- Email: [email protected]
Study of stained glass window using PIXE–PIGE
Nuclear Instruments and Methods in Physics Research B 240 (2005) 512–519 www.elsevier.com/locate/nimb
Study of stained glass window using PIXE–PIGE G. Weber
a,*
, Y. Vanden Bemden b, M. Pirotte c, B. Gilbert
d
a IPNAS, Universite´ de Lie`ge, Sart-Tilman B15, 4000 Lie`ge, Belgium Department of Histoire de lÕArt et Arche´ologie, FUNDP, Namur, Belgium c Lumie`re et couleur S.A., Richelle-Vise´, Belgium Chimie analytique, Universite´ de Lie`ge, CEA-Centre Europe´en d’Arche´ome´trie, Re´gion Wallonne–Division du Patrimoine, Belgium b
d
Available online 25 August 2005
Abstract We had the opportunity to study a large panel (100 · 80 cm) containing more than 40 stained glass pieces. Among them several come from restorations having taken place at different periods. The study of this rather complex arrangement has been processed by stages:
• the elemental composition of 16 zones were determined: several differences were identified and among them the Na/K ratio which allowed to set three groups of glass type; • the measurement of the Na concentrations by the two techniques give information in bulk (PIGE) and at the near surface (PIXE); the values defined by the (CPIGE–CPIXE))/CPIGE plotted in function of the historical estimation of the age of the stained glass pieces (original and restored) indicate a real correlation between the two variables; • the red-colored pieces were specially investigated in order to determine which coloration technique was employed (bulk coloration, superficial staining, multilayered flashing, etc.); • the corrosion was investigated by scanning two different worsened zones with a 0.5 mm diameter beam spot. This study shows the possibilities of the PIGE–PIXE association, but also points out some weaknesses, which have to be solved by other techniques; unfortunately, in that case, the non-destructive aspect could be lost. 2005 Elsevier B.V. All rights reserved. PACS: 29.30.Kv; 89.90.+m Keywords: Stained glass; External PIXE; RBS
*
Corresponding author. Tel.: +32 4366 3690; fax: +32 4366 2884. E-mail address: [email protected] (G. Weber).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.225
*
*
*
2397
2164
575 462 554
658 3034
639 437 2378 2682
806
92.23 101.47 98.75 93.98 110.47 90.09 100.16 103.96 98.11 107.61 93.40 99.85 84.84 82.06 74.81 95.56 467 415 486 497 2990 2442 2267 1521 2765
97 137 148 100 16 132 105 50 244 26 271 70 206 98 210 12 494
538
483
433
2565
570
591
2152
76 88 60 71 13 112 73 55 72 40 218 391 986 156 1395 15 5161 5692 6337 6056 1595 2496 6270 2300 3403 2351 3501 3756 3569 5112 3559 1136 1472 1643 1846 1748 277 9604 1855 1075 4599 1085 3334 2092 3793 1380 2674 222 1333 1416 1345 1303 195 1253 1244 298 953 329 908 903 733 1084 708 258 17.54 19.41 19.82 18.78 17.42 11.83 19.96 16.42 21.85 17.05 16.75 15.26 19.37 15.05 15.86 14.89 0.94 1.05 1.01 0.93 0.10 4.68 1.00 0.19 2.06 0.22 1.46 0.68 1.64 0.84 1.51 0.12 0.05 0.12 0.05 0.03 0.13 0.10 0.02 0.05 0.03 0.08 0.08 0.14 0.00 0.13 0.07 0.12 0.10 0.11 0.13 0.11 0.03 0.10 0.13 0.06 0.24 0.05 0.18 0.07 0.29 0.12 0.23 0.05 64.68 71.40 68.63 65.15 78.21 67.90 69.70 77.03 67.47 79.90 69.66 73.83 57.89 58.00 52.06 65.96 1.49 1.78 1.87 1.83 1.26 3.46 1.93 1.15 3.45 1.07 2.67 1.59 2.68 1.64 3.00 1.25 Non-quantified silver.
