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Portable EDXRF investigation of the patinas on the Riace Bronzes
Nuclear Instruments and Methods in Physics Research B 343 (2015) 101–109
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Portable EDXRF investigation of the patinas on the Riace Bronzes Giovanni Buccolieri a,⇑, Alessandro Buccolieri b, Paola Donati c, Maurizio Marabelli c, Alfredo Castellano a a
Università del Salento, Dipartimento di Matematica e Fisica, via Arnesano, 73100 Lecce, Italy Università del Salento, Dipartimento di Scienze e Tecnologie Biologiche e Ambientali, via Monteroni, 73100 Lecce, Italy c Istituto Superiore per la Conservazione e il Restauro, via di San Michele n. 23, 00153 Roma, Italy b
a r t i c l e
i n f o
Article history: Received 5 August 2014 Received in revised form 3 November 2014 Accepted 7 November 2014 Available online 5 December 2014 Keywords: Riace Bronzes Patina Non-destructive analysis EDXRF Statistical analysis
a b s t r a c t This paper summarizes the experimental results concerning the Energy Dispersive X-ray Fluorescence (EDXRF) analysis of patinas on two Riace Bronzes, kept in the National Archaeological Museum of Reggio Calabria (Calabria, Southern Italy). The two large Greek sculptures, famous nude bearded warriors both dated in the fifth century BC, are without a doubt, two masterpieces of inestimable historic and artistic value. EDXRF survey had the aim to determinate the chemical composition of the surface of these two bronze statues and to discriminate their different patinas. In particular, the concentration of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead was determined by using a portable apparatus. Multivariate statistical analysis was carried out in order to identify possible correlations and/or differences of elemental composition among the patinas of these two statues. The information obtained made it possible to improve knowledge about the patinas of the Riace Bronzes, and this may help further studies and subsequent methods of restoration and/or of preservation of the two celebrated Greek sculptures. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A valid analytical method for the study of statues of historical and artistic interest should be non-destructive, fast, universal, sensitive, multi-elemental and portable. EDXRF satisfies these requirements and, for this reason, it is widely used for qualitative and quantitative elements analysis in the field of cultural heritage. By using such method, objects can be analyzed with good repeatability without removing them from the exhibition, thus minimizing the inconvenience for the public. Moreover, the nondestructive character of the analysis and its short measurement times, allows to obtain a large number of analytical data and consequently the mapping of the monument [1–3]. On the other hand, in archaeometric the use of EDXRF for accurate quantitative analysis is severely inhibited by the fact that the irradiated area is usually large, the detector sensitivity for light elements is limited, the surface of the sample is irregular and its composition is non-homogeneous. These parameter are usually ⇑ Corresponding author. Tel.: +39 0832 297087; fax: +39 0832 297100. E-mail addresses: [email protected] (G. Buccolieri), alessandro. [email protected] (A. Buccolieri), [email protected] (P. Donati), [email protected] (M. Marabelli), [email protected] (A. Castellano). http://dx.doi.org/10.1016/j.nimb.2014.11.064 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
non-ideal and/or not well-defined, leading to errors in the quantification of the elements. Nowadays, however, thanks to the innumerable studies carried out in the field of cultural heritage these parameters can be efficaciously corrected by using fundamental parameters (FP) method and theoretical coefficient algorithms [4–7]. Bronze is an alloy based on copper and tin, which can often contain other elements such as zinc and lead at different concentration. The aesthetic appearance and the integrity of a bronze monument can be altered by various environmental factors, natural and anthropogenic, such as temperature, humidity, acid rain, atmospheric particulate matter, sulphur oxides, nitrogen oxides and marine aerosol. The most obvious result of the alteration of bronze objects is the appearance of a thin layer of corrosion called patina. Among the most famous bronze monuments in the world it is possible to remember the Riace Bronzes, two statues discovered on 16 August 1972 in the Ionian Sea, near the coast of Riace (Reggio Calabria, Southern Italy), at a distance of about 200 m from the coast and at a depth of about 8 m. The Riace Bronzes represent two bearded warrior and they are named respectively as Bronze A ‘‘The Young’’ and Bronze B ‘‘The Old’’ (Fig. 1). Statue A is 1.98 m high, whereas Statue B is 1.97 m. According to the most reliable sources, both statues are dated in the fifth century BC and, more specifically, Bronze A is dated from
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around 460 BC, and Bronze B refers to about 430 BC. The latest study of radiocarbon dating has allowed to obtain ages ranging between 517 and 394 BC for statue A, and between 509 and 394 BC for statue B [8,9]. The statues were probably made in Greece [10] and subsequently removed to be transported to Rome, where they never arrived because the boat that was carrying the precious cargo sank in Ionian Sea and it ended up submerged in sand until 1972, when the two sculptures were found. The very long stay underwater heavily corroded the two masterpieces and this has demanded extensive studies for their restoration and conservation. The first cleanings and the removal of the marine concretion were accomplished in 1975 in Reggio Calabria. The two statues were later transferred to Opificio delle Pietre Dure in Florence, where from 1975 to 1980 they were exposed to a total surface cleaning and analysis. In particular, the patinas on Bronze B were partially eliminated through a mechanical cleaning. Table 1 summarizes the median values of the concentration of elements determined on the alloy of the two statues during the restoration of 1975–1980 in Florence [11]. Both sculptures are made of a copper-tin binary alloy. The average thickness of the alloys is about 8.5 mm and 7.5 mm for Bronze A and for Bronze B respectively, even if the thickness varies widely between 4 and 12 mm. However, these values are typical of the Archaic and Classical age. This paper summarizes the experimental results concerning the Energy Dispersive X-ray Fluorescence (EDXRF) analysis of patinas on two Riace Bronzes. In particular, the EDXRF survey had the aim to determine the chemical composition of the patinas and to map their distribution. The EDXRF non-destructive technique represents an appropriate methodology for the analysis of the elemental composition of surface layers [12–17], for the individuation of corrosive components potentially harmful, for support the study of corrosion phenomena and finally for the individuation of the most proper environmental parameters suitable for the conservation of the above-mentioned Statues in the Museum exhibition Hall of Reggio Calabria. Furthermore, we would like to underline that the EDXRF procedure is an extremely efficient and relatively easy method of performing a large number of measurements quickly and in situ and, therefore, to map the entire surface of the monuments.
Table 1 Elements mean concentration (wt.%) determined in the alloy of the Riace Bronze during the restoration of 1975–1980 [11]. Bronze A
Fe Cu Sn Zn Pb Ag Ni Co Ca Mg
Bronze B
Body
Head
Body
Head
0.13 88.8 10.75 – 0.12 0.037 0.012 – 0.102 0.032
0.035 87.35 12.41 0.004 0.062 0.021 0.016 – 0.068 0.023
0.16 90.08 9.26 0.01 0.155 – 0.021 – 0.257 0.041
0.13 85.94 13.5 0.005 0.233 – 0.020 – 0.129 0.05
2. Materials and methods 2.1. EDXRF analysis EDXRF portable equipment, which was assembled in our laboratory, is mainly composed of an X-ray tube (MOXTEK Inc., USA) and of a detector Si-PIN (Amptek Inc., USA). The detector has a resolution of energy of about 180 eV at 5.9 keV. The output of the Xray tube is highly collimated and the analyzed area has a diameter of about 3 mm. For each measuring point, two EDXRF spectra were acquired, both with acquisition time of 30 s and with a tube voltage of 6 kV (at 40 lA) or of 20 kV (at 3 lA). A set of standard samples, with known chemical compositions, were used to calibrate the apparatus and to obtain reliable experimental data. For standards preparation were used copper sulphide (CuS), copper(I) chloride (CuCl), manganese oxide (MnO2), iron, zinc, lead(II) oxide (PbO), lead dioxide (PbO2), minimum or lead tetroxide (Pb3O4), tin dioxide (SnO2), copper(I) oxide (Cu2O) and copper(II) carbonate basic (CuCO3Cu(OH)2). All chemical compounds were purchased from Sigma–Aldrich with analytical grade. Each standards was prepared by mixing the compounds in different weight percentages. The use of different compounds with the same elements, but in different oxidation state, is irrelevant from the point of view of the XRF analysis, but has allowed to
Fig. 1. Image of two Riace Bronzes, Reggio Calabria, Italy. Bronze A named ‘‘The Young’’ (a) and Bronze B named ‘‘The Old’’ (b).
