 3 265 23 76; Tel: +32 3 265 23 63bCentre SMAArt, Department of Chemistry, University of Perugia, ViaElce di Sotto 8, 06123 Perugia, ItalycCNR-ISTM (Istituto di Scienze e Tecnologie Molecolari) c/o Departmentof Chemistry, University of Perugia, Via Elce di Sotto 8, 06123 Perugia,ItalydCentre de Recherche et de Restauration des Mus�ees de France, CNRSUMR171, Palais du Louvre, Porte des Lions, 14 Quai FrancoisMitterand, 75001 Paris, FranceeKoninklijk Museum voor Schone Kunsten Antwerpen, Leopold DeWaelplaats 2, B-2000 Antwerpen, BelgiumfPrivate conservator, Hof van Uythem, Remerstraat 143, B-3130Begijnendijk, Belgium

† This article is part of a themed issue highlighting the latest research inthe area of synchrotron radiation in art and archaeometry.

2216 | J. Anal. At. Spectrom., 2011, 26, 2216–2229

Introduction

The identification and study of painting materials is an aspect of

conservation science which received considerable attention in the

recent literature1–4. The study of the materials and techniques

used by the creative artists is of great interest, not only to

investigate the (material) history of these objects and the modus

operandi of the artists of that time, but also in view of the

conservation treatment itself. A thorough technical study

provides decisive information for selection of the appropriate

conservation strategy and enhances the overall knowledge on

the complex deterioration processes associated with ancient

works of art4. During the last decade, two divergent instrumental

trends were responsible for a significant progress in this

This journal is ª The Royal Society of Chemistry 2011

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field: the implementation of mobile non-invasive equipment on

one hand and research with synchrotron radiation (SR) on the

other hand. The first development brought spectroscopic anal-

ysis literally to the conservators, making it more tangible and

comprehensible for them. The advent of mobile instruments

with analytical performances close to or equal to the conven-

tional lab-based devices made in situ analysis feasible and rele-

vant5,6. Conservators and scientists benefited greatly from this

breakthrough as in the past, transportation of fragile and

precious works of art to a laboratory or extraction of samples

often restrained them from an extensive analytical campaign.

On the other hand, synchrotron light-based analysis

transfers research to large-scale facilities, rendering spectroscopic

analysis rather abstract and inaccessible for conservators.

Nevertheless, different authors demonstrated how the elevated

spatial resolution, brightness and energy-selectivity of the

primary beam makes SR techniques indispensable for gaining

new insights in the composition of painting materials and their

degradation phenomena7–15. The authors are convinced that

a combination of both instrumental advances (mobile equipment

and synchrotron radiation) will become essential for the

conservation treatment of important works of art, such as Hans

Memling’s (ca.1435–1494) ‘Christ with singing and music-

making Angels’ (see Fig. 1).

Since 2001, conservators of the Royal Museum of Fine Arts in

Antwerp are engaged with the examination and conservation

treatment of this masterpiece, dated ca. 1487-900. From an

analytical point of view, the conservation treatment period

provided a unique time window during which the paint layers

were accessible for direct analysis, without the disturbing inter-

ference of non-original, superimposed layers (e.g. old varnish,

restoration retouchings). Additionally, the removal of the frame

and the retouchings allowed taking samples at positions which

are supportable from a deontological point of view, e.g. at the

edge of the panels or at an existing lacuna. Therefore, an

extensive analytical campaign was initiated in the framework of

collaboration between the Royal Museum of Fine Arts in Ant-

werp and the Antwerp X-ray Instrumentation and Imaging

Laboratory (AXI2L) research group16 of the University of

Antwerp.

As it is generally known that each analytical technique offers

specific prospects and limitations17, a wide range of comple-

mentary techniques were employed. In order to supply a clear

survey of this diversified sequence of investigations, a schematic

Fig. 1 Picture of ‘Christ with singing and music-making Angels’ from the col

with numbering of the angels. Pictures by Ren�e Gerritsen, copyright KMSK


overview of the applied methodology is presented in Fig. 2. The

scheme demonstrates how the analytical campaign was divided

into two main phases: (1) research on the painting and (2)

research on samples. During the first phase, a large number of

analytical measurements were performed directly on the panels

by means of mobile and portable equipment. As Fig. 2 illustrates,

during the second phase, untreated samples were first studied by

means of various synchrotron radiation-based methods. In this

way, the broadly-based information obtained during the first

phase was supplemented with highly detailed and species-selec-

tive data collected on a few samples. The samples were subse-

quently embedded in cross-section and examined with

conventional lab-based instruments for a systematic study of the

spatial distribution of the identified materials over the different

paint layers. The analysis of embedded cross-sectioned samples

at synchrotron end stations is relevant as well, as it allows col-

lecting layer-specific information. However, due to the limited

access to synchrotron facilities, it was not possible to probe each

available sample with all relevant SR techniques, before and after

embedding. In view of the large amount of data resulting from

these analytical measurements, we were prompted to divide the

study into several parts. This article is confined to the analytical

measurements on the painting by means of portable and mobile

equipment (phase 1, see 1.3 in Fig. 2). The results of the

measurements on samples before embedding (see 2.1 in Fig. 2)

and after embedding (see 2.2 in Fig. 2), will be discussed in sequel

articles.

A first explorative and fairly extensive series of analyses was

performed by means of Portable X-Ray Fluorescence (PXRF)

spectrometry. After processing and interpretation of the

resulting data, some of the instrumentation of MOLAB was

employed in a second phase to complement the ensuing find-

ings. MOLAB is a mobile laboratory composed of a unique

collection of portable equipment which is available to art-

historians, conservators, and conservation scientists through

respectively the Eu-ARTECH and CHARISMA projects,

funded by the 6th and 7th FP18,19. During the last decade,

MOLAB developed a well-established expertise concerning in

situ, non-invasive characterization of works of art making use

of e.g., fibre-optic mid-20–22 and near-FTIR23 spectrometry,

UV fluorescence spectrometry in steady state24–26 and time

resolved27 and micro-Raman28 for local molecular finger-

printing of artwork materials. In summary, it is expected that

the intensive in situ and non-invasive campaign discussed in

lection of the Royal Museum of Fine Arts in Antwerp (inv. nrs. 778–780),

A.