7.15 7.28 6.88 6.82 13.18 1.44 7.11 8.95 2.56 9.13 2.30 8.04 2.40 5.93 1.70 13.07
*
Ti ppm Ca% K% S% P% Si% Al% Mg% Na%
A B C D E F G H I J K L M N O P
Sixteen (A to P) areas were analyzed. They concern the reverse side of the panel. This limited number of impacts has been chosen in function of the considered coloration and possible restorations. All these zones were free from grisaille and
Na g%
3.1. Quantitative analysis
Impact
3. Results and discussion
Table 1 Measured concentrations on impacts A to P
The experimental atmospheric PIXE–PIGE set-up of the IPNAS in Lie`ge has been described elsewhere [4], but its main specific feature is the possibility to rotate the sample around an axis passing through the beam path. The particle beam is extracted in air using a thin nickel foil (2.5 lm). This metal has been chosen because of its good thermal strength and because it does not produce c-rays under proton bombardment. The particle beam is monitored by counting the charged particles scattered by a gold layer (50 nm) deposited on a polycarbonate backing, which interrupts the beam at a 2 Hz frequency. A device allowing to replace the air between the beam impact and the detector by helium equips the X-ray detector. A large reduction of the low energy X-rays absorption is then obtained.
Mn ppm
2. Material and methods
0.27 0.31 0.36 0.33 0.13 0.59 0.31 0.10 0.43 0.10 0.31 0.24 0.56 0.35 0.39 0.11
Fe ppm
Co ppm
Ni ppm
Cu ppm
Zn ppm
As ppm
Sr ppm
In the field of archaeometry, the stained glass has been extensively studied by several techniques as ICP-MS, SEM-EDX, XRF [1–3] and very interesting results have been obtained. Unfortunately, all these tools need small glass specimens and sometimes they are destructive. The aim of the present paper is to show what is possible to collect as information by applying totally non-destructive techniques. Having the opportunity to study a large panel removed from its usual location for restoration, we applied PIXE and PIGE under normal and differential way, in order to get the maximum of data likely to be useful for the restorer.
6.27 6.75 6.35 5.98 14.56 0.73 6.39 9.21 1.38 9.37 0.59 6.62 1.04 5.26 0.95 12.17
Balance
Ag
1. Introduction
513
*
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
514
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
only cleaned with distilled water. The beam diameter was 0.8 mm. The results of these measurements are reported on Table 1. The two first columns give the concentrations respectively obtained by PIGE and PIXE. The major element concentrations are expressed in % of oxides, whereas minor and trace ones are expressed in ppm (lg/g) of elements.
In addition to the basic elements of glass, several ones are responsible for the coloration (Co for blue, Ag for yellow, Cu for red and green) Fig. 1. Fig. 2 shows the Na/K ratio for the 16 analyses. Three zones have been selected with respect to of that ratio. They correspond to three types of glass.
Fig. 1. Atmospheric PIXE–PIGE set-up.
Na/K 1000
Soda
126.7
100
105.1 46.1
40.6
11.9
10
7.6
6.9 6.8
7.3
7.1
1.6
1.2
1 0
2
4
6
8
7.0
Soda-Potash
10
12
1.5 14
1.1 16
Potash
0.3 0.1
Impact N˚
Fig. 2. Classification of impacts in function of the Na/K ratio.