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G. Buccolieri et al. / Nuclear Instruments and Methods in Physics Research B 343 (2015) 101–109 Table 2 Elements concentration (wt.%) determined on the Bronze A by using EDXRF. Label
Description of the patina
S
Cl
Sn
Mn
Fe
Cu
Zn
Pb
AF1 AF2 AF3 AF4 AF5 AF6 AF7 AF8 AC1 AC2 AC3 AC4 AC5 AC6 AC7 AC8 AC9 AA1 AA2 AA3 AA4 AA5 AL1 AL2 AL3 AL4 AL5 AL6 AL7 AB1 AB2 AB3 AFO1
Black patina on the face, the left cheekbone Green patina on the face, right cheekbone Black patina on the face Black patina on the hair band Black patina on the nose Black patina on the hair band Brown patina on the hair band Brown patina (and rough) on the neck Red patina (above black) on the chest Green patina on the chest Black patina on the chest Red patina (above black) on the chest Red patina (above black; green hue) on the chest Black patina (green hue) on the chest Black patina on the chest Green patina on the chest Green patina on the chest Red patina on the abdomen Black patina on the abdomen Green patina on the abdomen Red patina (above the black) on the abdomen Green patina on the abdomen (near the corrosion craters) Black patina on the left leg Red patina (above black) on the left leg Green patina on the left leg Green patina on the left leg Whitish patina on the left leg Red patina (above black) on the left leg Black patina on the left leg Black patina on the right bicep Green patina (above the black) on the right bicep Grey–green patina (under the black) on the right bicep Black patina on the left foot
7.4 4.0 6.6 8.4 6.2 7.1 2.2 4.6 4.9 3.7 10.1 5.3 3.6 9.8 11.1 3.7 <0.5 6.8 8.1 5.0 7.6 5.2 12.0 8.7 8.1 0.8 7.6 <0.5 12.5 11.8 <0.5 5.1 8.3
4.5 3.1 <1.5 2.0 2.1 2.9 4.9 3.9 3.9 2.6 2.1 4.2 2.3 1.8 2.6 1.7 10.9 4.6 3.4 <1.5 3.6 6.4 1.9 3.3 1.8 15.7 <1.5 5.3 2.8 2.0 14.9 2.3 2.6
13.6 23.0 25.4 33.5 27.8 8.2 23.9 18.7 5.4 31.1 15.9 <3.0 7.2 6.4 4.0 32.1 10.3 1.1 9.7 32.5 10.2 18.7 9.8 5.6 23.4 5.1 23.1 <3.0 5.3 2.3 4.1 27.4 26.6
0.6 0.5 0.6 0.7 0.6 0.5 0.3 0.3 <0.2 0.3 <0.2 <0.2 <0.2 <0.2 <0.2 1.2 <0.2 0.5 <0.2 0.5 <0.2 0.5 0.3 0.3 0.3 0.3 0.5 0.3 <0.2 0.3 0.3 0.4 0.7
7.1 6.4 7.4 10.7 6.0 4.4 4.6 3.9 3.5 5.6 4.7 2.5 1.8 7.9 2.7 10.0 3.3 10.1 3.9 5.7 5.4 6.7 4.1 4.0 9.0 4.5 5.9 4.3 3.2 3.1 4.9 6.7 8.1
58.1 59.0 49.7 23.4 45.9 74.0 61.2 66.0 73.5 53.4 63.9 79.6 72.1 28.6 73.3 36.6 69.5 72.4 73.5 53.4 64.6 62.0 69.5 74.6 29.9 62.7 57.5 81.7 73.6 72.2 71.9 57.6 50.2
<1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 4.3 <1.0 <1.0 <1.0
3.6 3.2 <2.0 3.1 2.0 2.6 2.6 2.2 3.3 3.0 3.1 3.1 2.8 3.8 3.2 3.2 3.5 <2.0 <2.0 <2.0 <2.0 <2.0 2.2 2.3 <2.0 2.0 2.5 3.0 2.4 <2.0 <2.0 <2.0 2.9
Table 3 Elements concentration (wt.%) determined on the Bronze B by using EDXRF. Label
Description of the patina
S
Cl
Sn
Mn
Fe
Cu
Zn
Pb
BF1 BF2 BF3 BF4 BF5 BF6 BC1 BC2 BC3 BC4 BC5 BC6 BA1 BA2 BA3 BA4 BA5 BA6 BA7 BAR1 BAR2 BL1 BL2 BL3 BL4 BL5 BL6 BFO1
Red on black patina on the helmet Green patina between the two eyes Dark brown patina on the nose Green patina on the left cheek Black patina on the left cheek Black patina on the right cheek Green patina on the right chest Brown patina on the right chest Green patina on the right chest Brown patina on the right chest Dark brown patina on the right chest Black patina on the right chest Green patina on the abdomen Brown–yellow–green patina on the abdomen Brown–yellow–green patina on the abdomen Green patina on the abdomen Black patina on the abdomen Brown–yellow–green patina on the abdomen Black patina on the abdomen Black patina on the right arm Green patina on the right arm Black patina on the left leg Yellow–green patina on the left leg Green patina on the left leg Green patina on the left leg Green patina on the left leg Whitish patina on the left leg Black patina on the left foot
<0.5 <0.5 <0.5 3.1 9.2 2.1 <0.5 3.2 <0.5 3.6 <0.5 7.0 2.2 1.1 <0.5 3.5 6.3 1.9 3.6 7.8 <0.5 9.6 2.9 2.5 1.0 5.1 0.7 8.6
4.0 1.9 2.1 1.5 2.3 3.0 2.9 1.9 <1.5 1.9 1.9 1.5 <1.5 <1.5 <1.5 <1.5 <1.5 1.7 1.5 2.9 <1.5 2.3 1.9 10.3 8.4 11.5 2.1 1.8
18.9 39.6 37.6 41.9 35.5 39.0 42.8 18.9 42.8 17.4 38.6 24.4 41.4 43.6 36.3 35.5 34.1 37.0 40.0 18.9 48.5 27.0 33.9 12.4 15.7 12.3 30.5 25.8
0.3 0.5 0.4 0.3 0.5 0.6 0.7 0.2 0.8 <0.2 0.7 0.7 0.3 0.8 0.5 0.3 0.6 0.7 0.8 0.5 0.3 <0.2 0.2 0.2 0.2 <0.2 <0.2 <0.2
2.0 3.9 2.2 1.7 4.0 6.8 5.7 3.4 5.0 1.3 6.0 5.7 4.6 5.0 5.3 3.6 5.3 5.4 6.0 5.9 4.2 1.6 3.8 4.4 2.0 1.3 5.1 1.6
56.5 18.6 44.6 41.7 25.9 27.1 18.2 64.2 19.2 62.3 23.5 16.7 10.0 14.4 13.5 5.9 9.6 9.0 12.3 47.3 11.1 13.0 6.1 17.4 14.7 19.4 2.5 15.6
2.8 4.0 <1.0 4.8 3.3 4.1 <1.0 4.6 4.5 4.7 3.5 2.8 2.5 4.5 5.7 4.9 1.8 5.6 1.6 3.7 <1.0 1.4 5.8 3.1 3.4 <1.0 9.7 1.5
3.1 3.2 2.7 3.9 3.8 4.6 2.8 3.3 3.8 3.5 3.5 3.8 4.2 3.9 4.0 4.6 3.5 4.6 3.1 4.2 3.9 3.0 3.3 2.6 2.4 2.0 4.2 2.8
obtain the calibration samples at different concentrations. In particular, the chemicals compounds have been weighed by using an analytical balance KERN model ABT 100-5M, subsequently mixed and homogenized in an agate mortar for ten minutes and finally compressed at 200 bar for ten minutes. The homogeneity of ele-
ments in the standard meets the requirements for the EDXRF quantitative analysis [18]. Moreover, the samples analyzed are supposed to ‘‘infinite thickness’’ and therefore the quantitative results are expressed in terms of weight percentage (% wt). A thickness of cuprite (Cu2O) from 40 lm reduces by a factor of 100 the
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Table 4 Minimum, maximum, mean and standard deviation values (wt.%) of elements determined on Bronze A. Element
Minimum
Maximum
Mean
Standard Deviation
S Cl Sn Mn Fe Cu Zn Pb
<0.5 <1.5 <3.0 <0.2 1.8 23.4 <1.0 <2.0
12.5 15.7 33.5 1.2 10.7 81.7 4.3 3.8
6.3 3.8 14.9 0.3 5.5 61.1 0.1 2.0
3.4 3.6 10.9 0.3 2.3 14.9 0.7 1.4
Table 5 Minimum, maximum, mean and standard deviation values (wt.%) of elements determined on Bronze B. Element
Minimum
Maximum
Mean
Standard Deviation
S Cl Sn Mn Fe Cu Zn Pb
<0.5 <1.5 12.3 <0.2 1.3 2.5 <1.0 2.0
9.6 11.5 48.5 0.8 6.8 64.2 9.7 4.6
3.0 2.5 31.8 0.4 4.0 22.9 3.4 3.5
3.1 2.9 10.6 0.3 1.7 17.3 2.2 0.7