J. Anal. At. Spectrom., 2011, 26, 2216–2229 | 2217


Fig. 2 Scheme presenting an overview of the research methodology applied toMemling’s ‘Christ with singing and music-making Angels’. In this article,

only the results of ‘1.3: analysis on the painting’ are discussed.

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this paper, will enable to extrapolate the highly specific

information obtained from a limited number of minute

samples to the whole surface of the sizeable panels. In this

way, our objective is to obtain a maximum of information with

a minimum of sampling.


The paintings

The Royal Museum of Fine Arts in Antwerp houses over 7600

works of art illustrative for the artistic evolution in the Southern

Netherlands from the 13th- until the 20th-C. The museum is well



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known for its rich collection of Mediaeval and Northern

Renaissance paintings, containing several important works of

Flemish Primitives (e.g. Rogier Van der Weyden, Jan Van Eyck,

Gerard David, etc.). One of the key masterpieces of this period is

‘Christ with singing and music-making Angels’ dated ca. 1487-

900. This sizeable work, attributed to Hans Memling and assis-

tants, depicts Christ, flanked by sixteen angels singing and/or

playing different musical instruments. The scene is presented

against a golden background and runs on three monumental

panel paintings with inventory numbers 778, 779 and 780,

measuring 167.7 � 212.7 cm, 170 � 231.5 cm and 170 � 231 cm

respectively. Most probably, this trinity represents only the top

register of what was once an even larger polyptych, probably

originally commissioned for the convent church of Santa Maria

la Real in N�ajera, Spain29. However, since 1895, this masterwork

is part of the collection of the Royal Museum of Fine Arts in

Antwerp. In what follows, throughout the three panels, each

angel has been assigned a code number from 1 to 16 (see Fig. 1).

Experimental

As Fig. 3 illustrates, the analyses during phase 1 were carried out

in situ by means of non-invasive and non-contact techniques:

Portable X-Ray Fluorescence (PXRF), Mobile X-Ray Diffrac-

tion (MXRD), mobile reflection mid-Fourier Transform

Infrared (mid-FTIR), mobile reflection near-Fourier Transform

Infrared (near-FTIR) and mobile reflection UV-vis spectroscopy

in absorption and emission.

PXRF analyses were carried out with a TRACeR III-V device,

a commercial system manufactured by Keymaster Technologies

(currently Bruker, Karlsruhe, Germany). This compact device is

equipped with a Rh-tube with a maximal acceleration voltage of

40 kV and a beam current of 2 to 25 mA. Its maximum irradiation

power is therefore 1 W. The measuring spot has a circular shape

with a surface of approximately 1 cm2. Spectra were collected by

means of a Peltier cooled Si-PIN diode detector with a resolution

of 169.4 eV, FWHM at Mn (5.9 keV). Detector and source are

orientated in 45� geometry. All measurements were performed in

air, at a voltage of 40 kV, a current of 2.3 mA and an acquisition

time of 200s. A limited amount of spots were analysed at 15kV

and 15mA to probe for low Z elements. After measurement, all

spectra were evaluated using the software package AXIL30. As

Fig. 3 demonstrates, the apparatus was mounted on a solid

studio camera stand, permitting controlled and accurate move-

ment of the device along the X-Y-Z axes. Consequently, the

device could be levered safely to the desired position, leaving

a few mm between its snout and the object. Technical features

(e.g.minimum detection limits) and the hands-on drawbacks and

benefits for the analysis of paintings, of the analytical method in

general and this instrument in particular are discussed

elsewhere31.

Mobile X-Ray Diffraction (MXRD) combined with XRF:

a single source is used to ascertain that the X-ray fluorescence is

collected from exactly the same spot where the diffracted beams

stem from. An air-cooled iMOXS-MFR (IFG, Adlershof, Ber-

lin, Germany) X-ray tube is used. The X-ray source provides

a maximum power of 30 W; during the experiments, a voltage of

40 kV was used with a current of 700 mA. In order to increase

the useful photon flux, the source is equipped with


a polycapillary semi-lens that provides a 4 mm (3.8–4.4 mm)

diameter parallel X-ray beam (total exit divergence of 0.25� or

4.36 mrad). The copper anode provides, through a 0.1 mm

beryllium window, polychromatic X-rays necessary for XRF

measurements. The source is equipped with a 15-mm Ni filter to

strongly attenuate the Cu Kb-line and avoid the presence of

secondary diffraction peaks in addition to the main peaks due

to Cu-Ka. XRD is therefore performed with the usual mono-

chromatic radiation (Cu-Ka; E ¼ 8.047 keV; ¼ 0.154 nm); the

bremstrahlung giving rise to some background. With the

MXRD instrument, diffractograms are collected in reflection

mode using an incident angle u of 10� to the specimen surface

allowing to reach 2q values larger than 10�. Fine-grain alumina

samples were used for the calibration. The FIT 2D software32

was used to transform the two-dimensional diffraction images

into 2q spectra. Based on the latter and a database of X-ray

powder diffraction patterns, the EVA33 and XRDUA34 software

programme were used to determine the crystalline phases that

were present. The components of the MXRD system are

assembled on a frame that can be moved along the surface of

the object to be analysed. Two laser pointers intersect at the

analysis position, where the X-ray beam impinges the surface of

the object. The relatively prolonged acquisition times,

amounting up to 30 min, limited the number of measurements.

The technical features and analytical performance of this

instrument are discussed elsewhere35.

Reflection mid-infrared (mid-FTIR) spectrometry: a portable

JASCO VIR 9500 spectrophotometer equipped with a Remspec

mid-infrared fibre optic sampling probe was used. It is made up

of a Midac Illuminator IR radiation source, a Michelson inter-

ferometer and a liquid nitrogen cooledMCT-detector. The probe

was a bifurcated cable containing 19 chalcogenide glass fibres

that allowed the collection of spectra in the range 4000–900 cm�1

at a resolution of 4 cm�1. The probe diameter was about 4 mm.

