Study of Italian Renaissance sculptures using an external beam nuclear microprobe

Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 699±703

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Study of Italian Renaissance sculptures using an external beam nuclear microprobe A. Zucchiatti

a,*

, A. Bouquillon b, B. Moignard b, J. Salomon b, J.R. Gaborit

c

a b

Istituto Nazionale di Fisica Nucleare Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy Centre de R echerche et Restauration des Mus ees de France, LRMF UMR CNRS 171, Paris, France c Departement des Sculptures, Mus ee du Louvre, Paris, France

Abstract The use of an extracted proton micro-beam for the PIXE analysis of glazes is discussed in the context of the growing interest in the creation of an analytical database on Italian Renaissance glazed terracotta sculptures. Some results concerning the frieze of an altarpiece of the Louvre museum, featuring white angels and cherubs heads, are presented. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 82.80E Keywords: Archaeometry; Micro-PIXE; Polished sections

1. Introduction One of the success stories of the Italian artistic Renaissance is that of the Della Robbia, a family of sculptors begun with Luca (1399/1400±1482) who re-invented the terracotta invetriata (glazed terracotta) as a form of sculpture that ¯ourished for almost a century under his nephew Andrea (1435±1525) and his sons. The great artistic value of their production, acknowledged all over Europe, as well as some peculiarities that made glazed terracotta quite appealing (vivid colours, solid structure, reasonable price), joint to the

*

Corresponding author. Tel.: +39-010-353-6384; fax: +39010-313-358. E-mail address: [email protected] (A. Zucchiatti).

commercial skill of the family, made the Florence pottery very active over several decades [1]. The commercial success of the Della Robbia stimulated contemporary imitation, some at a high artistic level like that of the Buglioni family. From the middle of the 19th century there is an increase of modern imitations from authentic models and also of fakes, not necessarily produced with fraudulent intentions. The attention of researchers, confronted by such a remarkable artistic production, is more and more addressed to the constitution of a database of analytical information, with emphasis on the properties and structure of the glazes that are the distinctive peculiarity of these sculptures. It should allow the solution of technological problems in the restoration work, that takes great advantage from the knowledge of the object composition in its

0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 9 0 5 - 2

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various parts. It should also considerably support historic and artistic debates. Nondestructive IBA techniques have a long record of applications to many ®elds of art studies [2,3] and are good candidate techniques for the acquisition of data in support to researchers, historians and restorers. Evidently such a database should be constituted from the e€orts of several laboratories, addressing the question of intercomparison of data. The Centre de Recherche et de Restauration des Musees de France operates with its in-house AGLAE accelerator in the context of Italian glazed terracotta since 1996 [4,5]. One of the novelties of the laboratory is a micro-beam, extracted in air [6], which is described in a companion paper [7]. Such a beam allows the application of IBA techniques, for example PIXE, to sets of artistic objects which need to be studied in a homogeneous way, but can include whole artefacts as well as fragments and micro-samples. Being capable of analysing, with the same apparatus, specimens having a wide range of sizes, prevents the need of larger visible sampling on whole objects as well as the occurrence of undesirable beam e€ects on glazed surfaces, like burns, opalescence and colour darkening. In comparison to other techniques, like the scanning electron microscopy (SEM), PIXE detects major, minor and trace elements with a much higher sensitivity and without any sample preparation (e.g., polishing, carbon coating). 2. Methodology The AGLAE microprobe allows analyses of samples in the 100 lm range. It is characterised by an exit nozzle with an aluminium window 0.75 lm thick, tightened to a 150 lm kapton foil, pierced with a 200 lm diameter hole and tilted by 45°. The beam, once extracted in air, has a diameter close to 20 lm at the target. If required, it can then be directed even within the glaze portion (50 to 150 lm thick) of the very same samples prepared in polished sections for SEM and available in appreciable numbers from the major restoration centres. It has to be remarked that PIXE should provide a ®ngerprint of the glaze elemental