Fig. 3. Measuring points on the abdomen and on the left leg of the Bronze A.
Fig. 2. Measuring points on the face and chest of the Bronze A.
intensity of a radiation at 10 keV, therefore both the standard samples (about 2 mm thick) and the bronze analyzed can be considered thick samples. Moreover, the standards were tested on certified materials of bronze purchased from Goodfellow (Cambridge Science Park, England). In particular, we used a foil of bronze Cu94/Sn6 (size 25 25 mm and thickness 0.25 mm) and three standards of bronze in powder (Cu89/Sn11, Cu80/Sn20 and Cu80/Pb20). The concentrations of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead were determined, for each measurement point, in order to individuate the chemical composition of the surface of two Riace Bronzes and to map their chemical distribution.
Fig. 4. Measuring points on the right bicep of the Bronze A.
The evaluation of the spectral interference between sulphur (K-line) and lead (M-line) is fundamental in their quantitative EDXRF analysis. The separation between K-line of sulphur and M-line of lead is equal to about 40 eV, a value which is lower than the resolution of the detector (equal to about 180 eV). To solve this problem, the separation of the two signals was obtained through the deconvolution of the peaks. This deconvolution is possible con-
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Fig. 5. Measuring points on the face, chest and abdomen of the Bronze B.
sidering both the higher sulphur concentration, compared to lead, and the different fluorescent yields between the two elements [19]: K-line of sulphur (0.061) and M-line of lead (0.032). The values of detection limit, all reported in wt.%, for sulphur, chlorine, tin, manganese, iron, copper, zinc and lead are equal respectively to 0.5, 1.5, 3.0, 0.2, 1.0, 1.0, 1.0 and 2.0. The software Microcal Origin Professional has been used to elaborate the experimental data obtained. 2.2. Statistical analysis Experimental results obtained by EDXRF analysis have been elaborated with multivariate statistical analysis [20–25] in order to identify possible correlations and/or differences existing among the elemental composition of the two Bronzes. The computer software package STATISTICA (StatSoft Inc., Tulsa, OK, USA) was used for statistical analysis and the Principal Component Analysis (PCA) and the Hierarchical Cluster Analysis (HCA) were used as statistical techniques. The PCA method calculates the orthogonal linear combinations of the autoscaled variables by using a correlation matrix based on the maximum variance criterion. Such linear combinations are called principal component scores and the coefficients of the linear combinations are called loadings. The numerical loading value, for each variable of a given principal component, shows how much the variable has in common with that component. PCA can give information about the similarities and groupings of the samples considered, and if a trend exists, it can evaluate the possibility of classifying the samples. The HCA method is a statistical treatment designed to reveal groupings (or clusters) within a data set that sometimes would not be apparent. The purpose of this analysis is to join together objects into successively larger clusters, by using some measure of similarity or dissimilarities.
Fig. 6. Measuring points on the left leg of the Bronze B.
The basic criterion for any clustering is distance and probably the most straightforward way of computing the distances among objects in a multidimensional space is to compute Euclidean distances. Objects that are near each other should belong to the same cluster, and objects that are far from each other should belong to different clusters. The dendrogram is the graphical summary of the cluster analysis. The cases (or the variables) are listed along the horizontal axis, while the distance among the clusters is showed along the vertical axis.
3. Results and discussion A close visual examination of both Riace Bronzes has allowed to differentiate the typologies of patinas present on the two statues. Statue A shows mainly three different patinas: a green patina with various hues, quite uneven and with reduced thickness, alternated with a second patina, brown, thin, and with a third one patina, black on the top, multilayer, smooth, apparently compact, dense and homogeneous. Sometimes, the third patina is concealed by a red layer of a compound rich of copper, probably caused by deposited cupper (then oxidized to cuprite, Cu2O), already analyzed at the time of the first investigations following the discovery of the statues [26,27]. Statue B is largely covered by a brown–yellow– green patina, that appeared unusual, probably at least partially
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Fig. 7. Loading of the variables on the first two principal components.
artificial, alternated with a thin brown patina and a few residual islands of the black patina. Tables 2 and 3 summarize the elements concentrations determined by using EDXRF respectively on the surfaces of statue A and of statue B. For each measurement point, the label, the description of the patina and the concentration of determined elements are shown in the tables. Overall, sixty-one patinas have been investigated (thirty-three patinas on the Bronze A and twenty-eight patinas on the Bronze B). Tables 4 and 5 show the values of minimum, maximum, mean and standard deviation for elements determined respectively on the surface of Bronze A and of Bronze B. The Figs. 2–6 show the positions of all points of measurement on the two Greek famous sculptures except AFO1, BAR1, BA2 and BFO1. The values of sulphur concentration are in general higher for Bronze A (<0.5–12.5% wt) in comparison with Bronze B (<0.5–9.6% wt) and in five areas of the Bronze A the sulphur concentration is higher than 10% wt. The black patina always shows prevailing sulphur and copper concentrations, corresponding to the presence of copper sulphides,
Fig. 8. Scatter plot of the scores for the first two principal components.