The non-contact probe is kept perpendicular to the painting

surface (0�/0� geometry) at a distance of about 3 mm. The total

reflectivity, R, due to the combined diffuse and specular

components, was collected over 400 scans using the spectrum

from an aluminium mirror plate for background correction.

reflection near-infrared (near-FTIR) spectrometry: Near

infrared spectra were recorded using a portable JASCO VIR

9600 spectrophotometer made up of a halogen lamp as the

source, a Michelson interferometer equipped with a CaF2 beam

splitter,and room temperature InGaAs detector. The spectral

range is 12500–4000 cm�1 with an energy resolution of 4 cm�1.

The spectrophotometer is equipped with a silica glass fibre optic

sampling probe (2 m-long, 200/300 mm of core) which can

remotely measure the reflection of a variety of solid surfaces with

a spatial resolution of about 10 mm2. Both for mid and near-IR,

the spectrum intensity was defined as the pseudo absorbance A0

where A0 ¼ log (1/R).

Reflectance UV-vis fluorimetry (RF): Fluorescence spectra

were collected using a portable fluorimeter, assembled as

a prototype from individual components at the University of

Perugia, and already described in a previous paper25. The exci-

tation was performed at 480 nm using a suitable couple of short-

(FWHM 10 nm, 30% transmittance at 480 nm) and long-

bandpass filters (constant transmittance above 520 nm). The

spectral resolution was about 25 nm. The emission spectra



Fig. 3 Composite image showing the different mobile, analytical techniques during measurement: A: PXRF, B: reflection near-FTIR, C: reflection mid-

FTIR, D: reflection UV-vis Fluorimetry and E: MXRD.

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obtained were then corrected for the detector sensitivity using

a suitable elaborated correction file. Reflection measurements

were undertaken using a portable spectrophotometer composed

of Avantes parts that is a deuterium-halogen lamp (AvaLight-

DH-2000-FHS) as a light source, an integrating sphere with


a 6 mm diameter viewing aperture (ISP-30-6) used to collect and

transfer the reflectance signals via a quartz fibre optic system

(diameter 600 mm) and an AvaSpec–2048 CCD detector. The

AvaSoft software controls the acquisition of the spectra in the

200–1100 nm range.



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Time resolved fluorescence measurements were carried out by

a portable single photon counting apparatus described else-

where27. The instrument is composed by a pulsed source with

inter-changeable diodes and LEDs a photocathode detector

working in the 350–850 nm range a FluoroHub electronic device

containing the TAC (time-amplitude converter) and a PC that

fully controls the system of data acquisition and elaboration. It

allows a spatial resolution of about 12 mm2 and a time resolution

of about 0.4–0.5 ns or 0.1 ns using LEDs or laser diodes

respectively.

Results and discussion

The non-invasive and non-contact characteristics of mobile

analytical techniques allowed carrying out a large number of

measurements in a relatively short amount of time. Although the

different analytical sessions were spread over several years in

accordance with the progression of the conservation treatment,

the actual analyses were completed in a total of ca. 12 working

days. The sizeable panel paintings were measured in the

conservation studio and stayed on the solid, metal easels that

were especially designed for the lengthy and elaborate conser-

vation treatment. In this way, more than 200 analyses were

performed using five different diagnostic methods without any

manipulation of the panels or surface preparation. In addition to

efficiency and flexibility, another important advantage of these

in situ techniques was the direct interaction between the conser-

vators of the panels and the (cultural heritage) scientists per-

forming the analyses. The operators of the analytical instrument

could assess the results immediately after each measurement,

enabling them to confer with the conservators for interpretation

of the results before continuing with another position or per-

forming another type of measurement on the current one. In the

next paragraphs, the results of the multimodal measurements are

discussed with respect to the materials and the pigments used for

realising the specific colour groups.

White

Substantial amounts of Pb were noticeable in all PXRF spectra

recorded on all three of the panels. MXRD and reflection mid-

FTIR complemented this outcome by identifying hydrocerussite

(2PbCO3$Pb(OH)2) and/or cerussite (PbCO3) in a number of

spots.

Although a variety of lead-based materials was used in

paintings for different purposes (pigments, fillers, siccatives .),

it was nevertheless expected that the detected Pb-L XRF signals

were mainly due to the presence of lead white. This lead

carbonate hydroxide was the only white pigment used in oil

painting until the 19th-C. Consequently, it has been intensively

used by artists since ancient times to realise white areas or to

lighten the shade of other colours by intermixture. Additionally,

this pigment was commonly added to paint with the aim of

compensating the drawbacks of other pigments, e.g. to enhance

the drying properties or the opacity of the paint36.

MXRD allowed identifying hydrocerussite, the basic form of

lead carbonate in several paint areas of the Memling panels,

including white zones (e.g. in the white dress of angel #2), while

both hydrocerussite and cerussite were found in the blue


undergarment of angel #6. Welcomme et al. demonstrated by

means of Synchrotron Radiation m-XRD that the 16th-C lead

white employed by Matthias Gr€unewald, consisted of a mixture

of hydrocerussite and cerussite in variable ratios12. These

changeable proportions of the different types of lead carbonate

are probably due to deviating recipes and/or the inadequate

control of the atmospheric conditions (temperature, carbon

dioxide, moisture, etc.) during the production process36. In

addition, mid-FTIR identified hydrocerussite in the white dress

of angel #4 and hydrocerussite or cerussite on a number of

differently coloured spots, based on the characteristic shape of

the n1 + n3 combination band of carbonate moiety at 2410 cm�1

and on the presence of the OH stretching fundamental band at

3537cm�16. Still, further analysis of cross-sectioned samples is

desirable to obtain a detailed specification of the mineral phases

of lead white and to situate the detected Pb-compound(s) in the

paint stratigraphy.

