"Christ with singing and music making angels", Hans Memling (1433-1495). Koninklijk Museum voor Schone Kunsten, Antwerpen. Photograph: René Gerritsen.
Cultura! Heritage Conservation and Environmental Impact Assessment by Non-Destructive Testing and Mi ero-Analysis
Edited by
René Van Grieken & Koen Janssens Department of Chemistry, University ofAntwerp, Belgium
A.A. BALKEMA PUBLISHERS
LEIDEN l LONDON l NEWYORK l PHILADELPHIA l SINGAPORE
Cover painting: "Christ with singing and music making angels", Hans Memling (1433-1495) Koninklijk Museum voor Schone Kunsten, Antwerpen Photograph: René Gerritsen
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ISBN 90 5809 681 5
Printed in Great Britain
Cultura/ Heritage Conservation and Environmentallmpact Assessment by Non-Destructive Testing and Micro-Analysis- Van Grieken & Janssens (eds) @ 2005 Tay/or & Francis Group, London, ISBN 90 5809 681 5
ustrial Application of X-Ray u:chaeology. X-Ray Spectrom. tandards. Swarthmore wistics, Volume 2. New York: Identifizierung. Graz: VEB
From Giotto to De Chirico: analysis ofpaintings with portable EDXRF equipment R. Cesareo Dept. ofMathematics and Physics, University of Sassari, Sassari, Ita/y
'igmente. Weinheim: Verlag :haracteristics, Volume 3. New Untersuchung von Bildaujbau,
A. Castellano, G. Buccolieri & S. Quarta Dept. of Maten·az Science, University of Lecce, Lecce, Ita/y
M. Marabelli, P. Santopadre & M. Ioele Centra/ Restoration Institute, Rome, Italy
G.E. Gigante & S. Ridolfi Dept. of Physics, University of Rome "La Sapienza", Rome, Italy
ABSTRACT: Energy-dispersive x-ray fluorescence (EDXRF) analysis is particularly suited to analyse paintings, because of its non destructive manner and multi-elemental capacity. Interestingly, it can be conducted with portable equipment. The mean penetration of incident and secondary radiation has the same order of magnitude as the pigments thickness; EDXRF-analysis ofpaintings generally provides the following informations: -
possible presence of elements on the surface due to pollution (sulphur, chlorine); identification of the elements and pigments, used by the artist; identification of previous restored areas, detected by the presence of "modero" elements; such as titaniurn, zinc, cadmium, seleniurn, etc; identification of fraudolent submissions.
Recently the famous "Chapel of the Scrovegni" painted by Giotto in Padua in 1303-05 was analysed in detail to obtain all the information as described above, ami, more specifically, to determine the superficial presence of sulphur due to pollution and to contro l its removal. Further the golden haloes were studied in details. As another example of application of EDXRF-analysis, Il assurned paintings of De Chirico (1888-1978) were recently analysed to create a basis of comparison with 15 paintings positively identified as authentic.
l
INTRODUCTION
Energy-dispersive x-ray fluorescence (EDXRF) analysis is a highly valuable technique for the study of works of art, because it is non destructive, multi elemental, simple and relatively inexpensive (Cesareo 1988). For these reasons EDXRF is a very popular analytical technique in the field of "archaeometry" (Cesareo 1997), even though there are various types of laboratory equipment dedicated to archaeometry, as well as portable equipment. Portability ofEDXRF equipment is, of course, extremely useful and almost mandatory in many cases, such as analysis of frescoes, large paintings, bronzes, brasses, gold alloys, etc., located in museurns, churches, excavations, etc. (Cesareo et al. 2000a). In fact, only in a few cases it is possible to study a work of art outside its normallocation and for these instances portable equipment is ofthe utrnost importance. 183