while the red thin superimposed layer is probably recognized as cuprite (Cu2O), deriving from deposited copper subsequently oxidized [28,29]. In general, the green patina shows moderate sulphur concentrations. Chlorine concentration for the two Riace Bronzes is comparable, but it is slightly higher for Bronze A (<1.5–15.7% wt) in comparison with Bronze B (<1.5–11.5% wt). The green patinas can be divided in three typologies, with different hues: grey–green, green and brown–yellow–green. Grey–green patina has an extensive presence and an uniform colour on the Bronze A: it is particularly evident in zones where the black patina was removed, due to the mechanical friction and wearing action of sand and marine sediments against the bronze surface. The green patinas, in some cases with hues light blue or whitish, are usually small and quite common on both Riace Bronzes. The green patinas AC9, AL4 and AB2 show higher concentration of chlorine and copper, indicating a significant accumulation of copper chlorides. The latters are very important chemical compound in corrosion processes of copper alloys. It is now known that there are four polymorph structures of copper trihydroxychloride (Cu2Cl(OH)3), such as, atacamite, paratacamite, botallackite and clinoatacamite and the nature of the chemical reactions in ‘‘bronze disease’’ is well described in literature [30–34]. Therefore, the presence of copper chloride must be carefully evaluated in order to perform a correct storage and this requires a periodic control of the surface of the two masterpieces. The brown–yellow–green patinas are present in extended areas on Bronze B, with low concentrations of chlorine (less than 2% wt) and of copper (9–15% wt), high percentages of tin (36–44% wt) and also significant values of zinc (4–6% wt). Tin concentration is higher on Bronze B (12.3–48.5% wt) than on Bronze A (<3.0–33.5% wt). Bronze B exhibits very high tin concentration (34.1–43.6% wt) especially in the zones of the abdomen. In particular, greater tin values are found in the brown–yellow–green areas, in some way sustaining the hypothesis of an intense electrochemical corrosion, but mainly of an artificial chemical surface treatment with consequent enrichment of this alloy component in the oxidized form, stable, of cassiterite (SnO2). Furthermore, in the marine environment, stable tin compounds are normally recognized, not only as tin dioxide, but also as some stannates (of iron, magnesium and copper). Even some whitish patinas have high percentage of tin (20–30% wt). Manganese concentration is very similar between the two Bronzes, in fact, it is equal to <0.2–1.2% wt on Bronze A and to <0.2–0.8% wt on Bronze B. Iron, present in variable concentration, 1.8–10.7% wt for Bronze A and 1.3–6.8% wt for Bronze B, cannot be certainly ascribed to a particular source. It is possible a deposition from the sea water or a provenance from some compound and/or treatment applied during restoration. Copper concentration is higher on Bronze A (23.4–81.7% wt) in comparison to Bronze B (2.5–64.2% wt). The thin brown patina is characterized by high levels of copper in both sculptures. Very high values of copper were measured for red patina that hides the black one, probably constituted by cuprous oxide (cuprite). On the other hand, the concentration of copper decreases in the green patina. Zinc concentration is lower on the surface of Bronze A (<1.0–4.3% wt), almost always less than 1% wt, while it is greater, and it is present in a diffuse manner, on patinas of Bronze B (<1.0–9.7% wt). In particular, this occurs in green–yellow–brown patinas (approximately 4–6% wt), but especially in whitish patinas, in which zinc concentration reaches a maximum value equal to 9.7% wt.
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Fig. 9. Dendrogram of the variables.
Fig. 10. Dendrogram of the patinas.
The unnatural appearance of these areas suggests an external contribution of zinc, in fact, the Art Bulletin [10] reports that the alloy has very low content of it (Table 1). Therefore, zinc concentration in the patinas of Bronze B is not imputable to marine sediments, but it is attributable to residues coming from brass brushes employed during the mechanical cleaning procedure in Florence in 1975–1980. Lead is present on the surface of both sculptures in lower-middle concentrations and it is slightly higher on Bronze B (2.0–4.6% wt) compared to Bronze A (<2.0–3.8% wt): it is obviously derived from the electrochemical corrosion of the alloys. Experimental results obtained by EDXRF analysis have been elaborated subsequently with the multivariate statistical treatment by using the method of the principal component analysis
(PCA). Principal component analysis was applied on a data set of sixty-one cases (thirty-three samples from Bronze A and twentyeight samples from Bronze B) and eight variables (the concentration of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead). As far as principal component analysis is concerned, four principal components were extracted covering 83.7% of the cumulative variance. The loading of the variables on the first two principal components (Fig. 7) shows that copper, sulphur and chlorine are the dominant variables for the positive values of the first principal component (Factor 1), tin is the dominant variable for the negative values of the first principal component, while iron and manganese are the dominant variables for the positive values of the second principal component (Factor 2). Lead and zinc are correlated
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between their and they have simultaneously negative value on Factor 1 and on Factor 2. The inverse correlation between copper and tin can be explained by copper selective dissolution (decuprification) considering both the higher mobility of copper cations and the greater stability of the tin oxide [35,36]. In particular, lower concentration of copper and higher amount of tin in Bronze B, rather than in Bronze A, are likely due to the mechanical cleaning during the restoration of the 1975 in Florence, which removed the protective patina on Bronze B, causing a superficial increasing of tin compared to copper. The scatter plot of the scores for the first two principal components PC1 and PC2 (Fig. 8) shows that most of the patinas of Bronze A have a positive score on the PC1 and, on the contrary, most of the patinas of Bronze B have a negative score for the same component. Thus, it is possible to discriminate the patinas of the two Bronzes by using the statistical treatment, whereas it is not possible to distinguish the patinas from different parts of the same statue. Hierarchical clustering analysis was carried out by using Complete Linkage procedure applied on the Euclidean distances and the dendrogram of the variables (Fig. 9) confirms the results obtained from the PCA. In fact, copper and tin have different behaviour from the remaining elements (lead, zinc, manganese, chlorine, iron and sulphur), which are collected in a large cluster. The graph also shows an elevated similarity between lead and zinc, a result already highlighted by the PCA. The dendrogram of the patinas (Fig. 10) shows the patinas divided into two main clusters, confirming the results obtained by the PCA analysis. To better highlight the discrimination among the sixty-one patinas, the patinas of Bronze A are written in grey, while the patinas of Bronze B are written in black. The first cluster contains all the patinas from Bronze B except four, which are from Bronze A (AC6, AL3, AC8 and AF4) and the second cluster contains all the patinas from Bronze A except four which are from B (BAR1, BC4, BC2 and BF1). Therefore, the results obtained confirm that the patinas of the two precious sculptures have different elemental composition.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13] [14]
[15] [16] [17]
[18]
[19]
4. Conclusions The analysis EDXRF, a non-destructive and in situ technique, has allowed us to determine the concentration of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead on the surface of the two Riace Bronzes and, therefore, to classify and to discriminate the different patinas. Furthermore, the EDXRF technique, allowing a large number of measures in a short time, made it possible to easily map the entire surface of both Bronzes. The presence in some patinas of copper chlorides suggests past events of corrosion and it also indicates a possible residual electrochemical risk, that must be monitored and controlled in order to ensure the conservation of these two Italian important monuments. Multivariate statistical analysis has allowed to show better the different chemical composition of the patinas from two Riace Bronzes. This confirms, once again, that the statistic techniques can be combined with the analysis archaeometric in order to obtain more information and/or to show more clearly the experimental results.