18
20
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
Potash glass Na/K < 2. Soda–potash glass 2 < Na/K < 20. Soda glass 20 < Na/K. This classification, at first glance empirical, can be justified by the difficulties to obtain native soda (sodium carbonate) from Egypt after the collapsing of the Roman Empire, although it is possible that during some time natron continued to be transported (perhaps as ‘‘chunk glass’’) to Northern Europe. Reuse of Roman soda glass was also evidenced. Finally, some time before the 12th century, ash from plants or trees becomes resolutely the main source of alkalis, but with K > > Na. The restorations of 18th and 19th centuries are indeed made with modern soda glass because, at this time, soda ash was replaced by artificial soda produced by the Leblanc process (1787). This classification can be compared with the one due to Foy and Nenna [5], who use the same criterion to define antic and medieval glass categories. As the PIXE–PIGE technique is able to give the very superficial Na2O concentration (PIXE) and the bulk one (PIGE), the corrosion status can be estimated by the following ratio: R¼
C PIGE C PIXE ; C PIGE
where CPIGE and CPIXE are the Na2O concentrations measured respectively by PIGE and PIXE. The difference between CPIGE and CPIXE is essentially due to the difference of the absorption coefficient between c-rays and X-rays. 0.8 0.7 0.6
R
0.5 0.4 0.3 0.2 0.1 0 15
16
17
18
19
20
21
22
23
24
25
515
On the graph reported in Fig. 3, the R values are plotted against the estimated age of the analyzed zones (century). These estimations have been done on the basis of several documented restorations. One can observe that the more recent the glass is the smaller is the R value and it seems then possible to use this parameter in order to help to estimate the age of the different pieces of glass which make up the panel. However, it is clear that the environmental conditions during the conservation time have to be taken into account (geographical localization, orientation, composition, etc). 3.2. Qualitative analysis After a quantitative analysis of the previous ‘‘clean’’ areas, 23 other areas were investigated. Because of the presence of interfering layers as painting (yellow stain, grisaille, etc), corrosion products and external deposits, the quantitative feature has been lost. In this paper, only some typical areas will be presented, but a full report with more details can be obtained [6]. 3.2.1. Grisaille The grisaille is dark enamel matted on the glass surface to delineate features. A mixture of highly fusible lead glass powder, iron oxide, copper oxide is painted on glass surface with the help of a medium as gum Arabic. The latter enables the enamel to flux and adhere to the glass substrate before firing. The ratio between copper and iron oxides varies at different periods, which explains the change in color from full black to reddish-brown [7]. Two specific grisaille areas were analyzed. The first corresponds to an original glass piece and the second to a restored one. We observe significant differences between the Cu/Pb ratios corresponding to the two samples. These two examples show that the Cu/Fe ratio in the grisaille could be another parameter that may help to date a stained window glass piece.
Century
Fig. 3. Relative difference between sodium oxide concentrations obtained by PIXE and PIGE as a function of the estimated age (century) of the different areas investigated.
3.2.2. Silver yellow glass Gold aspect in stained glass windows is generally obtained by firing transparent glass onto
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G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519 1.200
Normalized peak area
D (glass thickness)
Silver yellow
Experimental data D=0
1.000
D=1 D=1.5 D=2
0.800
0
10
20
30
40
50
60
70
80
Proton incident angle (degree)
Fig. 4. Determination of the depth reached by the silver yellow stain in the glass window.
which silver nitrate mixed with medium has been painted [8]. The result is a gold-colored zone due to a thin layer containing metallic silver at some distance from the surface. It is interesting to know that distance because silver yellow is often considered as a protective layer against the corrosion process. In Fig. 4, by comparing the theoretical evolution of the AgK a X-ray detected as a function of the incident angle with the experimental values [9], one can estimate the above-mentioned distance as about 1.5 lm. The theoretical values have been calculated using the different parameters as the stopping power in glass, the X-ray absorption coefficients, the X-ray production cross-sections, etc. 3.2.3. Red glass Except for grisaille and silver yellow stain, the color in window glass came from the glass itself, which is colored through the entire depth. For red color it is somewhat different: the red glass obtained by addition of Cu oxide in the pot-metal is so dense in tone that it would not transmit light sufficiently. For solving this problem, multilayered glasses were realized. The result was either a thin red glass layer on a thick uncolored glass substrate or a thin red glass layer sandwiched by two thick uncolored glass layers. In the first case, the detection of Cu by PIXE can be performed on the right side of the sample but in the second case, if the uncolored glass layers are too thick, PIXE analysis is impossible.
