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Silver surface enrichment controlled by simultaneous RBS for reliable PIXE analysis of ancient coins
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2320–2324 www.elsevier.com/locate/nimb
Silver surface enrichment controlled by simultaneous RBS for reliable PIXE analysis of ancient coins L. Beck a,c,*, E. Alloin b, C. Berthier c, S. Re´veillon c, V. Costa d a
Centre de Recherche et de Restauration des Muse´es de France (C2RMF) – Palais du Louvre, Porte des Lions, 14 Quai Francßois Mitterrand, 75001 Paris, France b Poˆle d’Arche´ologie Interde´partemental Rhe´nan – 2 Alle´e Thomas Edison ZA Sud – CIRSUD, 67600 Se´lestat, France c INSTN/UESMS – CEA Saclay, 91191 Gif sur Yvette Cedex, France d Institut de Restauration et de Recherches Arche´ologiques et Pale´ome´tallurgiques (IRRAP), 60200 Compie`gne, France Available online 18 March 2008
Abstract Evidence of silver surface enrichment of ancient silver–copper coins has been pointed out in the past years. Surface enrichment can be fortuitous or intentional. In this paper, we have investigated the cleaning procedures usually performed after excavation or in museums. We have shown that chemicals or commercial products routinely used dissolve preferentially the copper phase and consequently contribute to the silver surface enrichment. As a result, surface analyses such as PIXE or XRF can be strongly affected by this effect. By using simultaneously RBS and PIXE, it is possible to check through the silver surface enrichment and then select the reliable measurements, characteristic of the bulk composition. Results on coins recently discovered and mechanically or chemically cleaned are presented. Ó 2008 Elsevier B.V. All rights reserved. PACS: 82.80.Yc; 81.05.Bx; 82.80.Ej; 41.75.Ak Keywords: PIXE; RBS; Silver–copper alloys; Billon; Silver enrichment; Silver coins
1. Introduction Evidence of silver surface enrichment has been pointed out in the past years and the fortuitous or intentional origin of the surface modification has been discussed by several authors. Many hypotheses have been proposed to explain the silver surface enrichment of silver–copper alloys (also called billon): (a) segregation during casting or annealing [1–4], (b) deliberate thermal and/or chemical post-treatment such as pickling in acids [2,5] or blanching [6–8], (c) wearing [9] and (d) corrosion [2,6,10–14]. According to our knowledge, very few literature reports studies on post-excavation cleaning or cleaning procedures in museum. In this paper, *
Corresponding author. Address: Centre de Recherche et de Restauration des Muse´es de France (C2RMF) – Palais du Louvre, Porte des Lions, 14 Quai Francßois Mitterrand, 75001 Paris, France. Tel.: +33 1 40 20 24 82; fax: 33 1 47 03 32 46. E-mail address: [email protected] (L. Beck). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.084
we investigated the effects of mechanical and chemical cleaning procedures usually performed on silver-based objects during fieldwork or in museums. PIXE and RBS were simultaneously used in order to characterize the surface modification in silver and copper. PIXE results were viewed as reliable when no more enrichment was detected by RBS. 2. Samples, cleaning procedures and investigation techniques 2.1. Test samples Because destructive analyses are limited for ancient coins, a set of test samples was previously studied by microscopy techniques. Two compositions were investigated: 30% Ag/70% Cu and 80% Ag/20% Cu. The samples were cleaned with one of the most commonly used solutions called ‘‘silver dip” which is made up of thiourea ((NH2)2CS) and acid mixture. The samples were partially
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
dipped in the cleaning solution during 10 min to 48 h and rinsed with water. Each sample was sectioned and mounted in epoxy resin. Investigation were carried out by optical microscope, magnification 50–1000 (Leica MF 4M) and by scanning electron microscope (JEOL 6060 LV with backscattered and secondary electron detectors, and EDS). 2.2. Coins Silver coins of the 16th century coming from a recently excavated hoard (Preuschdorf, France) have been selected for this study. Some of the coins were cleaned just after the excavation with chemicals (Fig. 1). Another set of coins were mechanically cleaned by the restorer. The surface analysis of the coins was undertaken by simultaneous PIXE and RBS. PIXE and RBS analyses were performed with the 3 MeV proton external microbeam of AGLAE (Centre de Recherche et de Restauration des Muse´es de France-C2RMF) [15]. The external beam set-up allows to perform simultaneously PIXE, PIGE and RBS with two Si(Li) X-ray detectors, one HPGe c-ray detector and one charged particle detector [16]. A Peltier-cooled X-ray detector is used for monitoring the beam dose using the Si–K peak emitted by the exit window. Experiments were carried out by using a current beam of 1 nA approx. and each record lasted 5 min. PIXE spectra were fitted by GUPIX or GUPIXWIN [17], which extract elemental concentrations. RBS spectra were simulated with SIMNRA [18].