Ground layer(s)

After removal of the varnish and retouchings, the ground layer(s)

surfaced in small areas where the paint was damaged. PXRF

detected Ca all over the panels while MXRD and reflection mid-

FTIR were able to specify that calcite and gypsum are present in

the ground.

PXRF alone did not permit to analyse selectively the ground

layer(s), due to the relatively large diameter of the primary beam

(spot size ca. 1 cm2). Therefore, initially, the source of the

observed calcium compound was ambiguous as the incidence of

Ca can have various causes: a chalk- or gypsum-based ground

layer, the use of chalk as filler in paint or a component of

a pigment (e.g. bone or ivory black, a substrate for an organic

dye, etc.). Fortunately, the elevated lateral resolution of the

MXRD and mid-FTIR instrument (ca. 4 mm) permitted char-

acterising the materials in the ground layer(s) without interfer-

ence of the surface paint (see Fig. 4). Clear XRD patterns were

collected in these areas, demonstrating the presence of both

calcite (CaCO3) and gypsum (CaSO4$2H2O). The incidence of

both gypsum and calcite in the ground layers was confirmed by

means of mid-FTIR. Absorptions corresponding to the combi-

nation and overtone bands at circa 2200 cm�1 and at circa

2500 cm�1, indicative of the presence of gypsum37 and calcite20

respectively, were observed in areas with emerging ground layers

but also in several undamaged areas through the painting layer.

The discovery of gypsum was rather unexpected, since it is

a characteristic grounding material for Southern European easel

painting, while the use of chalk is usually associated with

Western and Northern European artists38,39. It remained unclear

whether the Spanish destination of the paintings is in some way

related with the occurrence of gypsum in the ground layers.

Surprisingly, gypsum was not found in all analysed spots. In

a number of areas, such as the gilded area above angel #15 and

the blue dress and brown instrument of angel #5 (panel 778),

only calcite was traced. However, it remained unclear if this

discrepancy is due to the fact that gypsum is inconsistently

distributed over the surface of the panels or whether it is asso-

ciated with technical limitations of the employed equipment. For

instance, it is not impossible that in some zones, the stack of

superimposed paint layers prevents the primary 8 keV photons of



Fig. 4 Middle: detail of the paint surface showing a reserve/defect where the ground layer(s) emerge. The lower rectangle (01) indicates a spot where the

preparation seems largely preserved, while the upper rectangle (02) indicates a spot where part of the ground layer(s) is missing. A + B: reflection mid-

FTIR spectra establishing the presence of calcite and gypsum in area (01) and (02). C + D: MXRD patterns confirming the presence of calcite and

gypsum; however, the presence of gypsum is less apparent in pattern C (damaged area). This analytical result suggests the presence of multiple ground

layers with a difference in composition.

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the MXRD instrument to access the gypsum-based ground layer

and/or to escape from the paint. In view of the relatively low

energy of the primary beam and the high Z value of the elements

in the paint, a substantial absorption of both the primary and

secondary radiation can be anticipated. In addition, the low

angle of both the incident as the diffracted beams with respect to

the panel surface lengthens their path through the set of paint

layers; hence the absorption of the radiation by the materials

present is increased. As a result, it was also not completely clear if

gypsum and calcite occur in separate layers or intermixed. The

FTIR and MXRD measurements seem to suggest a layered

structure, but also here, the impending analysis of embedded

samples promises complementary information on the build-up of

the preparation layer(s).

Blue to purple

The investigations established that a wide variety of blue shades,

ranging from light blue to deep purple, was achieved by

combining the blue pigment azurite, 2CuCO3$Cu(OH)2, with an

assortment of other pigments such as bone black, lead white, lead

tin yellow (Pb2SnO4) and/or an organic red lake. In addition, the

expensive pigment ultramarine, Na7Al6Si6O24S3, was employed

exclusively to apply highlights on the blue gems of Christ.

All PXRF spectra were dominated by substantial copper and

lead peaks. In particular, the copper content was detected in the

light to dark blue garment of angels #5, #15, the wings of the

angels #2, #4, #6, #12, #13, #15 and in the sleeve and caudae


(the ribbons on the tiara) of Christ. This outcome indicated an

intensive use of azurite.

Reflection mid-FTIR analyses substantiated the identification

of azurite as the combination bands of both the copper carbonate

(structured signal at 2500 cm�1) and copper hydroxide (doublet

at 4244 and 4373 cm�1) moiety21 were clearly visible (see Fig. 5).

MXRD ascertained the presence of the blue mineral in the wings

of angel #6 and the robe of angel #5. In the latter area, MXRD

established the presence of graphite as well, most probably

intermixed with the blue paint to produce a darker shade.

Accordingly, mid-FTIR spectra of dark blue areas showed

a small sharp signal at 2010 cm�1 that is typical of bone black6,

although its vibrational assignment is still lacking. The combined

use of PXRF and reflection mid-FTIR indicated that the blue-

green feathers in the rainbow-coloured wings of angels #4 and #8

were obtained by combining azurite with lead tin yellow, as

shown in Fig. 6.

Light blue paint was obtained by combining azurite with

variable ratios of lead white. The fact that no other relevant

elements than Cu were detected in the blue-purple to deep purple

areas, suggests that this colour was obtained by combining

azurite with a red organic pigment (laminated and/or inter-

mixed). Reflectance UV-vis emission and absorption spectros-

copy succeeded in validating this hypothesis, specifying the

vegetal origin of the anthraquinone dyestuff, representative for

madder lake. The emission spectrum is shown in Fig. 7; madder

lake is identified by the shape of absorption spectrum, structured

at 510 and 540 nm, and the maximum of emission feature at

600 nm40. The identification was also confirmed by the time



Fig. 5 Left: two spectra recorded by means of reflection mid-FTIR; spectrum A illustrates the presence of both azurite and natural lapis lazuli in the

highlights of the blue gems, while only azurite was employed to realise the blue colour of the garment, as demonstrated by spectrum B. Right: detail of

the garment of Christ; the rectangles indicate the areas where the spectra A and B, shown on the left, were recorded.