[20] [21] [22] [23] [24]
[25]
[26]
[27]
[28] [29]
[30]
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Portable EDXRF investigation of the patinas on the Riace Bronzes Giovanni Buccolieri a,⇑, Alessandro Buccolieri b, Paola Donati c, Maurizio Marabelli c, Alfredo Castellano a a
Università del Salento, Dipartimento di Matematica e Fisica, via Arnesano, 73100 Lecce, Italy Università del Salento, Dipartimento di Scienze e Tecnologie Biologiche e Ambientali, via Monteroni, 73100 Lecce, Italy c Istituto Superiore per la Conservazione e il Restauro, via di San Michele n. 23, 00153 Roma, Italy b
a r t i c l e
i n f o
Article history: Received 5 August 2014 Received in revised form 3 November 2014 Accepted 7 November 2014 Available online 5 December 2014 Keywords: Riace Bronzes Patina Non-destructive analysis EDXRF Statistical analysis
a b s t r a c t This paper summarizes the experimental results concerning the Energy Dispersive X-ray Fluorescence (EDXRF) analysis of patinas on two Riace Bronzes, kept in the National Archaeological Museum of Reggio Calabria (Calabria, Southern Italy). The two large Greek sculptures, famous nude bearded warriors both dated in the fifth century BC, are without a doubt, two masterpieces of inestimable historic and artistic value. EDXRF survey had the aim to determinate the chemical composition of the surface of these two bronze statues and to discriminate their different patinas. In particular, the concentration of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead was determined by using a portable apparatus. Multivariate statistical analysis was carried out in order to identify possible correlations and/or differences of elemental composition among the patinas of these two statues. The information obtained made it possible to improve knowledge about the patinas of the Riace Bronzes, and this may help further studies and subsequent methods of restoration and/or of preservation of the two celebrated Greek sculptures. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction A valid analytical method for the study of statues of historical and artistic interest should be non-destructive, fast, universal, sensitive, multi-elemental and portable. EDXRF satisfies these requirements and, for this reason, it is widely used for qualitative and quantitative elements analysis in the field of cultural heritage. By using such method, objects can be analyzed with good repeatability without removing them from the exhibition, thus minimizing the inconvenience for the public. Moreover, the nondestructive character of the analysis and its short measurement times, allows to obtain a large number of analytical data and consequently the mapping of the monument [1–3]. On the other hand, in archaeometric the use of EDXRF for accurate quantitative analysis is severely inhibited by the fact that the irradiated area is usually large, the detector sensitivity for light elements is limited, the surface of the sample is irregular and its composition is non-homogeneous. These parameter are usually ⇑ Corresponding author. Tel.: +39 0832 297087; fax: +39 0832 297100. E-mail addresses: [email protected] (G. Buccolieri), alessandro. [email protected] (A. Buccolieri), [email protected] (P. Donati), [email protected] (M. Marabelli), [email protected] (A. Castellano). http://dx.doi.org/10.1016/j.nimb.2014.11.064 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
non-ideal and/or not well-defined, leading to errors in the quantification of the elements. Nowadays, however, thanks to the innumerable studies carried out in the field of cultural heritage these parameters can be efficaciously corrected by using fundamental parameters (FP) method and theoretical coefficient algorithms [4–7]. Bronze is an alloy based on copper and tin, which can often contain other elements such as zinc and lead at different concentration. The aesthetic appearance and the integrity of a bronze monument can be altered by various environmental factors, natural and anthropogenic, such as temperature, humidity, acid rain, atmospheric particulate matter, sulphur oxides, nitrogen oxides and marine aerosol. The most obvious result of the alteration of bronze objects is the appearance of a thin layer of corrosion called patina. Among the most famous bronze monuments in the world it is possible to remember the Riace Bronzes, two statues discovered on 16 August 1972 in the Ionian Sea, near the coast of Riace (Reggio Calabria, Southern Italy), at a distance of about 200 m from the coast and at a depth of about 8 m. The Riace Bronzes represent two bearded warrior and they are named respectively as Bronze A ‘‘The Young’’ and Bronze B ‘‘The Old’’ (Fig. 1). Statue A is 1.98 m high, whereas Statue B is 1.97 m. According to the most reliable sources, both statues are dated in the fifth century BC and, more specifically, Bronze A is dated from
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around 460 BC, and Bronze B refers to about 430 BC. The latest study of radiocarbon dating has allowed to obtain ages ranging between 517 and 394 BC for statue A, and between 509 and 394 BC for statue B [8,9]. The statues were probably made in Greece [10] and subsequently removed to be transported to Rome, where they never arrived because the boat that was carrying the precious cargo sank in Ionian Sea and it ended up submerged in sand until 1972, when the two sculptures were found. The very long stay underwater heavily corroded the two masterpieces and this has demanded extensive studies for their restoration and conservation. The first cleanings and the removal of the marine concretion were accomplished in 1975 in Reggio Calabria. The two statues were later transferred to Opificio delle Pietre Dure in Florence, where from 1975 to 1980 they were exposed to a total surface cleaning and analysis. In particular, the patinas on Bronze B were partially eliminated through a mechanical cleaning. Table 1 summarizes the median values of the concentration of elements determined on the alloy of the two statues during the restoration of 1975–1980 in Florence [11]. Both sculptures are made of a copper-tin binary alloy. The average thickness of the alloys is about 8.5 mm and 7.5 mm for Bronze A and for Bronze B respectively, even if the thickness varies widely between 4 and 12 mm. However, these values are typical of the Archaic and Classical age. This paper summarizes the experimental results concerning the Energy Dispersive X-ray Fluorescence (EDXRF) analysis of patinas on two Riace Bronzes. In particular, the EDXRF survey had the aim to determine the chemical composition of the patinas and to map their distribution. The EDXRF non-destructive technique represents an appropriate methodology for the analysis of the elemental composition of surface layers [12–17], for the individuation of corrosive components potentially harmful, for support the study of corrosion phenomena and finally for the individuation of the most proper environmental parameters suitable for the conservation of the above-mentioned Statues in the Museum exhibition Hall of Reggio Calabria. Furthermore, we would like to underline that the EDXRF procedure is an extremely efficient and relatively easy method of performing a large number of measurements quickly and in situ and, therefore, to map the entire surface of the monuments.
Table 1 Elements mean concentration (wt.%) determined in the alloy of the Riace Bronze during the restoration of 1975–1980 [11]. Bronze A
Fe Cu Sn Zn Pb Ag Ni Co Ca Mg
Bronze B
Body
Head
Body
Head
0.13 88.8 10.75 – 0.12 0.037 0.012 – 0.102 0.032
0.035 87.35 12.41 0.004 0.062 0.021 0.016 – 0.068 0.023
0.16 90.08 9.26 0.01 0.155 – 0.021 – 0.257 0.041
0.13 85.94 13.5 0.005 0.233 – 0.020 – 0.129 0.05
2. Materials and methods 2.1. EDXRF analysis EDXRF portable equipment, which was assembled in our laboratory, is mainly composed of an X-ray tube (MOXTEK Inc., USA) and of a detector Si-PIN (Amptek Inc., USA). The detector has a resolution of energy of about 180 eV at 5.9 keV. The output of the Xray tube is highly collimated and the analyzed area has a diameter of about 3 mm. For each measuring point, two EDXRF spectra were acquired, both with acquisition time of 30 s and with a tube voltage of 6 kV (at 40 lA) or of 20 kV (at 3 lA). A set of standard samples, with known chemical compositions, were used to calibrate the apparatus and to obtain reliable experimental data. For standards preparation were used copper sulphide (CuS), copper(I) chloride (CuCl), manganese oxide (MnO2), iron, zinc, lead(II) oxide (PbO), lead dioxide (PbO2), minimum or lead tetroxide (Pb3O4), tin dioxide (SnO2), copper(I) oxide (Cu2O) and copper(II) carbonate basic (CuCO3Cu(OH)2). All chemical compounds were purchased from Sigma–Aldrich with analytical grade. Each standards was prepared by mixing the compounds in different weight percentages. The use of different compounds with the same elements, but in different oxidation state, is irrelevant from the point of view of the XRF analysis, but has allowed to
Fig. 1. Image of two Riace Bronzes, Reggio Calabria, Italy. Bronze A named ‘‘The Young’’ (a) and Bronze B named ‘‘The Old’’ (b).