2321
3. Results 3.1. Test samples Surface and cross-section were studied. Metallographic examination on cross-sections showed that the copper phase of the alloy was strongly affected after 48 h dipping in the silver dip solution (Fig. 2). Long time dipping has been used to simulate incomplete rinsing of the objects. Shorter time experiment samples were analyzed by nondestructive analyses (SEM-EDS), directly on the surface.
a
%Ag
%
%Cu
100 90
94 %
80 70 60 50 40 30 20 10 0 0
1
2
3
4
5
48
Steeping time (h)
b 100
97 %
80 60 40 20 0 0
1
2
3
4
48
Steeping time (h) Fig. 1. Medieval coins from the Preuschdorf’s hoard: (a) corroded, as found; (b) after cleaning with chemicals.
Fig. 3. Surface composition in silver and copper, measured by SEM-EDS, as a function of the dipping time. Initial composition: (a) 30% Ag/70% Cu and (b) 80% Ag/20% Cu.
Fig. 2. Effect of the silver dip solution (48 h) on the surface of two test samples containing: (a) 30% Ag/70% Cu and (b) 80% Ag/20% Cu.
2322
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
one was undertaken by the restorer by using a mechanical tool. The external corrosion products were removed according to aesthetic criteria. The corresponding RBS spectra and PIXE results are presented in Fig. 4(a). After the mechanical removal, a thin corroded layer still remains, corresponding to the internal corrosion of the silver–copper alloy To reach the unaltered metal, a deeper abrasion was necessary (Fig. 4(b)). The other cleanings were made by using commercial chemicals, mostly based on acid solutions. The surface of the coins was analyzed as-cleaned and on small areas abraded in order to reach the unaltered coin core alloy. RBS spectra of a coin cleaned with a commercial product containing HCl are presented in Fig. 5. RBS spectra clearly
Surface compositions are reported in Fig. 3. The copper content decreases whereas the silver content increases as a function of the dipping time. These results give evidence of a severe dissolution of the copper-rich phase induced by the silver dip solution, leading to an artificial silver surface enrichment. 3.2. Coins PIXE and RBS were simultaneously carried out to study the surface of the cleaned coins. PIXE was used to determine the coin fineness whereas RBS was used to control the surface homogeneity in order to get reliable bulk composition. Two cleaning procedures were tested. The first
a
Energy [keV] 600
1000
1400
1800
2200
2600
3000
6000
5000
Counts
4000 Cu
3000
Ag
2000 1000
CO
0
400 350
Channel
Experimental Simulated
Energy [keV]
b 600
1000
1400
2200
1800
2600
3000
6000
Counts
5000
4000
3000
2000 1000 0 50
Cu
Ag
100
150
200
250
300
350
400
450
Channel Fig. 4. RBS spectra of a 16th c. coin: (a) after mechanical removal of the external corrosion products; PIXE Ag content = 50.7%; (b) after mechanical removal of the external and internal corrosion products; PIXE Ag content = 39.6%.
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
a
2323
Energy [keV] 600
1000
1400
2200
1800
2600
3000
600 0
Counts
500 0 4000
Ag 3000
2000
Cu CO
1000 0 50
100
150
20 0
250
300
350
400
450
Channel
b
Ag
Cu
CO
c
Ag Cu
Fig. 5. RBS spectra of a 16th c. coin cleaned with a commercial cleaner containing HCl: (a) after chemical cleaning; PIXE Ag content = 73%; (b) after a 1st abrasion; PIXE Ag content = 64%; (c) after a 2nd abrasion; PIXE Ag content = 39.8%.