There are a wide variety ofmaterials that can be studied by using a portable EDXRF apparatus: to lista few, paintings and frescoes, alloys, ceramics, illuminated manuscripts and papers, ena~els, stones and marbles. This type of portable equipment is beneficiai in its diversity of a.ppiications, such as, instances in which a qualitative analysis is sufficient (for example the individuatimì of a pigment in a painting) and others in which a quantitative approach is required (for examp~ in the case of alloys). EDXRF anaiysis generally involves an area of mm2 , and a thickness between ~-tm and a few mm. Therefore the anaiysis is superficiai and dependent on the surface conditions. In some cases 2 "capillary collimators" are emp1oyed, to focus the radiation into an area approximateiy 0.01 mm (Janssens 200I). Due to reduced analysed thickness, EDXRF ana1ysis is representative ofthe bulk composition oniy for homogeneous sampies. EDXRF analysis is specifically suited for ana1ysis ofpigments in any type ofpaintings. In this particu1ar case only a semi-quantitative anaiysis was required, and the penetration ofthe radiation was ofthe same order ofmagnitude ofthe pigment's thickness. From the analysis of paintings, following information may be deduced: a. the effects of pollution on the surfaces, generally indicated by the presence ofsuiphur or chlorine; the first element may be present, mainiy as gypsum (CaS0 4 ) at the surface of monuments, frescos etc. due to buming ofwood, cokes, gasoline, during the centuries. Gypsum produces blackening and heavy damages; chiorine, as N aCI, is sometimes present in works of art close to the sea; b. the identification of "modem" restoration areas, is indicated by the presence of eiements such as zinc, titanium, chromium, cadmium, se1enium and others; for examp1e titanium, as titanium white (oxide) was systematically employed starting from 1918, and zinc, as zinc white, starting from about 1870; c. the characterisation of the pigments commoniy used by the artist; d. the identification of fakes, for exampie on the basis of points b and c. In this paper a review is presented of recent applications of portabie EDXRF equipment for analysis ofpaintings, from Giotto in the Chapei ofthe Scrovegni (painted in the years 1303-1305) and De Chirico (1888-1978). In the chapei ofthe Scrovegni, the above outlined points a. to c. were emphasised, and the golden haioes were systematically analysed. In the case ofDe Chirico, 26 paintings were examined, including Il ofwhich were ofuncertain attribution, possibiy frauduient. This Iast hypothesis was in fact confirmed by EDXRF-analysis, based on an in-depth comparative analysis of the pigments used by the artist (point c) with the authentified De Chirico paintings.
whe1
-N -k
-w,
- aF
-m
-A; Aì is
wher - /-Li
flt
In The ~ prese cases attem Fo in the
when
-Ro
on
- /-Lo
La inc -p i -x il
Di l heavy given
2 THEORETICAL BACKGROUND When radiation from a x-ray tube penetrates the pigments of a fresco or of a painting, it is absorbed aiong its path. A fraction ofthe energy ofthe absorbed photons is converted into fluorescent photons ofthe various e1ements, and some ofthem, according to the thickness ofthe involved Iayers, reach the surface of the fresco and are detected. In the case of a fresco, the deepest Iayer is indicated by the piaster. Superimposed is the preparation, and above that one or more pigment layers. In the case of a painting, the deepest layer is given by the canvas or wood. Superimposed is the preparation, and above one or more pigment layers. As an exampie, in the case of Giotto 's haioes in the chapei of the Scrovegni the compiexity ofthe x-ray spectra indicated the presence ofvarious pigment Iayers below the golden Ieaf. Each layer behaves as a thin Iayer (Cesareo 1988); eiements are visib1e, such as strontium, coming from the deepest Iayer, which corresponds to the piaster. In the hypothesis of a sequence of thin Iayers, fluorescent counts Na from a generic element a may be written in the form: (l)
184
where [La !L,
Th~
negati Th< theau pigme Fig the ra1 discus discus
gnxRF apparatus: enamels, ity of applications, individuation of a (for example in the ~d papers,
1
·een 11m and a few ions. In some cases rximately 0.01 mm2 entative ofthe bulk suited for analysis itative analysis was tde of the pigment's
:sulphur or chlorine; monuments, frescos produces blackening tt close to the sea; ice of elements such titanium, as titanium s zinc white, starting
)XRF equipment for me years 1303-1305) ed points a. to c. were
tich were ofuncertain by EDXRF-analysis, tist (point c) with the
painting, it is absorbed
~to fluorescent photons
where: - No is the incident p ho ton flux; - k is an overall geometrica! and detector factor; - w. is the fluorescent yield; - aph.a is the cross section of element !!: for photoelectric effect; - ma is the mass per unitary area of element !!: in the sample. - A; gives the attenuati o n of incident and output radiation if element !!: is in the intemallayer j.