Furthermore, the thickness of ancient window glass is far from being constant along its surface. The detection of Cu will be then much more difficult. In Fig. 5, the two PIXE spectra correspond to different impacts 5 mm apart on the same side of a red-colored glass piece. Cu as a coloring agent is only detected at one impact. Furthermore, Ag appears at the second impact because the yellow layer under the red one is also reached in this case. This shows that care must to be taken when analyzing this kind of multilayered red glass sample. 3.2.4. Corrosion Two different areas have been scanned with a 2.5 MeV proton beam (0.8 mm B). Scanning 1: corroded area distant from the window leads. Fig. 6 shows the scanned path, which starts in a ‘‘virgin’’ area, crosses a corrosion crater, leaves it and enters another corrosion crater. The following graphs give the values of the X-ray peak areas for several elements. It should be noticed that all the values are monitored by RBS measurements and normalized function according to the values of the first impact. The graph in Fig. 7 indicates the evolution of K, Ca and Si along the scanning path. In the corroded zone, K and Ca decrease due to the extraction by water responsible for the corrosion process. On the other hand, Si presents a slight increase in the same area. That is due to the formation of silica after the water molecules leave the glass network. This behaviour corresponds to the usual glass corrosion process. In Fig. 8, the increase of S could be responsible of an enhancement of the corrosion process. SO2 could be the external source of this element. The similar behaviour of Pb could be due to the fact that in the crater, the surface is far from being smooth. This could help to trap lead salts coming from windows leached by rain or moisture water. Scanning 2: corroded area close to the window leads. As shown in Fig. 9, the scanning starts in a white area and leave it to enter a ‘‘virgin’’ part. In Fig. 9, one observes a decrease of Ca with a simultaneous increase of Si and Mn. This is due to the fact that the white area is not a corroded one but rather a layer of calcium compound coming
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
517
Fig. 5. PIXE spectra corresponding to two different beam spots on a red areas.
1.2
Pic/RBS
1 Na g Si K
0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
7
8
9
DISTANCE (mm)
Fig. 7. Evolution of Ca, K and Si along the scanning path.
Fig. 6. View of scanning 1. The beam spot diameter is 0.8 mm.
from the window leads. The thickness of this layer is reducing more and more until arriving in the glass itself. This is the reason why Ca decreases and Si and Mn increase: the X-rays from these elements are less and less absorbed. Absorption is in-
deed less important for Mn than for Si. These elements belong to the glass itself as shown in Fig. 10. The nature of the white compound was determined by Raman analysis on a small fragment on the white layer: it is calcium carbonate. This material probably comes from the mastic used for mounting the glass pieces with leads.
518
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519 10
8
9
7
1
3
2
8 7
Pic/RBS
Pic/RBS
6 S Pb
5 4
6 Si Ca Mn
5 4 3
3
2
2
1
1
0 0
CaCO3
5
10
15
20
25
0 0
1
2
3
4
5
6
7
8
9
Glass
DISTANCE (mm)
Fig. 8. Evolution of S and Pb along the scanning path.
DISTANCE (mm)
Fig. 10. Evolution of Si, Mn and Ca along the scanning path.
niques. It enabled to display several problems, which have been generally solved, but some questions to be answered induce the use of other techniques. Unfortunately, these techniques are often destructive or need small samples. From the quantitative analyses of glass pieces free of ‘‘painting’’ (grisaille, silver yellow), the glasses have been sorted as a function of the ratio Na/K. Three categories have been obtained: soda, soda-potash and potash glasses which correspond to Na/K > 20, 2 < Na/K < 20, Na/K < 2. Another classification has been also deduced from the relative ratio between the superficial Na concentration (PIXE: 1 lm) and the bulk Na concentration (PIGE 0–30 lm). The analysis of several areas showing different and/or painting allowed obtaining information without taking apart the window glass pieces. The differential PIXE has been applied to measure the depth of the silver yellow stained layer. Finally, two deteriorated areas have been scanned with the proton beam and some information has been deduced from the evolution of several elements along the scanned path. Fig. 9. View of scanning 2. The beam spot diameter is 0.8 mm.
Acknowledgements 4. Conclusions This study has been essentially achieved with the couple of non-destructive PIXE–PIGE tech-
We are indebted to the Institut Universitaire des Sciences Nucle´aires and the Re´gion Wallonne: division du Patrimoine (Belgium) for their scientific and financial support.
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
References [1] I. Borbely-Kiss, Z. Fueloep, A.Z. Kiss, E. Koltay, G. Szabo, Nucl. Instr. and Meth. B 785 (1994) 836. [2] M.J. Baxter, H.E.M. Cool, M. Heyworth, C. Jackson, Archaeometry 37 (1995) 129. [3] M. Schreiner, Proceedings of the Institute of Archaeometry Jubilee Conservation Conference (1987), 73. [4] G. Weber, J.M. Delbrouck, D. Strivay, F. Kerff et, L. Martinot, Nucl. Instr. and Meth. B 139 (1998) 196. [5] D. Foy, D.D. Nenna, Tout feu tout sable, Muse´e de Marseille, Editions Edisud (2000), 23.