2324
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
Table 1 Silver concentration (in wt%) determined from the RBS or PIXE spectra for a chemically cleaned coin Coin state Chemically cleaned
Chemically cleaned and slightly abraded
Chemically cleaned and deeply abraded
RBS Enriched surface layer Substrate
80.5%, thickness 3.5 lm 42.1%
86%, thickness 2 lm 42.1% Ag
39.8%, homogenous
PIXE
73%
64%
39.6%
Comment
Presence of a silver-enriched layer at the surface of the coin, mainly induced by the chemical cleaning treatment
show an enriched layer in silver at the surface of the cleaned coins with some remains of corroded products (Fig. 5(a) and (b)). The presence of this silver-rich layer is also detected by PIXE which provides overestimated values (Table 1). After removing the enriched layer, the RBS spectrum shows a homogeneous composition corresponding to the bulk composition (Fig. 5(c)). Only in that last case, PIXE and RBS results are in agreement. Similar results were obtained for a coin cleaned with a chemical containing H2NSO3H (sulphamic acid).
4. Conclusion Previous investigations have pointed out the surface enrichment of the silver–copper alloys and the issue for reliable coin analysis. We have shown that usual chemical cleaning procedures contribute to this surface enrichment. This paper proposes a method to get reliable PIXE data by using simultaneous RBS acquisition. This method was successfully tested on corroded and cleaned ancient silver coins. The complementarity of the IBA techniques seems to be essential to determine the ancient coin fineness with accuracy. This study should be extended to innovative cleaning procedures such as laser or plasma techniques.
Acknowledgements The authors thank Karim Chaouki and Daniel Eliot for test sample preparation and characterization.
Reliable bulk analysis
References [1] L.H. Cope, in: E.T. Hall, D.M. Metcalf (Eds.), Methods of Chemical and Metallurgical Investigation of Ancient Coinage, RNS Special Publication No. 8, 1972, p. 261. [2] Z. Smit, P. Kos, Nucl. Instr. and Meth. B 3 (1984) 416. [3] G. Weber, J. Guillaume, D. Strivay, H.P. Garnir, A. Marchal, L. Martinot, Nucl. Instr. and Meth. B 161–163 (2000) 724. [4] L. Beck, S. Bosonnet, S. Re´veillon, D. Eliot, F. Pilon, Nucl. Instr. and Meth. B 226 (2004) 153. [5] S. La Niece, in: S. La Niece, P. Craddock (Eds.), Metal, Plating and Platination, Butterworth-Heinemann, London, 1993, p. 201. [6] R. Linke, M. Schreiner, Mikrochim. Acta 133 (2000) 165. [7] M. Bompaire, F. Dumas, in: Turnhout (Ed.), Numismatique me´die´vale, monnaies et documents d’origine francßaise, 2000, ISBN: 2503509517, 9782503509518. [8] H. Gitler, M. Ponting, Glaux 16, Edizioni ennerre, Milano, 2003. [9] A.-M. Mayer, G. Demortier, Nucl. Instr. and Meth. B 49 (1990) 300. [10] E.T. Hall, Archaeometry 4 (1961) 62. [11] G.F. Carter, Archaeometry 19 (1977) 67. [12] M. Basutcßu, PhD Thesis, Universite´ Paris VI, 1980. [13] I. Brissaud, P. Chevallier, C. Dardenne, N. Deschamps, J.P. Frontier, K. Gruel, A. Taccoen, A. Tarrats, J.X. Wang, Nucl. Instr. and Meth. B 49 (1990) 305. [14] R. Klockenka¨mper, H. Bubert, K. Hasler, Archaeometry 41 (2) (1999) 311. [15] J.-C. Dran, J. Salomon, T. Calligaro, Ph. Walter, Nucl. Instr. and Meth. B 219–220 (2004) 7. [16] T. Calligaro, J.-C. Dran, J. Salomon, Ph. Walter, Nucl. Instr. and Meth. B 226 (2004) 29. [17] N. Boyd, A.Weatherstone, J.L. Campbell, J.A. Maxwell, M. Vormwald, in: Proceedings of the XI International Conference on PIXE and its Analytical Applications, Puebla, Mexico, 25–29 May 2007. [18] M. Mayer, Nucl. Instr. and Meth. B 194 (2002) 177.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2320–2324 www.elsevier.com/locate/nimb
Silver surface enrichment controlled by simultaneous RBS for reliable PIXE analysis of ancient coins L. Beck a,c,*, E. Alloin b, C. Berthier c, S. Re´veillon c, V. Costa d a
Centre de Recherche et de Restauration des Muse´es de France (C2RMF) – Palais du Louvre, Porte des Lions, 14 Quai Francßois Mitterrand, 75001 Paris, France b Poˆle d’Arche´ologie Interde´partemental Rhe´nan – 2 Alle´e Thomas Edison ZA Sud – CIRSUD, 67600 Se´lestat, France c INSTN/UESMS – CEA Saclay, 91191 Gif sur Yvette Cedex, France d Institut de Restauration et de Recherches Arche´ologiques et Pale´ome´tallurgiques (IRRAP), 60200 Compie`gne, France Available online 18 March 2008
