Radioluminescence study of surface and ceramic body of lustred majolicasby A. Galli, M. Martini, L. Panzeri, E. Sibilia

Surface Engineering

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Radioluminescence study of surface and ceramic body of lustred majolicas

A. Galli1, M. Martini2, L. Panzeri2 and E. Sibilia2

Radioluminescence (RL), the light emission observed during the excitation of a material by ionising radiation, has been applied to a set of ceramic samples, chosen as representatives of the three main classes of lustred majolicas: Egyptian Fatimid (the tenth to eleventh centuries),

Hispano–Moresque (the eleventh to fourteenth centuries) and Italian (the fifteenth to seventeenth centuries). Before the RL characterisation, all the samples were dated by thermoluminescence.

The present study was aiming at characterising the three components of the lustred ceramics: the ceramic body, the glaze and the surface lustre, in order to find, if any, the RL associated emissions.

Keywords: Lustre, surface properties, thermoluminescence dating, radioluminescence, ancient ceramics

Introduction

Lustre is a decorative metal-like film applied to a white opacified surface of glazed pottery.1,2 The decoration with this technology began in the early Islamic period and continued in the Mediterranean area until present days. In central Italy, the production of lustred majolica achieved its maximum development during the fifteenth and the sixteenth centuries. Figure 1 shows scanning electron microscopy image acquired from a polished section of a typical lustred ceramics.

This typology of pottery consists of a ceramic body covered with a decorative glaze on which a further layer is applied, the lustre. To obtain this decoration, the samples undergo a triple heating treatment in the oven.

The glaze is a glass-like material, usually 100–300 mm thick, made of a mixture of silica, soda and lead-alkali.

The flux and colour formers are different, depending on manufacturing technology, period and area of production. The lustre film (0?1–2?0 mm thick) is the external layer, characterised by the presence of copper and silver nanocrystals embedded in a glassy matrix together with cassiterite (SnO2), silver and copper oxides and cosalite (Pb2Bi2S5). The presence of silver nanocrystal gives the lustre a golden iridescence.1,2

In this paper, the authors describe the radioluminescence (RL) properties of ceramic body, glaze and lustred glaze surfaces of samples representative of the three main class of lustres: Fatimid (Egypt, the eleventh to twelfth centuries), Hispano–Moresque (Spain, the thirteenth century, the fifteenth century) and Italian (the fifteenth to seventeenth centuries). Radioluminescence is the luminescence emission arising from excitation by ionising radiation, observed during the irradiation itself.

It is correlated to the presence and structures of defects in the material. Radioluminescence can be successfully used for radiation dosimetry and for dating geological sediments,3,4 but it has never been used for characterising historical ceramics until now. before the RL measurements, thermoluminescence dating (TL) of 22 samples found during excavations in

Egypt, in central Italy or provided by private collectors was performed. This step was done to surely assign each sample to a specific production period and therefore to a particular geographic area.

Experimental

All the samples analysed, already fully characterised from the chemical and mineralogical point of view,2 are described in Table 1, columns 1–3. Those labelled Fx were supposed to be Fatimid (the eleventh to twelfth centuries), those named IMMA, LIM1 and LIM2

Hispano–Moresque (the thirteenth to fifteenth centuries) while those labelled Lx and Ux came from Umbria (central Italy) and were dated from the fifteenth to seventeenth centuries. Samples S1 and S2 were probably produced in Italy imitating the Hispano–Moresque style.5

The method of dating by thermoluminescence is in principle quite simple,6 being expressed by the following equation

Age(years)~Paleodose(Gy)=annual dose(Gy per year) where the paleodose is the total dose absorbed by the pottery since its last heating, which generally corresponds to the firing of the ceramics in the oven, while the annual dose is the sum of the internal and external contributions from alpha and beta radiation and from gamma and cosmic radiation, principally due to the presence of uranium2238, thorium2232 and potassium240 in the pottery and its surrounding environment. In the pottery the accumulated dose is recorded by quartz, feldspars and other TL emitting minerals. 1CNR-INFM and Dipartimento di Scienza dei Materiali, Universita` di

Milano-Bicocca, via R. Cozzi 53, 20125 Milano, Italy 2INFN and Dipartimento di Scienza dei Materiali, Universita` di MilanoBicocca, via R. Cozzi 53, 20125 Milano, Italy *Corresponding author, email m.martini@unimib.it  2008 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 24 September 2007; accepted 3 January 2008 118 Surface Engineering 2008 VOL 24 NO 2 DOI 10.1179/174329408X286088

Thermoluminescence dating was performed following the standard fine-grain technique7 using a home made system based on the photon counting technique with a photomultiplier tube (EMI 9635QB) coupled to blue filters (Corning BG12). Artificial irradiations were carried out by a 1400 MBq 90Sr-90Y beta source (dose rate: 1?4 Gy min21), a 37 MBq 241Am alpha source (dose rate: 14?8 Gy min21).

Internal annual alpha and beta dose rates due to the radioactivity content of the ceramics were obtained by total alpha counting with ZnS scintillator discs6 and inductively coupled plasma optical emission spectrometry analysis.1

In absence of information about the external dose experienced by the samples, the same concentrations of radioactivity measured in the ceramics were assumed for the environment, with errors covering possible wide deviations.

Radioluminescence measurements were performed under X-ray irradiation at room temperature using a

Philips 2274 X-ray tube operating at 20 kV. The detection system was a CCD (Jobin-Yvon Spectrum

One 3000) coupled to a monochromator operating in the 300–900 nm range. The following RL spectra were recorded: sample untreated (ceramic body, glaze and lustre), glazed surface (measured where the lustre was lacking) and ceramic body alone (inner core).