519
[6] G. Weber, Etude par PIXE dÕun panneau de vitrail de la Chapelle Castrale dÕEnghien, Feneˆtre IV Panneau 2 A, Rapport final, Re´gion Walonne (Belgium) – Division Patrimoine (2004), 1. [7] S. Davison, Conservation and restoration of glass, Butterworth-Heinemann, ISBN 07 506 43412 (2003), 123. [8] D. Jembrih-Simbu¨rger, C. Neelmeijer, O. Schalm, P. Fredrickx, M. Schreiner, K. De Vis, M. Ma¨der, D. Schreyvers, J. Caen, J. Anal. At. Spectrom. 17 (2002) 321. [9] G. Weber, D. Strivay, L. Martinot, H.P. Garnir, Nucl. Instr. and Meth. B 189 (2002) 350.
Study of stained glass window using PIXE–PIGE G. Weber
a,*
, Y. Vanden Bemden b, M. Pirotte c, B. Gilbert
d
a IPNAS, Universite´ de Lie`ge, Sart-Tilman B15, 4000 Lie`ge, Belgium Department of Histoire de lÕArt et Arche´ologie, FUNDP, Namur, Belgium c Lumie`re et couleur S.A., Richelle-Vise´, Belgium Chimie analytique, Universite´ de Lie`ge, CEA-Centre Europe´en d’Arche´ome´trie, Re´gion Wallonne–Division du Patrimoine, Belgium b
d
Available online 25 August 2005
Abstract We had the opportunity to study a large panel (100 · 80 cm) containing more than 40 stained glass pieces. Among them several come from restorations having taken place at different periods. The study of this rather complex arrangement has been processed by stages:
• the elemental composition of 16 zones were determined: several differences were identified and among them the Na/K ratio which allowed to set three groups of glass type; • the measurement of the Na concentrations by the two techniques give information in bulk (PIGE) and at the near surface (PIXE); the values defined by the (CPIGE–CPIXE))/CPIGE plotted in function of the historical estimation of the age of the stained glass pieces (original and restored) indicate a real correlation between the two variables; • the red-colored pieces were specially investigated in order to determine which coloration technique was employed (bulk coloration, superficial staining, multilayered flashing, etc.); • the corrosion was investigated by scanning two different worsened zones with a 0.5 mm diameter beam spot. This study shows the possibilities of the PIGE–PIXE association, but also points out some weaknesses, which have to be solved by other techniques; unfortunately, in that case, the non-destructive aspect could be lost. 2005 Elsevier B.V. All rights reserved. PACS: 29.30.Kv; 89.90.+m Keywords: Stained glass; External PIXE; RBS
*
Corresponding author. Tel.: +32 4366 3690; fax: +32 4366 2884. E-mail address: [email protected] (G. Weber).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.225
*
*
*
2397
2164
575 462 554
658 3034
639 437 2378 2682
806
92.23 101.47 98.75 93.98 110.47 90.09 100.16 103.96 98.11 107.61 93.40 99.85 84.84 82.06 74.81 95.56 467 415 486 497 2990 2442 2267 1521 2765
97 137 148 100 16 132 105 50 244 26 271 70 206 98 210 12 494
538
483
433
2565
570
591
2152
76 88 60 71 13 112 73 55 72 40 218 391 986 156 1395 15 5161 5692 6337 6056 1595 2496 6270 2300 3403 2351 3501 3756 3569 5112 3559 1136 1472 1643 1846 1748 277 9604 1855 1075 4599 1085 3334 2092 3793 1380 2674 222 1333 1416 1345 1303 195 1253 1244 298 953 329 908 903 733 1084 708 258 17.54 19.41 19.82 18.78 17.42 11.83 19.96 16.42 21.85 17.05 16.75 15.26 19.37 15.05 15.86 14.89 0.94 1.05 1.01 0.93 0.10 4.68 1.00 0.19 2.06 0.22 1.46 0.68 1.64 0.84 1.51 0.12 0.05 0.12 0.05 0.03 0.13 0.10 0.02 0.05 0.03 0.08 0.08 0.14 0.00 0.13 0.07 0.12 0.10 0.11 0.13 0.11 0.03 0.10 0.13 0.06 0.24 0.05 0.18 0.07 0.29 0.12 0.23 0.05 64.68 71.40 68.63 65.15 78.21 67.90 69.70 77.03 67.47 79.90 69.66 73.83 57.89 58.00 52.06 65.96 1.49 1.78 1.87 1.83 1.26 3.46 1.93 1.15 3.45 1.07 2.67 1.59 2.68 1.64 3.00 1.25 Non-quantified silver.