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G. Buccolieri et al. / Nuclear Instruments and Methods in Physics Research B 343 (2015) 101–109 Table 2 Elements concentration (wt.%) determined on the Bronze A by using EDXRF. Label
Description of the patina
S
Cl
Sn
Mn
Fe
Cu
Zn
Pb
AF1 AF2 AF3 AF4 AF5 AF6 AF7 AF8 AC1 AC2 AC3 AC4 AC5 AC6 AC7 AC8 AC9 AA1 AA2 AA3 AA4 AA5 AL1 AL2 AL3 AL4 AL5 AL6 AL7 AB1 AB2 AB3 AFO1
Black patina on the face, the left cheekbone Green patina on the face, right cheekbone Black patina on the face Black patina on the hair band Black patina on the nose Black patina on the hair band Brown patina on the hair band Brown patina (and rough) on the neck Red patina (above black) on the chest Green patina on the chest Black patina on the chest Red patina (above black) on the chest Red patina (above black; green hue) on the chest Black patina (green hue) on the chest Black patina on the chest Green patina on the chest Green patina on the chest Red patina on the abdomen Black patina on the abdomen Green patina on the abdomen Red patina (above the black) on the abdomen Green patina on the abdomen (near the corrosion craters) Black patina on the left leg Red patina (above black) on the left leg Green patina on the left leg Green patina on the left leg Whitish patina on the left leg Red patina (above black) on the left leg Black patina on the left leg Black patina on the right bicep Green patina (above the black) on the right bicep Grey–green patina (under the black) on the right bicep Black patina on the left foot
7.4 4.0 6.6 8.4 6.2 7.1 2.2 4.6 4.9 3.7 10.1 5.3 3.6 9.8 11.1 3.7 <0.5 6.8 8.1 5.0 7.6 5.2 12.0 8.7 8.1 0.8 7.6 <0.5 12.5 11.8 <0.5 5.1 8.3
4.5 3.1 <1.5 2.0 2.1 2.9 4.9 3.9 3.9 2.6 2.1 4.2 2.3 1.8 2.6 1.7 10.9 4.6 3.4 <1.5 3.6 6.4 1.9 3.3 1.8 15.7 <1.5 5.3 2.8 2.0 14.9 2.3 2.6
13.6 23.0 25.4 33.5 27.8 8.2 23.9 18.7 5.4 31.1 15.9 <3.0 7.2 6.4 4.0 32.1 10.3 1.1 9.7 32.5 10.2 18.7 9.8 5.6 23.4 5.1 23.1 <3.0 5.3 2.3 4.1 27.4 26.6
0.6 0.5 0.6 0.7 0.6 0.5 0.3 0.3 <0.2 0.3 <0.2 <0.2 <0.2 <0.2 <0.2 1.2 <0.2 0.5 <0.2 0.5 <0.2 0.5 0.3 0.3 0.3 0.3 0.5 0.3 <0.2 0.3 0.3 0.4 0.7
7.1 6.4 7.4 10.7 6.0 4.4 4.6 3.9 3.5 5.6 4.7 2.5 1.8 7.9 2.7 10.0 3.3 10.1 3.9 5.7 5.4 6.7 4.1 4.0 9.0 4.5 5.9 4.3 3.2 3.1 4.9 6.7 8.1
58.1 59.0 49.7 23.4 45.9 74.0 61.2 66.0 73.5 53.4 63.9 79.6 72.1 28.6 73.3 36.6 69.5 72.4 73.5 53.4 64.6 62.0 69.5 74.6 29.9 62.7 57.5 81.7 73.6 72.2 71.9 57.6 50.2
<1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 <1.0 4.3 <1.0 <1.0 <1.0
3.6 3.2 <2.0 3.1 2.0 2.6 2.6 2.2 3.3 3.0 3.1 3.1 2.8 3.8 3.2 3.2 3.5 <2.0 <2.0 <2.0 <2.0 <2.0 2.2 2.3 <2.0 2.0 2.5 3.0 2.4 <2.0 <2.0 <2.0 2.9
Table 3 Elements concentration (wt.%) determined on the Bronze B by using EDXRF. Label
Description of the patina
S
Cl
Sn
Mn
Fe
Cu
Zn
Pb
BF1 BF2 BF3 BF4 BF5 BF6 BC1 BC2 BC3 BC4 BC5 BC6 BA1 BA2 BA3 BA4 BA5 BA6 BA7 BAR1 BAR2 BL1 BL2 BL3 BL4 BL5 BL6 BFO1
Red on black patina on the helmet Green patina between the two eyes Dark brown patina on the nose Green patina on the left cheek Black patina on the left cheek Black patina on the right cheek Green patina on the right chest Brown patina on the right chest Green patina on the right chest Brown patina on the right chest Dark brown patina on the right chest Black patina on the right chest Green patina on the abdomen Brown–yellow–green patina on the abdomen Brown–yellow–green patina on the abdomen Green patina on the abdomen Black patina on the abdomen Brown–yellow–green patina on the abdomen Black patina on the abdomen Black patina on the right arm Green patina on the right arm Black patina on the left leg Yellow–green patina on the left leg Green patina on the left leg Green patina on the left leg Green patina on the left leg Whitish patina on the left leg Black patina on the left foot
<0.5 <0.5 <0.5 3.1 9.2 2.1 <0.5 3.2 <0.5 3.6 <0.5 7.0 2.2 1.1 <0.5 3.5 6.3 1.9 3.6 7.8 <0.5 9.6 2.9 2.5 1.0 5.1 0.7 8.6
4.0 1.9 2.1 1.5 2.3 3.0 2.9 1.9 <1.5 1.9 1.9 1.5 <1.5 <1.5 <1.5 <1.5 <1.5 1.7 1.5 2.9 <1.5 2.3 1.9 10.3 8.4 11.5 2.1 1.8
18.9 39.6 37.6 41.9 35.5 39.0 42.8 18.9 42.8 17.4 38.6 24.4 41.4 43.6 36.3 35.5 34.1 37.0 40.0 18.9 48.5 27.0 33.9 12.4 15.7 12.3 30.5 25.8
0.3 0.5 0.4 0.3 0.5 0.6 0.7 0.2 0.8 <0.2 0.7 0.7 0.3 0.8 0.5 0.3 0.6 0.7 0.8 0.5 0.3 <0.2 0.2 0.2 0.2 <0.2 <0.2 <0.2
2.0 3.9 2.2 1.7 4.0 6.8 5.7 3.4 5.0 1.3 6.0 5.7 4.6 5.0 5.3 3.6 5.3 5.4 6.0 5.9 4.2 1.6 3.8 4.4 2.0 1.3 5.1 1.6
56.5 18.6 44.6 41.7 25.9 27.1 18.2 64.2 19.2 62.3 23.5 16.7 10.0 14.4 13.5 5.9 9.6 9.0 12.3 47.3 11.1 13.0 6.1 17.4 14.7 19.4 2.5 15.6
2.8 4.0 <1.0 4.8 3.3 4.1 <1.0 4.6 4.5 4.7 3.5 2.8 2.5 4.5 5.7 4.9 1.8 5.6 1.6 3.7 <1.0 1.4 5.8 3.1 3.4 <1.0 9.7 1.5
3.1 3.2 2.7 3.9 3.8 4.6 2.8 3.3 3.8 3.5 3.5 3.8 4.2 3.9 4.0 4.6 3.5 4.6 3.1 4.2 3.9 3.0 3.3 2.6 2.4 2.0 4.2 2.8
obtain the calibration samples at different concentrations. In particular, the chemicals compounds have been weighed by using an analytical balance KERN model ABT 100-5M, subsequently mixed and homogenized in an agate mortar for ten minutes and finally compressed at 200 bar for ten minutes. The homogeneity of ele-
ments in the standard meets the requirements for the EDXRF quantitative analysis [18]. Moreover, the samples analyzed are supposed to ‘‘infinite thickness’’ and therefore the quantitative results are expressed in terms of weight percentage (% wt). A thickness of cuprite (Cu2O) from 40 lm reduces by a factor of 100 the
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Table 4 Minimum, maximum, mean and standard deviation values (wt.%) of elements determined on Bronze A. Element
Minimum
Maximum
Mean
Standard Deviation
S Cl Sn Mn Fe Cu Zn Pb
<0.5 <1.5 <3.0 <0.2 1.8 23.4 <1.0 <2.0
12.5 15.7 33.5 1.2 10.7 81.7 4.3 3.8
6.3 3.8 14.9 0.3 5.5 61.1 0.1 2.0
3.4 3.6 10.9 0.3 2.3 14.9 0.7 1.4
Table 5 Minimum, maximum, mean and standard deviation values (wt.%) of elements determined on Bronze B. Element
Minimum
Maximum
Mean
Standard Deviation
S Cl Sn Mn Fe Cu Zn Pb
<0.5 <1.5 12.3 <0.2 1.3 2.5 <1.0 2.0
9.6 11.5 48.5 0.8 6.8 64.2 9.7 4.6
3.0 2.5 31.8 0.4 4.0 22.9 3.4 3.5
3.1 2.9 10.6 0.3 1.7 17.3 2.2 0.7