Abstract Evidence of silver surface enrichment of ancient silver–copper coins has been pointed out in the past years. Surface enrichment can be fortuitous or intentional. In this paper, we have investigated the cleaning procedures usually performed after excavation or in museums. We have shown that chemicals or commercial products routinely used dissolve preferentially the copper phase and consequently contribute to the silver surface enrichment. As a result, surface analyses such as PIXE or XRF can be strongly affected by this effect. By using simultaneously RBS and PIXE, it is possible to check through the silver surface enrichment and then select the reliable measurements, characteristic of the bulk composition. Results on coins recently discovered and mechanically or chemically cleaned are presented. Ó 2008 Elsevier B.V. All rights reserved. PACS: 82.80.Yc; 81.05.Bx; 82.80.Ej; 41.75.Ak Keywords: PIXE; RBS; Silver–copper alloys; Billon; Silver enrichment; Silver coins
1. Introduction Evidence of silver surface enrichment has been pointed out in the past years and the fortuitous or intentional origin of the surface modification has been discussed by several authors. Many hypotheses have been proposed to explain the silver surface enrichment of silver–copper alloys (also called billon): (a) segregation during casting or annealing [1–4], (b) deliberate thermal and/or chemical post-treatment such as pickling in acids [2,5] or blanching [6–8], (c) wearing [9] and (d) corrosion [2,6,10–14]. According to our knowledge, very few literature reports studies on post-excavation cleaning or cleaning procedures in museum. In this paper, *
Corresponding author. Address: Centre de Recherche et de Restauration des Muse´es de France (C2RMF) – Palais du Louvre, Porte des Lions, 14 Quai Francßois Mitterrand, 75001 Paris, France. Tel.: +33 1 40 20 24 82; fax: 33 1 47 03 32 46. E-mail address: [email protected] (L. Beck). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.084
we investigated the effects of mechanical and chemical cleaning procedures usually performed on silver-based objects during fieldwork or in museums. PIXE and RBS were simultaneously used in order to characterize the surface modification in silver and copper. PIXE results were viewed as reliable when no more enrichment was detected by RBS. 2. Samples, cleaning procedures and investigation techniques 2.1. Test samples Because destructive analyses are limited for ancient coins, a set of test samples was previously studied by microscopy techniques. Two compositions were investigated: 30% Ag/70% Cu and 80% Ag/20% Cu. The samples were cleaned with one of the most commonly used solutions called ‘‘silver dip” which is made up of thiourea ((NH2)2CS) and acid mixture. The samples were partially
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
dipped in the cleaning solution during 10 min to 48 h and rinsed with water. Each sample was sectioned and mounted in epoxy resin. Investigation were carried out by optical microscope, magnification 50–1000 (Leica MF 4M) and by scanning electron microscope (JEOL 6060 LV with backscattered and secondary electron detectors, and EDS). 2.2. Coins Silver coins of the 16th century coming from a recently excavated hoard (Preuschdorf, France) have been selected for this study. Some of the coins were cleaned just after the excavation with chemicals (Fig. 1). Another set of coins were mechanically cleaned by the restorer. The surface analysis of the coins was undertaken by simultaneous PIXE and RBS. PIXE and RBS analyses were performed with the 3 MeV proton external microbeam of AGLAE (Centre de Recherche et de Restauration des Muse´es de France-C2RMF) [15]. The external beam set-up allows to perform simultaneously PIXE, PIGE and RBS with two Si(Li) X-ray detectors, one HPGe c-ray detector and one charged particle detector [16]. A Peltier-cooled X-ray detector is used for monitoring the beam dose using the Si–K peak emitted by the exit window. Experiments were carried out by using a current beam of 1 nA approx. and each record lasted 5 min. PIXE spectra were fitted by GUPIX or GUPIXWIN [17], which extract elemental concentrations. RBS spectra were simulated with SIMNRA [18].