7.15 7.28 6.88 6.82 13.18 1.44 7.11 8.95 2.56 9.13 2.30 8.04 2.40 5.93 1.70 13.07
*
Ti ppm Ca% K% S% P% Si% Al% Mg% Na%
A B C D E F G H I J K L M N O P
Sixteen (A to P) areas were analyzed. They concern the reverse side of the panel. This limited number of impacts has been chosen in function of the considered coloration and possible restorations. All these zones were free from grisaille and
Na g%
3.1. Quantitative analysis
Impact
3. Results and discussion
Table 1 Measured concentrations on impacts A to P
The experimental atmospheric PIXE–PIGE set-up of the IPNAS in Lie`ge has been described elsewhere [4], but its main specific feature is the possibility to rotate the sample around an axis passing through the beam path. The particle beam is extracted in air using a thin nickel foil (2.5 lm). This metal has been chosen because of its good thermal strength and because it does not produce c-rays under proton bombardment. The particle beam is monitored by counting the charged particles scattered by a gold layer (50 nm) deposited on a polycarbonate backing, which interrupts the beam at a 2 Hz frequency. A device allowing to replace the air between the beam impact and the detector by helium equips the X-ray detector. A large reduction of the low energy X-rays absorption is then obtained.
Mn ppm
2. Material and methods
0.27 0.31 0.36 0.33 0.13 0.59 0.31 0.10 0.43 0.10 0.31 0.24 0.56 0.35 0.39 0.11
Fe ppm
Co ppm
Ni ppm
Cu ppm
Zn ppm
As ppm
Sr ppm
In the field of archaeometry, the stained glass has been extensively studied by several techniques as ICP-MS, SEM-EDX, XRF [1–3] and very interesting results have been obtained. Unfortunately, all these tools need small glass specimens and sometimes they are destructive. The aim of the present paper is to show what is possible to collect as information by applying totally non-destructive techniques. Having the opportunity to study a large panel removed from its usual location for restoration, we applied PIXE and PIGE under normal and differential way, in order to get the maximum of data likely to be useful for the restorer.
6.27 6.75 6.35 5.98 14.56 0.73 6.39 9.21 1.38 9.37 0.59 6.62 1.04 5.26 0.95 12.17
Balance
Ag
1. Introduction
513
*
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
514
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
only cleaned with distilled water. The beam diameter was 0.8 mm. The results of these measurements are reported on Table 1. The two first columns give the concentrations respectively obtained by PIGE and PIXE. The major element concentrations are expressed in % of oxides, whereas minor and trace ones are expressed in ppm (lg/g) of elements.
In addition to the basic elements of glass, several ones are responsible for the coloration (Co for blue, Ag for yellow, Cu for red and green) Fig. 1. Fig. 2 shows the Na/K ratio for the 16 analyses. Three zones have been selected with respect to of that ratio. They correspond to three types of glass.
Fig. 1. Atmospheric PIXE–PIGE set-up.
Na/K 1000
Soda
126.7
100
105.1 46.1
40.6
11.9
10
7.6
6.9 6.8
7.3
7.1
1.6
1.2
1 0
2
4
6
8
7.0
Soda-Potash
10
12
1.5 14
1.1 16
Potash
0.3 0.1
Impact N˚
Fig. 2. Classification of impacts in function of the Na/K ratio.