Fig. 3. Measuring points on the abdomen and on the left leg of the Bronze A.
Fig. 2. Measuring points on the face and chest of the Bronze A.
intensity of a radiation at 10 keV, therefore both the standard samples (about 2 mm thick) and the bronze analyzed can be considered thick samples. Moreover, the standards were tested on certified materials of bronze purchased from Goodfellow (Cambridge Science Park, England). In particular, we used a foil of bronze Cu94/Sn6 (size 25 25 mm and thickness 0.25 mm) and three standards of bronze in powder (Cu89/Sn11, Cu80/Sn20 and Cu80/Pb20). The concentrations of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead were determined, for each measurement point, in order to individuate the chemical composition of the surface of two Riace Bronzes and to map their chemical distribution.
Fig. 4. Measuring points on the right bicep of the Bronze A.
The evaluation of the spectral interference between sulphur (K-line) and lead (M-line) is fundamental in their quantitative EDXRF analysis. The separation between K-line of sulphur and M-line of lead is equal to about 40 eV, a value which is lower than the resolution of the detector (equal to about 180 eV). To solve this problem, the separation of the two signals was obtained through the deconvolution of the peaks. This deconvolution is possible con-
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Fig. 5. Measuring points on the face, chest and abdomen of the Bronze B.
sidering both the higher sulphur concentration, compared to lead, and the different fluorescent yields between the two elements [19]: K-line of sulphur (0.061) and M-line of lead (0.032). The values of detection limit, all reported in wt.%, for sulphur, chlorine, tin, manganese, iron, copper, zinc and lead are equal respectively to 0.5, 1.5, 3.0, 0.2, 1.0, 1.0, 1.0 and 2.0. The software Microcal Origin Professional has been used to elaborate the experimental data obtained. 2.2. Statistical analysis Experimental results obtained by EDXRF analysis have been elaborated with multivariate statistical analysis [20–25] in order to identify possible correlations and/or differences existing among the elemental composition of the two Bronzes. The computer software package STATISTICA (StatSoft Inc., Tulsa, OK, USA) was used for statistical analysis and the Principal Component Analysis (PCA) and the Hierarchical Cluster Analysis (HCA) were used as statistical techniques. The PCA method calculates the orthogonal linear combinations of the autoscaled variables by using a correlation matrix based on the maximum variance criterion. Such linear combinations are called principal component scores and the coefficients of the linear combinations are called loadings. The numerical loading value, for each variable of a given principal component, shows how much the variable has in common with that component. PCA can give information about the similarities and groupings of the samples considered, and if a trend exists, it can evaluate the possibility of classifying the samples. The HCA method is a statistical treatment designed to reveal groupings (or clusters) within a data set that sometimes would not be apparent. The purpose of this analysis is to join together objects into successively larger clusters, by using some measure of similarity or dissimilarities.
Fig. 6. Measuring points on the left leg of the Bronze B.
The basic criterion for any clustering is distance and probably the most straightforward way of computing the distances among objects in a multidimensional space is to compute Euclidean distances. Objects that are near each other should belong to the same cluster, and objects that are far from each other should belong to different clusters. The dendrogram is the graphical summary of the cluster analysis. The cases (or the variables) are listed along the horizontal axis, while the distance among the clusters is showed along the vertical axis.
3. Results and discussion A close visual examination of both Riace Bronzes has allowed to differentiate the typologies of patinas present on the two statues. Statue A shows mainly three different patinas: a green patina with various hues, quite uneven and with reduced thickness, alternated with a second patina, brown, thin, and with a third one patina, black on the top, multilayer, smooth, apparently compact, dense and homogeneous. Sometimes, the third patina is concealed by a red layer of a compound rich of copper, probably caused by deposited cupper (then oxidized to cuprite, Cu2O), already analyzed at the time of the first investigations following the discovery of the statues [26,27]. Statue B is largely covered by a brown–yellow– green patina, that appeared unusual, probably at least partially
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Fig. 7. Loading of the variables on the first two principal components.
artificial, alternated with a thin brown patina and a few residual islands of the black patina. Tables 2 and 3 summarize the elements concentrations determined by using EDXRF respectively on the surfaces of statue A and of statue B. For each measurement point, the label, the description of the patina and the concentration of determined elements are shown in the tables. Overall, sixty-one patinas have been investigated (thirty-three patinas on the Bronze A and twenty-eight patinas on the Bronze B). Tables 4 and 5 show the values of minimum, maximum, mean and standard deviation for elements determined respectively on the surface of Bronze A and of Bronze B. The Figs. 2–6 show the positions of all points of measurement on the two Greek famous sculptures except AFO1, BAR1, BA2 and BFO1. The values of sulphur concentration are in general higher for Bronze A (<0.5–12.5% wt) in comparison with Bronze B (<0.5–9.6% wt) and in five areas of the Bronze A the sulphur concentration is higher than 10% wt. The black patina always shows prevailing sulphur and copper concentrations, corresponding to the presence of copper sulphides,
Fig. 8. Scatter plot of the scores for the first two principal components.