2321
3. Results 3.1. Test samples Surface and cross-section were studied. Metallographic examination on cross-sections showed that the copper phase of the alloy was strongly affected after 48 h dipping in the silver dip solution (Fig. 2). Long time dipping has been used to simulate incomplete rinsing of the objects. Shorter time experiment samples were analyzed by nondestructive analyses (SEM-EDS), directly on the surface.
a
%Ag
%
%Cu
100 90
94 %
80 70 60 50 40 30 20 10 0 0
1
2
3
4
5
48
Steeping time (h)
b 100
97 %
80 60 40 20 0 0
1
2
3
4
48
Steeping time (h) Fig. 1. Medieval coins from the Preuschdorf’s hoard: (a) corroded, as found; (b) after cleaning with chemicals.
Fig. 3. Surface composition in silver and copper, measured by SEM-EDS, as a function of the dipping time. Initial composition: (a) 30% Ag/70% Cu and (b) 80% Ag/20% Cu.
Fig. 2. Effect of the silver dip solution (48 h) on the surface of two test samples containing: (a) 30% Ag/70% Cu and (b) 80% Ag/20% Cu.
2322
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
one was undertaken by the restorer by using a mechanical tool. The external corrosion products were removed according to aesthetic criteria. The corresponding RBS spectra and PIXE results are presented in Fig. 4(a). After the mechanical removal, a thin corroded layer still remains, corresponding to the internal corrosion of the silver–copper alloy To reach the unaltered metal, a deeper abrasion was necessary (Fig. 4(b)). The other cleanings were made by using commercial chemicals, mostly based on acid solutions. The surface of the coins was analyzed as-cleaned and on small areas abraded in order to reach the unaltered coin core alloy. RBS spectra of a coin cleaned with a commercial product containing HCl are presented in Fig. 5. RBS spectra clearly
Surface compositions are reported in Fig. 3. The copper content decreases whereas the silver content increases as a function of the dipping time. These results give evidence of a severe dissolution of the copper-rich phase induced by the silver dip solution, leading to an artificial silver surface enrichment. 3.2. Coins PIXE and RBS were simultaneously carried out to study the surface of the cleaned coins. PIXE was used to determine the coin fineness whereas RBS was used to control the surface homogeneity in order to get reliable bulk composition. Two cleaning procedures were tested. The first
a
Energy [keV] 600
1000
1400
1800
2200
2600
3000
6000
5000
Counts
4000 Cu
3000
Ag
2000 1000
CO
0
400 350
Channel
Experimental Simulated
Energy [keV]
b 600
1000
1400
2200
1800
2600
3000
6000
Counts
5000
4000
3000
2000 1000 0 50
Cu
Ag
100
150
200
250
300
350
400
450
Channel Fig. 4. RBS spectra of a 16th c. coin: (a) after mechanical removal of the external corrosion products; PIXE Ag content = 50.7%; (b) after mechanical removal of the external and internal corrosion products; PIXE Ag content = 39.6%.
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
a
2323
Energy [keV] 600
1000
1400
2200
1800
2600
3000
600 0
Counts
500 0 4000
Ag 3000
2000
Cu CO
1000 0 50
100
150
20 0
250
300
350
400
450
Channel
b
Ag
Cu
CO
c
Ag Cu
Fig. 5. RBS spectra of a 16th c. coin cleaned with a commercial cleaner containing HCl: (a) after chemical cleaning; PIXE Ag content = 73%; (b) after a 1st abrasion; PIXE Ag content = 64%; (c) after a 2nd abrasion; PIXE Ag content = 39.8%.