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decay values that were of circa 1 and 4 ns (1, 2 and 4.3 ns of

standard madder lake in oil)27. This type of lakes was prepared

by precipitating an extract of the roots from various plants of the

Rubiaceae family onto an insoluble base, usually aluminium

hydroxide36. However, the relatively low concentration of the

latter commonly prevents detection by means of PXRF40.

Usually, analysis of paint samples (e.g. with a scanning electron

microscope equipped with an energy dispersive X-ray spec-

trometer) allows to demonstrate the metallic content of the

precipitant.

In addition, IR spectra collected on the highlights of the blue

gems decorating the cloak of Christ, revealed a sharp stretching

band at 2340 cm�1 assigned to CO2 and a broader absorption

band inverted by restrahlen effect, in the range 900–1100 cm�1.

The latter corresponds to a Si-O stretching vibration; the

distinctive incidence of silicates is indicative for ultramarine blue.

Moreover, as reported byMiliani et al., the entrapment of CO2 in

the sodalite site of lazurite is related to the geological genesis of

the mineral and consequently allows discriminating between

synthetic and natural ultramarine41. Regarding the Memling

panels, the finding of CO2 assured the natural origin of this

pigment. The constituent elements of ultramarine have low Z

values and were not detected during the PXRF analyses in

ambient air. As generally known, this highly appreciated (and

expensive) pigment was extracted from a composite rock


mineral, named lapis-lazuli, following a labour-intensive sepa-

ration method42. The lapis lazuli used in mediaeval European

paintings was most probably mined in the Kokcha River valley

in Afghanistan. The bright blue lazurite particles, which prevail

in the mineral, were isolated by subsequently heating, quenching,

grinding, kneading into a paste (made of wax, resins, etc.) and

finally by washing36. Apparently, for the Memling panels, the

application of this particularly expensive pigment was limited to

the highlights on the gems of Christ (central panel) as no ultra-

marine was found in the other blue areas. Since in all the loca-

tions where the presence of lazurite was demonstrated, also clear

signals of azurite were present (see Fig. 5), it is likely that for the

realisation of the highlights, a paint stroke containing ultrama-

rine was applied over a less expensive, opaque azurite layer43.

Green

The PXRF spectra recorded in the green paint point towards

a combined use of verdigris, Cu(CH3COO)2$nCu(OH)2, or

a derivative organo-copper complex, and lead tin yellow. The

presence of the latter pigment was confirmed by means of

MXRD, while reflection mid-FTIR did not yield any relevant

data on the pigments in these areas.

The deep green areas in the garment and wings of the angels

#1, 4, 5, 12 and 16 are characterised by intense XRF signals of



Fig. 6 The pigments identified in angel #8 demonstrate how the artists achieved a particular rich colour palette using a limited amount of colouring

substances. The PXRF spectra A to D exemplify how an array of blue to purple tints was obtained by combining a blue copper-based pigment, i.e.

azurite, with other pigments (organic lakes, lead tin yellow, carbon black and lead white).

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Cu, Sn and Pb. This is typical for a well-known green paint

stratigraphy used by mediaeval painters to realise the intense

green colour of foliage or fabrics. In this context, various authors

reported paint layers containing lead-tin yellow, lead white and

a green copper-based pigment in paintings of the 15th-


16th-C13,44. In line with this theory, the green copper-based

pigment would be verdigris or a derivative organo-copper

complex as, according to K€uhn, no other pigment (e.g.malachite

or green earth) or combination of pigments available at that time

could have created an equal intense green colour45.



Fig. 7 UV-vis emission and absorption spectra illustrating the combined

use of vermillion and madder lake to realise the red gems. UV-vis features

of a reddish area: absorption (pink line) and emission (red dotted line)

spectra. Insert: Fluorescence decay curves (red dots), excitation source

time profile (black dots), fitting curve (light grey line) and distribution of

residuals (bottom plot), lexc ¼ 455 nm, lem ¼ 620 nm.

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Verdigris was produced by reactions of metallic copper in the

presence of acetic acid. The above mentioned derivative organo-

copper complex was usually manufactured by dissolution of

verdigris in drying oil and/or resin, resulting in an amorphous

pigment containing copper carboxylate complexes of Cu(II) with

acetic, oleaginous and/or resinous acids15. In addition, analogous

compounds are also known to exist between copper and proteins

and from the reaction between copper compounds and

beeswax45. In older literature, this group of pigments is often

incorrectly referred to as ‘copper resinate’. As this type of

pigment does not necessarily contain a resin, the term ‘organo-

copper complex’ was proposed36. To complicate things even

more, it is not always clear to what extent mediaeval artists

employed this organo-copper complex intentionally or if it was

formed by the spontaneous dissolution of green copper pigments

in their organic binding medium, over time. For instance, Gunn

et al. revealed that when verdigris is mixed with an oleaginous

and/or resinous binder, the pigment starts to transform imme-

diately by the exchange of the acetato ligands of the pigment with

the carboxylic groups of the medium46.

MXRD identified lead stannate (lead-tin yellow, type I) and

hydrocerussite (lead white) in the green paint (green dress angel

#12) but failed to pinpoint the nature of the green pigment.

Nevertheless, this negative outcome is consistent with the

assumed presence of an amorphous organo-copper complex as

XRD is only sensitive towards crystalline substances. Lipid

signals from carbonyl (1745 cm�1) and C–H stretching (2845 &

2920 cm�1), both indicative of natural fats and oils, dominate the

FTIR spectra collected in green areas where conservators

removed the superimposed degradation (see further) and varnish

layers hampering any possible signals from the copper resinate

and/or acetate. The function of the detected lead-tin yellow and

lead white consists in increasing both the drying properties and

the covering power of the paint, as this type of green pigment is

known to produce a (semi-) transparent paint layer. Moreover,

lead-tin yellow compensates the bluish shade of verdigris45.