while the red thin superimposed layer is probably recognized as cuprite (Cu2O), deriving from deposited copper subsequently oxidized [28,29]. In general, the green patina shows moderate sulphur concentrations. Chlorine concentration for the two Riace Bronzes is comparable, but it is slightly higher for Bronze A (<1.5–15.7% wt) in comparison with Bronze B (<1.5–11.5% wt). The green patinas can be divided in three typologies, with different hues: grey–green, green and brown–yellow–green. Grey–green patina has an extensive presence and an uniform colour on the Bronze A: it is particularly evident in zones where the black patina was removed, due to the mechanical friction and wearing action of sand and marine sediments against the bronze surface. The green patinas, in some cases with hues light blue or whitish, are usually small and quite common on both Riace Bronzes. The green patinas AC9, AL4 and AB2 show higher concentration of chlorine and copper, indicating a significant accumulation of copper chlorides. The latters are very important chemical compound in corrosion processes of copper alloys. It is now known that there are four polymorph structures of copper trihydroxychloride (Cu2Cl(OH)3), such as, atacamite, paratacamite, botallackite and clinoatacamite and the nature of the chemical reactions in ‘‘bronze disease’’ is well described in literature [30–34]. Therefore, the presence of copper chloride must be carefully evaluated in order to perform a correct storage and this requires a periodic control of the surface of the two masterpieces. The brown–yellow–green patinas are present in extended areas on Bronze B, with low concentrations of chlorine (less than 2% wt) and of copper (9–15% wt), high percentages of tin (36–44% wt) and also significant values of zinc (4–6% wt). Tin concentration is higher on Bronze B (12.3–48.5% wt) than on Bronze A (<3.0–33.5% wt). Bronze B exhibits very high tin concentration (34.1–43.6% wt) especially in the zones of the abdomen. In particular, greater tin values are found in the brown–yellow–green areas, in some way sustaining the hypothesis of an intense electrochemical corrosion, but mainly of an artificial chemical surface treatment with consequent enrichment of this alloy component in the oxidized form, stable, of cassiterite (SnO2). Furthermore, in the marine environment, stable tin compounds are normally recognized, not only as tin dioxide, but also as some stannates (of iron, magnesium and copper). Even some whitish patinas have high percentage of tin (20–30% wt). Manganese concentration is very similar between the two Bronzes, in fact, it is equal to <0.2–1.2% wt on Bronze A and to <0.2–0.8% wt on Bronze B. Iron, present in variable concentration, 1.8–10.7% wt for Bronze A and 1.3–6.8% wt for Bronze B, cannot be certainly ascribed to a particular source. It is possible a deposition from the sea water or a provenance from some compound and/or treatment applied during restoration. Copper concentration is higher on Bronze A (23.4–81.7% wt) in comparison to Bronze B (2.5–64.2% wt). The thin brown patina is characterized by high levels of copper in both sculptures. Very high values of copper were measured for red patina that hides the black one, probably constituted by cuprous oxide (cuprite). On the other hand, the concentration of copper decreases in the green patina. Zinc concentration is lower on the surface of Bronze A (<1.0–4.3% wt), almost always less than 1% wt, while it is greater, and it is present in a diffuse manner, on patinas of Bronze B (<1.0–9.7% wt). In particular, this occurs in green–yellow–brown patinas (approximately 4–6% wt), but especially in whitish patinas, in which zinc concentration reaches a maximum value equal to 9.7% wt.
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Fig. 9. Dendrogram of the variables.
Fig. 10. Dendrogram of the patinas.
The unnatural appearance of these areas suggests an external contribution of zinc, in fact, the Art Bulletin [10] reports that the alloy has very low content of it (Table 1). Therefore, zinc concentration in the patinas of Bronze B is not imputable to marine sediments, but it is attributable to residues coming from brass brushes employed during the mechanical cleaning procedure in Florence in 1975–1980. Lead is present on the surface of both sculptures in lower-middle concentrations and it is slightly higher on Bronze B (2.0–4.6% wt) compared to Bronze A (<2.0–3.8% wt): it is obviously derived from the electrochemical corrosion of the alloys. Experimental results obtained by EDXRF analysis have been elaborated subsequently with the multivariate statistical treatment by using the method of the principal component analysis
(PCA). Principal component analysis was applied on a data set of sixty-one cases (thirty-three samples from Bronze A and twentyeight samples from Bronze B) and eight variables (the concentration of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead). As far as principal component analysis is concerned, four principal components were extracted covering 83.7% of the cumulative variance. The loading of the variables on the first two principal components (Fig. 7) shows that copper, sulphur and chlorine are the dominant variables for the positive values of the first principal component (Factor 1), tin is the dominant variable for the negative values of the first principal component, while iron and manganese are the dominant variables for the positive values of the second principal component (Factor 2). Lead and zinc are correlated
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between their and they have simultaneously negative value on Factor 1 and on Factor 2. The inverse correlation between copper and tin can be explained by copper selective dissolution (decuprification) considering both the higher mobility of copper cations and the greater stability of the tin oxide [35,36]. In particular, lower concentration of copper and higher amount of tin in Bronze B, rather than in Bronze A, are likely due to the mechanical cleaning during the restoration of the 1975 in Florence, which removed the protective patina on Bronze B, causing a superficial increasing of tin compared to copper. The scatter plot of the scores for the first two principal components PC1 and PC2 (Fig. 8) shows that most of the patinas of Bronze A have a positive score on the PC1 and, on the contrary, most of the patinas of Bronze B have a negative score for the same component. Thus, it is possible to discriminate the patinas of the two Bronzes by using the statistical treatment, whereas it is not possible to distinguish the patinas from different parts of the same statue. Hierarchical clustering analysis was carried out by using Complete Linkage procedure applied on the Euclidean distances and the dendrogram of the variables (Fig. 9) confirms the results obtained from the PCA. In fact, copper and tin have different behaviour from the remaining elements (lead, zinc, manganese, chlorine, iron and sulphur), which are collected in a large cluster. The graph also shows an elevated similarity between lead and zinc, a result already highlighted by the PCA. The dendrogram of the patinas (Fig. 10) shows the patinas divided into two main clusters, confirming the results obtained by the PCA analysis. To better highlight the discrimination among the sixty-one patinas, the patinas of Bronze A are written in grey, while the patinas of Bronze B are written in black. The first cluster contains all the patinas from Bronze B except four, which are from Bronze A (AC6, AL3, AC8 and AF4) and the second cluster contains all the patinas from Bronze A except four which are from B (BAR1, BC4, BC2 and BF1). Therefore, the results obtained confirm that the patinas of the two precious sculptures have different elemental composition.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13] [14]
[15] [16] [17]
[18]
[19]
4. Conclusions The analysis EDXRF, a non-destructive and in situ technique, has allowed us to determine the concentration of sulphur, chlorine, tin, manganese, iron, copper, zinc and lead on the surface of the two Riace Bronzes and, therefore, to classify and to discriminate the different patinas. Furthermore, the EDXRF technique, allowing a large number of measures in a short time, made it possible to easily map the entire surface of both Bronzes. The presence in some patinas of copper chlorides suggests past events of corrosion and it also indicates a possible residual electrochemical risk, that must be monitored and controlled in order to ensure the conservation of these two Italian important monuments. Multivariate statistical analysis has allowed to show better the different chemical composition of the patinas from two Riace Bronzes. This confirms, once again, that the statistic techniques can be combined with the analysis archaeometric in order to obtain more information and/or to show more clearly the experimental results.
[20] [21] [22] [23] [24]
[25]
[26]
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