composition, averaged to some extent over the glaze microstructures. These could consist of colorant or opaque grains, quartz or feldspar crystals, secondary minerals and gas bubbles, and could have a size comparable to the beam diameter. In our case such an averaging should be achieved since, in normal conditions, the major elements are completely fused in the glaze and the occurrence of nodules and crystals has a clear signature in the PIXE spectra through the increase of typical elements concentration. A relative accumulated charge is measured through the Al Ka line emitted by the exit window and detected by a dedicated, Peltier cooled, Si(Li) detector. In PIXE runs the X-rays are detected by two di€erent LN2 cooled Si(Li) detectors. One is optimised for the detection of X-rays up to 15 keV (small ± 10 mm2 area, 141 eV FWHM resolution at 5.894 keV, He ¯ow). The other is optimised from 5 to 30 keV (big ± 50 mm2 area, 163 eV FWHM resolution at 5.894 keV). Data sorting is performed by the GUPIX [8] de-convolution program which is used in its thick target con®guration. Elemental concentrations are extracted for all elements seen in the small detector, assuming that they come in oxide form and constraining their sum (including the associated oxygen) to 106 ppm. The matrix is then used to extract concentrations for heavier elements in the other detector. A ®rst normalisation is made by imposing equal concentrations of iron in the two detectors and scaling accordingly the concentrations of all other elements in the big detector. For elements contributing to both spectra we have generally found good consistency of results. However, we have chosen to retain the concentration extracted from the spectrum where the element line was better visible amongst possible interference. This holds in particular for elements that are known to be characteristic of the pigments (e.g., Sn Ka, As Kb, Co Ka) and essential for their precise identi®cation. A ®nal re-normalisation to 106 ppm has been made for the retained elements. A total of 17 elements were above detection limits in blue and in white glazes, namely: Na, Mg, Al, Si, Cl, K, Ca, Ti, Mn, Fe, Co, Ni, Cu, Zn, As, Sn and Pb, while Sb and Ba were seen only in the few yellow and green specimen of Buglioni.

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The detection and de-convolution procedure was controlled by examining a set of certi®ed glass standards having a Pb concentration from 10% to 40%. The Pb concentrations were overestimated in all standards by about 20%. Since it was not observed in a cross-check PIXE analysis performed at the INFN Florence laboratory, this discrepancy appeared to be due to an instrumental e€ect and was corrected. After correction of Pb in AGLAE data, a good agreement with certi®ed values was generally found, both for the AGLAE and the INFN runs, as seen in Fig. 1 for some elements relevant to glaze analysis. The concentration of Na2 O appears underestimated in INFN runs, probably because the helium ¯ow in front of the detector was incomplete and left an air layer of uncontrolled variable thickness along the X-ray

Fig. 1. The oxide concentration for Si, Pb, Cu and Na, extracted by PIXE both at the Louvre and INFN Florence laboratories, compared to the certi®ed values for three standard glasses: British Glass Industry Research Association (BG3 and BG4) and Corning Museum of Glass Standards (CC).

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path. We observe that the Cu concentration is well measured in sample BG4 while it is overestimated for samples BG3 and CC. 3. Results SEM samples and fragments from artefacts attributed to Della Robbia and Buglioni were analysed by PIXE together with a series of large fragments from an altarpiece featuring white heads of angels and cherubs on a blue background. A reference list is given in Table 1. A total of 65 spectra were collected. Almost all fragments were analysed at two di€erent positions to investigate possible clay body interference and to assess the ability of interpreting micro-beam results as glaze average ®ngerprints. With SEM polished sections, we ®rst positioned each section by means of a micro-camera and then we improved the alignment by maximising the ratio between the Pb La from the glaze and the Al Ka from the relative charge monitor. Some of the polished sections were analysed at two di€erent positions. For the polished sections, PIXE and SEM data are compatible, as in the example of Table 2, concerning the minor and trace elements typical of a blue glaze (from the S. Anne tympanum of Santi Buglioni). We will report on PIXE data concerning blue glazes only, since samples of other colours were too few to allow statistically signi®cant considerations for each colour. We have paid attention to the distinctive elements of the blue glaze: Si, Pb and Sn as the major glass components and Co, Cu and As as the characteristic elements of the pigments [9]. The concentration obtained for our di€erent specimen is plotted in Fig. 2 for some of the major glass constituents and some of the pigment elements. All the pairs of points taken on the same fragment gave equal concentrations, within the experimental uncertainties, and did not show evidence of anomalous increases of Ca and Al (major clay body constituents). Thus it is excluded that the thickness of the glaze was smaller than the 3 MeV proton range and it is con®rmed that, within the limits stated before, the ®ngerprint given by a PIXE analysis using a 20 lm proton