Additional measurements on cross-sectioned samples will be


necessary to clarify the chemical nature of the Cu-compounds

observed and to situate these elements in the stratigraphy.

Degradation phenomena

A number of the above mentioned green areas were of particular

interest to the conservators, not only in view of the unclear

nature of the green pigment, but especially considering the local

occurrence of a dark, opaque crust that could only be mechan-

ically removed with the aid of a scalpel and a microscope. During

cleaning tests, all of the conventional solvents failed in elimi-

nating this solid degradation product. Fig. 8 exemplifies how the

green colour of the maniple of angel #4 is concealed by the

greyish crust. In addition, a set of brownish, semi-transparent

layers seemed to be situated in between the crust and the green

paint. At first sight, this stack resembled a series of super-

imposed, old varnish layers. Nevertheless, the conservators were

questioning the originality of these layers as in the past, authors

had reported the presence of original, brownish layers on top of

green copper paint. In some cases, the green paint itself was

partly discoloured, but now and again the discoloration was

present as a separate layer resembling a glaze47,49. In addition,

Gunn et al. proved that copper(II) ions can diffuse from copper-

based paint into superimposed, organic (varnish) layers,

rendering it extremely difficult to differentiate the original paint

layers from later varnishes47. In spite of various scientific inves-

tigations, the exact nature of this type of degradation mechanism

of the green copper paint is still open for discussion15,45,47,48.

In this crust, it was possible to identify two related Ca-based

oxalate salts by means of MXRD: whewellite (CaC2O4$H2O)

and weddellite (C2CaO4$2H2O). This analytical outcome

explains the rigid and opaque aspect of the crusts and confirmed

earlier, preliminary FTIR analyses performed on surface scrap-

ings by Catherine Higgitt49. Surprisingly, the in situ mid-FTIR

analyses evidenced the widespread presence of traces of oxalates

all over the three panels, even in areas where no disturbing crust

was observed by the conservators. The characterization of

calcium oxalates by reflectance infrared is possible through the

derivative bands at 1315 cm�1 and 1620 cm�1 corresponding to ns

(C–O) + d (O–C]O) and n(C]O) respectively50. In the past,

calcium oxalates have been encountered commonly on weathered

surfaces such as outdoor stone materials of various natures

(marble, sandstone, granite, etc.)51,52, exterior wall paintings4,7,

rock art53 and even stained glass windows54. The importance of

this problem and the omnipresence of these compounds are

reflected by the large number of papers addressing this issue and

the fact that a dedicated conference was held in 199655. Never-

theless, the cause for the development of these particularly stable,

inorganic crystals is still subject of debate. In general, the

different opinions can be condensed into a chemical and a bio-

logical mechanism, both resulting from the reaction of oxalic

acid with a calcium-rich substrate: (a) the oxalate film is formed

by an oxidative degradation of organic material, or (b) the oxalic

acid is produced as a metabolic by-product of micro-biological

organisms (lichens, algae, bacteria, etc.), cultivating on organic

material. According to the conservators, the proposed prove-

nance of the organic material is most probably a non-original,

organic layer, or a stack of organic layers, applied in the course

of time with the aim of improving the appearance of the painting



Fig. 8 A: detail of a green paint surface in the maniple of angel #4. The white dotted line indicates the interface between an area where the oxalate crust

was mechanically removed by the conservators (lower part) and an area prior to cleaning (upper part). The white circle indicates the area where spectra

B, C andDwere collected. B: PXRF spectrum showing intense Cu (green copper pigment), Pb + Sn (lead tin yellow), Fe (earth pigment) and Ca (calcium

oxalate) emission lines. C: MXRD pattern establishing the presence of (among others) weddellite and whewellite. D: reflection mid-FTIR spectrum

confirming the presence of calcium oxalates.

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by saturating the deep and dark colours. Application of layers of

oil, animal fat and/or egg white with the aim of ‘reviving’ the

paint surface was a common practice at the end of the 19th-C [Y].

This would explain why the crust is especially present on the deep

green paint as well, as these areas benefit the most from satura-

tion. Apart from that, additional factors such as the deposition

of dead organisms, animal excrements or other protective

conservation treatments cannot be excluded.52,53

Nevertheless, the occurrence of calcium oxalates on easel

paintings has been rarely reported in literature. The calcium

source is less apparent and microbiological attack is less expected

due to the relatively protected indoor environment (as opposed

to open-air monuments). However, recently, degradation prod-

ucts similar to the oxalates found on the Memling panels were

encountered on 15th-C paintings from the Museu Nacional

d’Art de Catalunya in Barcelona. Salvad�o et al. employed SR

m-XRD and SR m-FTIR to establish the presence of weddellite

and whewellite on embedded samples14. In addition, Kahrim

et al. documented the removal of a calcium oxalate crust from

a 20th-C. oil painting56. In both cases, the authors indicated that

the oxalic acid was predominantly stemming from varnish layers,

a finding that corroborated with the observations on the Mem-

ling panels. In addition, the authors suggest that the calcium


compound was caused by deposition of gypsum particles from

the (museum) atmosphere. Still, the origin of calcium on the

surface of theMemling panels remains uncertain, both accidental

(e.g. environmental deposition15, application of lime wash or

other protective coatings in the church57, etc.) or deliberate

deposit of calcium compounds (ancient conservation treatment

with milk or casein products, calcium phosphate; etc.) should be

considered. The organic breeding ground could be the above

mentioned set of old varnish layers, but also here, the possible

presence of old protective coatings or consolidation treatments

(oil, resin, glue, etc.) should not be overlooked. In addition, the

imminent analysis of paint samples will be required in order to

supply information on the stratigraphical distribution of these

alteration products.