2324
L. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2320–2324
Table 1 Silver concentration (in wt%) determined from the RBS or PIXE spectra for a chemically cleaned coin Coin state Chemically cleaned
Chemically cleaned and slightly abraded
Chemically cleaned and deeply abraded
RBS Enriched surface layer Substrate
80.5%, thickness 3.5 lm 42.1%
86%, thickness 2 lm 42.1% Ag
39.8%, homogenous
PIXE
73%
64%
39.6%
Comment
Presence of a silver-enriched layer at the surface of the coin, mainly induced by the chemical cleaning treatment
show an enriched layer in silver at the surface of the cleaned coins with some remains of corroded products (Fig. 5(a) and (b)). The presence of this silver-rich layer is also detected by PIXE which provides overestimated values (Table 1). After removing the enriched layer, the RBS spectrum shows a homogeneous composition corresponding to the bulk composition (Fig. 5(c)). Only in that last case, PIXE and RBS results are in agreement. Similar results were obtained for a coin cleaned with a chemical containing H2NSO3H (sulphamic acid).
4. Conclusion Previous investigations have pointed out the surface enrichment of the silver–copper alloys and the issue for reliable coin analysis. We have shown that usual chemical cleaning procedures contribute to this surface enrichment. This paper proposes a method to get reliable PIXE data by using simultaneous RBS acquisition. This method was successfully tested on corroded and cleaned ancient silver coins. The complementarity of the IBA techniques seems to be essential to determine the ancient coin fineness with accuracy. This study should be extended to innovative cleaning procedures such as laser or plasma techniques.
Acknowledgements The authors thank Karim Chaouki and Daniel Eliot for test sample preparation and characterization.
Reliable bulk analysis
References [1] L.H. Cope, in: E.T. Hall, D.M. Metcalf (Eds.), Methods of Chemical and Metallurgical Investigation of Ancient Coinage, RNS Special Publication No. 8, 1972, p. 261. [2] Z. Smit, P. Kos, Nucl. Instr. and Meth. B 3 (1984) 416. [3] G. Weber, J. Guillaume, D. Strivay, H.P. Garnir, A. Marchal, L. Martinot, Nucl. Instr. and Meth. B 161–163 (2000) 724. [4] L. Beck, S. Bosonnet, S. Re´veillon, D. Eliot, F. Pilon, Nucl. Instr. and Meth. B 226 (2004) 153. [5] S. La Niece, in: S. La Niece, P. Craddock (Eds.), Metal, Plating and Platination, Butterworth-Heinemann, London, 1993, p. 201. [6] R. Linke, M. Schreiner, Mikrochim. Acta 133 (2000) 165. [7] M. Bompaire, F. Dumas, in: Turnhout (Ed.), Numismatique me´die´vale, monnaies et documents d’origine francßaise, 2000, ISBN: 2503509517, 9782503509518. [8] H. Gitler, M. Ponting, Glaux 16, Edizioni ennerre, Milano, 2003. [9] A.-M. Mayer, G. Demortier, Nucl. Instr. and Meth. B 49 (1990) 300. [10] E.T. Hall, Archaeometry 4 (1961) 62. [11] G.F. Carter, Archaeometry 19 (1977) 67. [12] M. Basutcßu, PhD Thesis, Universite´ Paris VI, 1980. [13] I. Brissaud, P. Chevallier, C. Dardenne, N. Deschamps, J.P. Frontier, K. Gruel, A. Taccoen, A. Tarrats, J.X. Wang, Nucl. Instr. and Meth. B 49 (1990) 305. [14] R. Klockenka¨mper, H. Bubert, K. Hasler, Archaeometry 41 (2) (1999) 311. [15] J.-C. Dran, J. Salomon, T. Calligaro, Ph. Walter, Nucl. Instr. and Meth. B 219–220 (2004) 7. [16] T. Calligaro, J.-C. Dran, J. Salomon, Ph. Walter, Nucl. Instr. and Meth. B 226 (2004) 29. [17] N. Boyd, A.Weatherstone, J.L. Campbell, J.A. Maxwell, M. Vormwald, in: Proceedings of the XI International Conference on PIXE and its Analytical Applications, Puebla, Mexico, 25–29 May 2007. [18] M. Mayer, Nucl. Instr. and Meth. B 194 (2002) 177.