A; is given by: 1
A;= exp[ -~t' !l; (Eo)x; ] exp [-~j-t !-!(Eph.a)
X;]
(2)
where: - p,;(E0 ) and p,(Eph.a) is the attenuation coefficient of the i-th layer at incident energy E 0 and fluorescent energy Eph.a respectively; - x; represents the thickness of the i-th layer. In the case ofthin layers, elements from the various layers will be visible by EDXRF-analysis. The attribution to the correct layer is in many cases possible, especially when heavy elements are present in a layer and L-lines of these elements (gold, mercury, lead) are clearly visible. In these cases the approximate thickness of the pigment may be calculated through the calculation of auto attenuation, so as the thickness of a super imposed pigment or metallayer. For example the ratio R = La/Ltl of a heavy element of thickness x changes by auto attenuation in the following manner (Cesareo 2000b):
where:
- Ro is the ratio R = La!Ltl for an infinitely thin thickness; Ro depends on its theoretical value and on the detector efficiency at the two energy values ofLa and Ltl radiation (Cesareo, 2000a). - p, 0 , p, 1 , p, 2 are the mass attenuation coefficients (in cm2 /g) at incident energy and at energy of La and Ltl radiation respectively; in the case of incident Bremsstrahlung radiation the energy of incident radiation is not well defined, and an approximate mean value should be defined; - p is the physical density ofthe sample (in g/cm 3 ); - x is the thickness ofthe sample (in cm). Differential attenuation ofL" and Lf:l x-rays of a heavy element!!: (for example lead) by another heavy element Q (for example gold, as in the golden haloes of the Chapel of the Scrovegni) is given by: [La /Lil]a =
[
La /Lil]ao exp-([!lz-!ll]px
(4)
involved layers, reach
is the preparadeepest layer is given pigment layers. the complexity the golden leaf. Each tronti.1nn, coming from QIUeJ!lce of thin layers,
(l)
where: [La/Ltl]ao represents the La!Lf:l ratio (for example oflead) simply auto attenuated. The term (P- 2 - p,!) is positive for example for gold attenuation, because ofthe gold edges, and negative for example for tin. The measured La!Ltl ratio fora heavy element in a pigment is finally depending on two effects: a. the auto attenuation in the pigment containing this element and b. the attenuation in a superimposed pigment or metallayer. Figure l shows the attenuation coefficients of gold, lead and tin versus energy, and Figure 2 the ratio R = La/Ltl for lead L-lines attenuated by a gold leaf or by a tin sheet. Examples will be discussed later, conceming both pigments employed by Giotto and by De Chirico, where the above discussed cases will be present. 185
Sn 1000
---- Pb Au
~ ~
'
------on
l'l
E u
~
&
_)
~·
'
100
.... ..... l
a.
:t
'
8
3"
Q..
Cl..
.o
.o
10
12
Cl..
6
......
l
...J
l
4
......
.........
t
co.
tS ...J
2
,...
•
'-"
10
,... ,...
·~
l
.o
14
16
18
20
24
22
26
28
30
Energy (ke V) Figure l. Attenuation coefficient of gold, lead and tin versus energy, showing that Pb-L~ are more absorbed than Pb-La X-rays by Au, less absorbed by Sn.
2,5
2,0
-g
1,5
.5
2
3
4
Gold thickness (!-Lm) Figure 2.
Ratio
La!L~
oflead lines attenuated by a gold leafversus Au thickness x, following Eq. (4).
186
It should be observed that the above. described differential attenuation effects'is quantitatively evident for L-X rays ofheavy elements, but are also present in the case ofK-X rays. However this effect is much more reduced both for low-Z elements (calcium, iron, copper), because K-lines are very close in energy, and for medium-Z elements (tin) because the attenuation coefficient is very slowly changing.
Sn ---- Pb Au
3 EXPERIMENTAL SET-UP
..... ....
22
24
26
28
30
w:ing that Pb-L~ are more absorbed
For analysis of low atomic number elements (sulphur, chlorine, argon, potassium, calcium) a portable EDXRF-equipment was assemblee!, composed of a Ca-anode x-ray tube working at about 6kV (Cesareo 1999) (Fig. 3). In this case the incident radiatio n is composed ofthe Ca-K lines at 3. 7 keV, and o'f Bremsstrahlung radiation extending up to 6 keV. This radiation is able to excite in a very efficient manner sulphur, and also calcium c an be excited, but only by the last and smaller tail of the Bremsstrahlung radiation. Alternatively, also a Pd-anode x-ray tube was employed, working at 4-6kV (Cesareo 2000b). In this case the incident radiation is composed of the P d-L lines at 2.9 keV and of the Bremsstrahlung radiation. Aiso in this case sulphur is excited in a very efficient manner, while, on the contrary, calcium is excited only by the last and smaller part ofthe Bremsstrahlung spectrum In both cases an AMPTEK thermoelectrically cooled Si-PIN detector was employed, having a beryllium window of 25 ~-ttn, an energy resolution of 200 eV at 5.9 keV and coupled to a pocket AMPTEK multi channel analyser (AMPTEK 2003) For analysis of ali elements with atomic number Z higher than that of silicon approximately (including therefore elements from sulphur to calcium, however excited with low efficiency), i.e. analysis of pigments from both Giotto's Chapel of the Scrovegni, and De Chirico paintings, a portable EDXRF-equipment was employed, composed of a small size, low weight W-anode Oxford x-ray tube, working at 30 kV and l 0-50 ~-tA (OxfordAnal. Systems 2003) and a small size, thermoelectrically cooled AMPTEK detector. Si-PIN detector having a Be-window of75~-tm, with an energy resolution of200 eV at 5.9 keV and a pocket AMPTEK multi channel analyser (Fig. 4). For analysis of the 14 true De Chirico paintings, a thermoelectrically cooled Si-drift detector from EIS was employed, having an energy resolution of 150eV at 5.9keV (Fiorini 1998). The x-ray output from the W-anode x-ray tube is composed of a Bremsstrahlung radiation, with a photon intensity starting from about 4--5 keV, having a maximum at about lOkeV and decreasing up to 30kV. The order ofmagnitude ofthe fluorescence excitation efficiency was approximately the same from elements from iron to lead, a factor of 4-5 less for calcium and of20 for sulphur (Cesareo 2000). Both x-ray tube and detector were placed as close as possible to the painting, for increasing the geometrica! efficiency of the system, but without touching it. This corresponds to x-ray tube and detector at a distance of about 5-1 Omm from the painting, each at an angle of about 20o with respect to the painting normal. An area of approximately 3-5 mm2 was irradiated, varying with the distance. A typical measuring time of about 100-200 s is generally needed to obtain a x-ray spectrum with a sufficiently good statistics.