Yellow

PXRF measurements in the yellow paint of the robe of the third

angel revealed a combination of Pb and Sn, usually associated

with the pigment lead-tin yellow. This deduction was ascertained

by means ofMXRD, as the presence of a crystalline phase of lead

tin oxide (lead-tin yellow type 1) was demonstrated. On the other

hand, the artist(s) clearly preferred a yellow earth pigment



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(ochre) to paint the blond hair of the angels as all corresponding

PXRF spectra were typified by substantial iron X-ray emission

lines. Yellow earth deposits are found in numerous localities all

over Europe. These naturally occurring, stable pigments all

contain the iron oxide hydroxide goethite (FeOOH) as the key

colouring matter and are accompanied by various contaminants

such as clays, silicates, carbonates and other phases dependent of

their provenance36. However, the latter often consist of low Z

elements and/or are present in small quantities. Therefore, these

compounds were not detected by means of PXRF in ambient air.

Still, mid-FTIR corroborated the terrestrial source of supply as

kaolin, Al4[Si4O10](OH)8, was discovered in these zones. Kaolin

is a white compound with a very fine grain (<2 mm) usually

associated with art from the Middle and Far East36, but here it is

most probably present as an impurity, related to the clay

compound of the yellow earth.

Brown

According to the PXRFmeasurements, the brown paint contains

a significant concentration of iron. Consequently, an earth

pigment was employed to realise the brown areas such as the hair

of Christ or the wood of the musical instruments. The absence of

Mn-XRF signals appears to exclude the presence of (burnt)

sienna’s or umbers. The latter are both iron oxides and

hydroxides similar to ochre but discriminated by a distinct

content of manganese oxides. An alternative for sienna and

umber would be yellow ochres that can be turned in to brown or

red pigments by a thermal transformation. Basically Fe(II)

hydroxides (goethite: FeOOH) are converted into Fe(III)oxides

(such as hematite, Fe2O3) by burning them at temperatures

around 300 �C. Burnt ochres demonstrate a disordered crystal

structure, different from naturally occurring hematite, due to the

roasting [36, X]. Unfortunately, apart from quartz, MXRD did

not yield any identifiable patterns in these areas possibly due to

the significant distortion of the lattice during roasting, which

resulted in a poorly crystalline material.

Red

The flesh tones and the bright red paint areas in the wings and

garments of the angels and Christ feature intense XRF signals of

mercury. This finding points towards the use of HgS, a pigment

that was either obtained as a mineral (cinnabar) or manufactured

(vermillion) from sulphur and liquid mercury. The presence of

minium (Pb3O4), another red pigment known at that time, could

not be confirmed nor denied by PXRF. The lead emission lines of

minium would be concealed by the presence of the above-

mentioned (see title ‘white’) intense Pb-peaks lines, observed in

all spectra. However, the use of minium was not anticipated, as it

is a highly instable pigment. Colours tending to orange, such as

the feathers in the wing of the angel #9, were obtained by

a combination of vermillion with lead-tin yellow, as demon-

strated by MXRD and PXRF.

The conservators also observed a reddish glaze on top of the

vermillion-based paint. In addition, certain areas, such as the

lining of the cloak of the first angel or the clouds, present pink to

burgundy tints. Here, no relevant elements were observed in the

PXRF spectra, excluding the application of a red, inorganic


pigment. In both cases, the use of a red lake pigment was

assumed. Lake pigments are pigments manufactured by precip-

itating dyestuffs, derived from various vegetable and animal

sources, on an inert substrate (e.g. alum). Usually, lake

compounds were used for glazing purposes, modifying the colour

or adding depth to the underlying paint. Analogous with the

purple tints (see Fig. 7) the pigment was characterised as madder

lake by means of reflection UV-vis fluorimetry27,41. However, no

fluorescence was detected in the more red-brownish clouds,

possibly owing to a degraded madder lake pigment.

Black

The panels present a limited number of black areas, such as the

black trumpet played by the fourth angel. As no relevant

elements were detected by PXRF, it was deduced that the black

colour was produced by means of a carbon-based pigment

obtained by burning organic material (e.g. oil, bones, wood,

etc.). Carbon blacks of vegetal origin can be distinguished from

carbon black of animal origin as the latter are characterised by

the presence of calcium and phosphor (stemming from the burnt

bones or ivory). PXRF spectra show the emission lines of

calcium but it remained unclear whether this is due to the

underlying gypsum/chalk ground layer(s) or to the employment

of a Ca-rich carbon black pigment. The concentration and

atomic number of phosphor is usually too low for detection with

ambient air PXRF. Nevertheless, mid-FTIR spectra display

a small peak at 2010 cm�1, indicative for carbon black of animal

origin, in several black and blue areas such as the black trumpet

of angel #13 and the blue garment of Christ.

Gilding

One of the most striking features of this work of art is the large

gilded surface. Apart from lead, PXRF did not detect any metals

indicative for a gold alloy (Ag, Cu, etc.). Usually, relatively pure

gold was used for manufacturing the very thin gold leaf.

Consequently, it was assumed that the detected lead was asso-

ciated with an underlying adhesive layer. Based on the binding

medium used for the adhesive layer, historic gilding techniques

can be divided into two main procedures: (a) water gilding and

(b) mordant gilding. For the latter, the gold leaf is applied over

a thin layer containing siccative oils and/or resins. Lead-based

materials are commonly added to speed up the drying process

and to obtain the desired colour. Usually, a red-brown colour is

preferred as the underlying layer shows through the micrometric

gold leaf, giving it a warm glow and masking possible defects.

Water gilding (a) involves adhesion of the gold leaf on a so-called

bole, a fine-grained clay coloured by iron oxide pigments and

bound by water-based glue11. This clay provides a mouldable,

soft material that permits polishing of the gilded surface with an

agate stone. Consequently, water gilding leads to a shinier

surface than the mordant technique58. Bearing in mind the

dimensions of the gilt surface and considering the particularly

labour-intensive aspect of the water gilding technique, employ-

ment of a mordant (oil technique) appears to be the most plau-

sible here. The fact that PXRF detected Pb in the gilded area

seems to corroborate this theory. As already mentioned, lead-

based materials have siccative properties and are therefore



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related to oil-based, mordant gilding. Nevertheless, apart from

gold, MXRD documented the presence of goethite and quartz.