18
20
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
Potash glass Na/K < 2. Soda–potash glass 2 < Na/K < 20. Soda glass 20 < Na/K. This classification, at first glance empirical, can be justified by the difficulties to obtain native soda (sodium carbonate) from Egypt after the collapsing of the Roman Empire, although it is possible that during some time natron continued to be transported (perhaps as ‘‘chunk glass’’) to Northern Europe. Reuse of Roman soda glass was also evidenced. Finally, some time before the 12th century, ash from plants or trees becomes resolutely the main source of alkalis, but with K > > Na. The restorations of 18th and 19th centuries are indeed made with modern soda glass because, at this time, soda ash was replaced by artificial soda produced by the Leblanc process (1787). This classification can be compared with the one due to Foy and Nenna [5], who use the same criterion to define antic and medieval glass categories. As the PIXE–PIGE technique is able to give the very superficial Na2O concentration (PIXE) and the bulk one (PIGE), the corrosion status can be estimated by the following ratio: R¼
C PIGE C PIXE ; C PIGE
where CPIGE and CPIXE are the Na2O concentrations measured respectively by PIGE and PIXE. The difference between CPIGE and CPIXE is essentially due to the difference of the absorption coefficient between c-rays and X-rays. 0.8 0.7 0.6
R
0.5 0.4 0.3 0.2 0.1 0 15
16
17
18
19
20
21
22
23
24
25
515
On the graph reported in Fig. 3, the R values are plotted against the estimated age of the analyzed zones (century). These estimations have been done on the basis of several documented restorations. One can observe that the more recent the glass is the smaller is the R value and it seems then possible to use this parameter in order to help to estimate the age of the different pieces of glass which make up the panel. However, it is clear that the environmental conditions during the conservation time have to be taken into account (geographical localization, orientation, composition, etc). 3.2. Qualitative analysis After a quantitative analysis of the previous ‘‘clean’’ areas, 23 other areas were investigated. Because of the presence of interfering layers as painting (yellow stain, grisaille, etc), corrosion products and external deposits, the quantitative feature has been lost. In this paper, only some typical areas will be presented, but a full report with more details can be obtained [6]. 3.2.1. Grisaille The grisaille is dark enamel matted on the glass surface to delineate features. A mixture of highly fusible lead glass powder, iron oxide, copper oxide is painted on glass surface with the help of a medium as gum Arabic. The latter enables the enamel to flux and adhere to the glass substrate before firing. The ratio between copper and iron oxides varies at different periods, which explains the change in color from full black to reddish-brown [7]. Two specific grisaille areas were analyzed. The first corresponds to an original glass piece and the second to a restored one. We observe significant differences between the Cu/Pb ratios corresponding to the two samples. These two examples show that the Cu/Fe ratio in the grisaille could be another parameter that may help to date a stained window glass piece.
Century
Fig. 3. Relative difference between sodium oxide concentrations obtained by PIXE and PIGE as a function of the estimated age (century) of the different areas investigated.
3.2.2. Silver yellow glass Gold aspect in stained glass windows is generally obtained by firing transparent glass onto
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Normalized peak area
D (glass thickness)
Silver yellow
Experimental data D=0
1.000
D=1 D=1.5 D=2
0.800
0
10
20
30
40
50
60
70
80
Proton incident angle (degree)
Fig. 4. Determination of the depth reached by the silver yellow stain in the glass window.
which silver nitrate mixed with medium has been painted [8]. The result is a gold-colored zone due to a thin layer containing metallic silver at some distance from the surface. It is interesting to know that distance because silver yellow is often considered as a protective layer against the corrosion process. In Fig. 4, by comparing the theoretical evolution of the AgK a X-ray detected as a function of the incident angle with the experimental values [9], one can estimate the above-mentioned distance as about 1.5 lm. The theoretical values have been calculated using the different parameters as the stopping power in glass, the X-ray absorption coefficients, the X-ray production cross-sections, etc. 3.2.3. Red glass Except for grisaille and silver yellow stain, the color in window glass came from the glass itself, which is colored through the entire depth. For red color it is somewhat different: the red glass obtained by addition of Cu oxide in the pot-metal is so dense in tone that it would not transmit light sufficiently. For solving this problem, multilayered glasses were realized. The result was either a thin red glass layer on a thick uncolored glass substrate or a thin red glass layer sandwiched by two thick uncolored glass layers. In the first case, the detection of Cu by PIXE can be performed on the right side of the sample but in the second case, if the uncolored glass layers are too thick, PIXE analysis is impossible.