4 RESULTS 4.1 3
4
thickness x, following Eq. (4).
Determination of sulphur in frescos and especially in Giotto sfrescoes in the Chapel of the Scrovegni in Padua.
Superficial sulphur in monuments and frescos, mainly in the form of gypsum (CaS04), are the indicators of pollution. It c an be often found on the surface of frescoes and monuments, producing black colouring and damages (Laurenzi Tabasso 1992). These elements, and especially sulphur must be removed to avoid extensive damage 187
Figure 3. Equipment for sulphur and chlorine analysis, composed of a Ca-anode x-ray tube working at 6 kV and 0.1 mA, and a thermoelectrically cooled Si-PIN detector. (This figure is presented in the signature in colour.s at the end ofthis volume, Appendix, pg. 324)
188
f Ca-K...
500
/
400
S-K
300
l 200
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
Energy Figure 5. X-ray spectrum of an area ofthe lastjudgement (flame on the top left) ofthe Chapel ofthe Scrovegni obtained with the equipment shown in Figure 4. The x-ray peaks of S, Ar, K and Ca are vjsible.
e. the famous frescos by Giotto in the "chapel of the Scrovegni" in Padua were systematically analysed in July and September 2001, and again in January 2002, in about 300 points, before, during and after restoration, in order to detect the possible presence of sulphur and to test various cleaning procedures. Concerning this Iast example, as described in Section 2, sulphur was determined with two different types of equipment: one using the Ca-anode x-ray tube, the second one using the Pd-anode working at Iow voltages, to selectively excite Pd-L Iines. The fresco-pigments were analysed with the same Pd x-ray tube working at about 10 kV, and with a W-X ray tube working at 30 keV The following results were obtained: - sulphur was detected everywhere, at a concentration leve! from about l% to about 10%, depending on the exposition and on the underlying pigment, possibly due to porosity; sulphur content for example was lower in the case of azurite pigments, higher in the white and green pigments; the use ofthe Ca-anode x-ray tube (Fig. 5) gave rise to a "cleaner" spectrum with respect to the Pd-L x-ray tube, but the counting rates are much Iower, due to the much larger window ofthe Ca-anode tube; - the S-cleaning process was of great importance to the restoration of the frescoes and was continuously monitored with the EDXRF portable equipment. Various cleaning procedures were carried out, and the S-content is reported in Figure 6. The use of a cleaning process ba'!ìed on ion-exchange resins gave the best results, compatibly with the requirement to not touch the pigments lying below; - chlorine was detected only once, in an area that was possibly recently cleaned with a chlorine solution; - titanium was detected in many white areas, indicating recent restoration. 190
Ca
2600 2400
i
~a-K.._
X·RA V SPECTRUM (NO TREA. TMENT)
2200 2000
JAI'ANESE PA.PER
1800
-
1600
1/)
c
1400
:l
1200
(..)
1000
o
AMMONIUM CARBONATE (5 MI N)
s Ca
800 RESINS (5 MI N)
600 400
--J~
RESINS(IOMIN)
~ left) ofthe Chapel ofthe Scrovegni 1K and Ca are visible.
K
A
\/ ' "
200
o
4,0
'
RESINS (15 MI N)
1.5
2.0
Al'
..'\
2.5
3.5
3.0
Energy (keV)
4.0 •
l
figme 6. Sulphm x- haloex in tho Chapel oftho Sa