The later mineral is indicative for clay; an outcome which is

rather consistent with the presence of a bole layer (water gilding)

below the gold. Also here, it is expected that the forthcoming

analyses of samples will pinpoint the exact build-up of the gilding

and the nature of the constituting materials.

Binding media and varnish

The near-FTIR investigations registered C–H combination

bands at 4260 (nsCH2 + dCH2) and 4340 (naCH2 + dCH2) cm�1,

most probably due to lipids23, on all of the points measured on

the three panels. The presence of lipids points towards the use of

an oil and/or egg yolk as binding medium. Weak protein signals

(4595 cm�1, first overtone nCO amide I + amide II, 4880 cm�1,

nsNH + dNH)23 were exclusively registered in some small,

damaged paint areas where the ground layer emerged. As

a result, the material in the ground layers was probably bound by

glue and/or egg white. Apart from that, mid-FTIR measure-

ments demonstrated that the old organic varnish (partially

removed at the time of the measurements) contained a natural

terpenic resin. In addition, the collected spectra established the

presence of wax, mainly on the central panel 780 and to a lesser

extent on the other panels.

Conclusions

The combination of a variety of mobile instruments leads to

a far-reaching insight in the materials of a 15th-C polyptych

‘Christ surrounded by Angels playing Music’ by Hans Memling.

The advantages of these non-destructive, non-contact instru-

ments were fully exploited as a large number of spots were

analysed, allowing a systematic study of the full surface of the

sizeable panels. In addition, the in situ measurements assured

a direct interaction between conservators and (conservation)

scientists, resulting in a more effective campaign of analyses and

material identification. The supplementation of the elemental

data, gained by means of PXRF, with the species-selective

information derived from MXRD and FTIR analysis, is the key

to a more comprehensive identification of both organic and

inorganic materials used by the artists and of their current state.

In addition, the relatively short acquisition times of PXRF and

FTIR spectrometry allowed their use as swift screening tools.

The ensuing results were completed with a limited number of

MXRD measurements, which, though more time-consuming,

provided highly specific crystallographic data.

In particular, the interpretation of the analytical results

permitted to document the presence of the pigments lead tin

yellow, azurite, ultramarine, lead white, a green organo-copper

complex, brown and yellow earth, vermillion and madder lake.

Using this relatively limited pallet, Memling succeeded in

creating a wide scale of colours and optical effects. In addition,

the measurements demonstrated that the blue gems on the cloak

of Christ were worked with a combination of high-priced ultra-

marine with less expensive azurite. This set of complementary

diagnostic techniques supplied unexpected information on the

ground layers as well: both chalk and gypsum were detected in

various locations on all panels; these two materials were


probably applied in separate layers. The use of gypsum was

unexpected as this is a grounding material rather associated with

Southern European easel painting than with the Northern

European painting tradition of Memling’s era. Another note-

worthy outcome was the finding of quartz and goethite in the

gilding, suggesting the use of labour intensive water gilding as

application technique.

The capabilities of the analytical instrumentation that was

employed became particularly clear during the characterisation of

the degradation products and related phenomena. MXRD did not

only identify calcium-oxalate in the observed, opaque crust, but

also permitted detecting whewellite and weddellite separately.

Additionally, mid-FTIR revealed the distribution of these oxalates

over the entire surface of the panels, even in areas where no crust

was observed. In this way, this study did not only contribute to the

technical knowledge on early-Netherlandish painting but also

supported the ongoing conservation treatment. Moreover, the

omnipresence of alteration products on ancient paintings and the

need for better understanding their genesis and formation mech-

anisms was demonstrated once more by this study.

In conclusion, the implementation of this array of comple-

mentary and completely non-invasive analyses provided

a detailed overview of the materials used and the techniques

employed by Memling and provided some insights into the

ageing phenomena that have taken place as well. Usually this

information can only be achieved by (destructive) analysis of

samples, extracted from the panels.

Nevertheless, analysis of cross-sectioned samples remains

desirable to pinpoint the nature of all materials and their

distribution over the various layers. In this framework, the

above-mentioned investigations allowed defining relevant

regions of interests and related questions for further research in

a substantiated manner while minimising the number of samples

required for these studies. Especially, sampling of the green, blue-

purple, gilded and altered areas is considered, in order to allow

a deconvolution of the paint stratigraphy in these areas and to

better understand the development of degradation products.

Therefore, in the second phase of the study, the results obtained

in this first phase will be correlated to, confronted with and

supplemented by findings obtained via laboratory and synchro-

tron radiation based microanalysis of untreated and cross-

sectioned samples.

Acknowledgements

This research was supported by the Interuniversity Attraction

Poles Programme - Belgian Science Policy (IUAP VI/16). The

text also presents results of GOA ‘‘XANES meets ELNES’’

(Research Fund University of Antwerp, Belgium) and from

FWO (Brussels, Belgium) projects no. G.0103.04, G.0689.06 and

G.0704.08. The staff of the Royal Museum of Fine Arts Antwerp

is acknowledged for this pleasant cooperation and the author-

isation for the publication of the images in this article. Therefore,

a word of gratitude to Paul Huvenne, Yolande Deckers, Stef

Antonissen and Gwen Borms. In addition, the authors would

like to thank the MOLAB’s team operators Chiari Anselmi and

Federica Presciutti. MOLAB analyses have been carried out

through the support of the EU within the 6th Framework Pro-

gramme (Contract Eu-ARTECH, RII3-CT-2004-506171).



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Notes and references

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3 C. Aib�eo, S. Goffin, O. Schalm, G. Van der Snickt, N. Laquiere,P. Eyskens and K. Janssens, J. Raman Spectrosc., 2008, 39, 1091–1098.

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Material analyses of ‘Christ with singing and music-making Angels’, a late 15th-C panel painting...

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Transcript of Material analyses of ‘Christ with singing and music-making Angels’, a late 15th-C panel painting...