Furthermore, the thickness of ancient window glass is far from being constant along its surface. The detection of Cu will be then much more difficult. In Fig. 5, the two PIXE spectra correspond to different impacts 5 mm apart on the same side of a red-colored glass piece. Cu as a coloring agent is only detected at one impact. Furthermore, Ag appears at the second impact because the yellow layer under the red one is also reached in this case. This shows that care must to be taken when analyzing this kind of multilayered red glass sample. 3.2.4. Corrosion Two different areas have been scanned with a 2.5 MeV proton beam (0.8 mm B). Scanning 1: corroded area distant from the window leads. Fig. 6 shows the scanned path, which starts in a ‘‘virgin’’ area, crosses a corrosion crater, leaves it and enters another corrosion crater. The following graphs give the values of the X-ray peak areas for several elements. It should be noticed that all the values are monitored by RBS measurements and normalized function according to the values of the first impact. The graph in Fig. 7 indicates the evolution of K, Ca and Si along the scanning path. In the corroded zone, K and Ca decrease due to the extraction by water responsible for the corrosion process. On the other hand, Si presents a slight increase in the same area. That is due to the formation of silica after the water molecules leave the glass network. This behaviour corresponds to the usual glass corrosion process. In Fig. 8, the increase of S could be responsible of an enhancement of the corrosion process. SO2 could be the external source of this element. The similar behaviour of Pb could be due to the fact that in the crater, the surface is far from being smooth. This could help to trap lead salts coming from windows leached by rain or moisture water. Scanning 2: corroded area close to the window leads. As shown in Fig. 9, the scanning starts in a white area and leave it to enter a ‘‘virgin’’ part. In Fig. 9, one observes a decrease of Ca with a simultaneous increase of Si and Mn. This is due to the fact that the white area is not a corroded one but rather a layer of calcium compound coming
G. Weber et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 512–519
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Fig. 5. PIXE spectra corresponding to two different beam spots on a red areas.
1.2
Pic/RBS
1 Na g Si K
0.8 0.6 0.4 0.2 0 0
1
2
3
4
5
6
7
8
9
DISTANCE (mm)
Fig. 7. Evolution of Ca, K and Si along the scanning path.
Fig. 6. View of scanning 1. The beam spot diameter is 0.8 mm.
from the window leads. The thickness of this layer is reducing more and more until arriving in the glass itself. This is the reason why Ca decreases and Si and Mn increase: the X-rays from these elements are less and less absorbed. Absorption is in-
deed less important for Mn than for Si. These elements belong to the glass itself as shown in Fig. 10. The nature of the white compound was determined by Raman analysis on a small fragment on the white layer: it is calcium carbonate. This material probably comes from the mastic used for mounting the glass pieces with leads.
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8
9
7
1
3
2
8 7
Pic/RBS
Pic/RBS
6 S Pb
5 4
6 Si Ca Mn
5 4 3
3
2
2
1
1
0 0
CaCO3
5
10
15
20
25
0 0
1
2
3
4
5
6
7
8
9
Glass
DISTANCE (mm)
Fig. 8. Evolution of S and Pb along the scanning path.
DISTANCE (mm)
Fig. 10. Evolution of Si, Mn and Ca along the scanning path.
niques. It enabled to display several problems, which have been generally solved, but some questions to be answered induce the use of other techniques. Unfortunately, these techniques are often destructive or need small samples. From the quantitative analyses of glass pieces free of ‘‘painting’’ (grisaille, silver yellow), the glasses have been sorted as a function of the ratio Na/K. Three categories have been obtained: soda, soda-potash and potash glasses which correspond to Na/K > 20, 2 < Na/K < 20, Na/K < 2. Another classification has been also deduced from the relative ratio between the superficial Na concentration (PIXE: 1 lm) and the bulk Na concentration (PIGE 0–30 lm). The analysis of several areas showing different and/or painting allowed obtaining information without taking apart the window glass pieces. The differential PIXE has been applied to measure the depth of the silver yellow stained layer. Finally, two deteriorated areas have been scanned with the proton beam and some information has been deduced from the evolution of several elements along the scanned path. Fig. 9. View of scanning 2. The beam spot diameter is 0.8 mm.
Acknowledgements 4. Conclusions This study has been essentially achieved with the couple of non-destructive PIXE–PIGE tech-
We are indebted to the Institut Universitaire des Sciences Nucle´aires and the Re´gion Wallonne: division du Patrimoine (Belgium) for their scientific and financial support.
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