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Table 1 Reference list of samples Object description

Origin

Type

Code

Colour

Number of samples

Altarpiece frieze: angels and cherubs Altarpiece frieze: angels and cherubs Tympanum: S. Anne, Santi Buglioni Deposition of Christ, Santi Buglioni

Musee du Louvre, Paris

Fragment

CB

Blue

16

Musee du Louvre, Paris

Fragment

CW

White

Musee du Louvre, Paris

SEM

LBT

White, blue

Musee des Arts Decoratifs, Paris

SEM

ADB

Pieta, Andrea Della Robbia Medaillon, probably of Santi Buglioni Pieta, Santi Buglioni

Notre Dame, Marseilles

SEM

LRM

White, blue, green, yellow, black, violet Blue

1

Aix en Provence

SEM

AIX

Blue

1

Finalpia Abbey, Italy

Fragment

BFL

Blue, yellow

2

4 11 7

Table 2 Mass concentrations determined by PIXE and SEM on the same polished section, for some minor elements speci®c of a blue glaze Element oxide

SEM concentration (%)

PIXE concentration (%)

TiO2 MnO CoO NiO As2 O5 Fe2 O3

0 0.3 0.4 0 0 2.6

0.3 0.2 0.4 0.2 0.3 1.7

beam is representative of the average glaze composition. For the altarpiece fragments, the values tend to split into two groups for many detected elements as the data of Fig. 3 indicate. This is true in particular for cobalt, which is the base ingredient of the blue colour. The attributed Della Robbia and Buglioni artefacts do not allow, for the moment, any distinction based on a single element concentration. For the totality of blue glazes we have performed, in the 17-dimensional space described by all the detected elements, a cluster association as shown in the dendrogram of Fig. 3. The existence of two distinct groups of blue samples from the altarpiece is con®rmed as well as the diversity of these two groups with respect to artefacts attributed to Della Robbia or Buglioni. As it is seen nowadays, the altarpiece is the result of a restor-

ation that combined two similar objects that were received, with considerable alterations, from the same private collection, but were believed to belong to the same artefact. The present results shed some new light on the history of this artefact and its past remakes. 4. Conclusions The extracted micro-beam of the AGLAE accelerator can be used for the analysis of glazes in specimen having a wide range of sizes. The extracted PIXE elemental concentrations allow to address artistic questions as in the case of a Louvre altarpiece that we considered in detail. Two distinct groups of blue samples from the altarpiece were identi®ed and will certainly stimulate

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Fig. 3. A dendrogram produced from the oxide concentration of 17 elements detected by PIXE in all blue samples. Three major clusters are identi®ed by this analysis. Points are labelled according to the coding of Table 1.

References

Fig. 2. The oxide concentration extracted by PIXE for some glaze constituent elements (Si, Pb, Sn) and for some blue pigment characteristic elements (Co, As, Cu) for the whole set of specimens, indicated by a progressive measurement number. Full dots identify the altarpiece fragments, open dots identify the artefacts attributed to Della Robbia and Buglioni.

discussion and new research on the history of this artefact and its past remakes. The overall results encourage e€orts for the creation of a database of analytical information based not only on large fragments or whole artefacts but also on the micro-sample catalogues of Italian Renaissance terracotta sculptures that several restoration centres have available. This work has been carried on under the auspices and with the support of the European action COST_G1.

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