Thermally and optically stimulated luminescence of early medieval blue-green glass mosaics

Radiation Measurements 38 (2004) 799 – 803 www.elsevier.com/locate/radmeas

Thermally and optically stimulated luminescence of early medieval blue-green glass mosaics A. Galli∗ , M. Martini, C. Montanari, E. Sibilia Dipartimento di Scienza dei Materiali, INFM and Universita di Milano Bicocca, via R. Cozzi, 53, Milano 20125, Italy Received 17 November 2003; received in revised form 17 November 2003; accepted 15 March 2004

Abstract The preliminary results of a study related to luminescent mechanisms in glass mosaic tesserae are presented. The samples came from a medieval glass deposit found during archaeological excavations in the S. Lorenzo Church in Milan. Energy Dispersive X-rays Fluorescence (EDXRF) measurements were performed to obtain information on the elemental composition of the materials. Thermally Stimulated Luminescence (TSL, both conventional and wavelength resolved) and Optically Stimulated Luminescence (OSL) analyses allowed to get information about traps and luminescence centres. The observed luminescence characteristics were close to that of quartz, showing the presence of an easy to bleach trap (300◦ C, 1:95 eV) and of a hard to bleach trap (350◦ C, 2:20 eV); charge transfer phenomena, involving the low-temperature peaks have been observed. There is a strong indication that the easy to bleach traps are responsible for both OSL and TSL emission at 300◦ C. c 2004 Elsevier Ltd. All rights reserved.  Keywords: Glass mosaic; Thermally and optically stimulated luminescence

1. Introduction Mosaics consist of small pieces (tesserae) of glass or other materials (stone, ceramics) held in place by mortar. Glass mosaic tesserae, variously coloured, are semi-transparent or opaque because they used to be looked at in re:ected light. The glassy phase is associated in variable ratio to the crystalline phase. Crystals can be added to the fused phase as ;ne powder or separated by devetri;cation from the cooling of homogeneous fused glass containing suitable components. Glass colouration was obtained combining to the glassy matrix base, in variable ratios, metal oxides with colouring and opacifying properties (Verit
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green, blue, azure and turquoise, cobalt for blue (Co2+ ) and purple (Co3+ ), iron for dark or light green (Fe2+ or Fe3+ ). Antimony and tin act as opaci;ers, forming crystals dispersed in the base. Ancient glass tesserae systematically showed TSL sensitivity higher than normal glasses. The impurities present or added to the glass network create colour centres in the vitreous base that are likely to act as electron traps and recombination centres (Chiavari et al., 2000). It has also been proposed (Muller and Schvoerer, 1993) that the TL sensitivity increases with the crystallinity degree: the formation of microcrystals consequent to the addition of opaci;er oxides (in particular antimony oxide, III and V oxidation states, Mirti et al., 2000) could account for the relatively high TSL sensitivity observed in some antimony-enriched materials (Galli et al., 2003). Appropriate TSL protocols have been developed to test sensitivity, extent of sensitivity changes, optical bleaching and signal regeneration by sunlight of such materials. Our previous results showed that mosaic tesserae are a class of glasses possibly suitable for TSL dating (Chiavari et al., 2001).

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A. Galli et al. / Radiation Measurements 38 (2004) 799 – 803

The preliminary result of a study performed on medieval mosaic tesserae with the aim of a deeper understanding of their luminescent mechanisms are presented.

number higher than 19) are reported in Table 1. The sample SL27 could not be analysed, being too thin.

2. Experimental and results

The studied glass mosaics could be grouped into two classes according to the features of their TSL glow-curves. Class A (8 samples): the natural glow-curve (induced by the exposure to the natural irradiation ;eld since the making of glass), showed a broad, sometimes intense emission between 300◦ C and 450◦ C, while the glow-curves due to laboratory irradiation are characterised by several well resolved peaks (100◦ C, 200◦ C, 230◦ C, 300◦ C and 400◦ C approximately, Fig. 1a). No sensitisation was observed when repeatedly irradiating and measuring the same aliquot, at least up to 200 Gy of integral dose. Class B (4 samples): the natural signal had a peak at about 300◦ C and a broad, intense emission, centred at about 400◦ C. The arti;cial glow-curves exhibited peaks at 100◦ C, 230◦ C and 300◦ C and a low or very low emission at higher temperature (Fig. 1b). Again, no sensitisation was observed (200 Gy integral dose). The maximum intensity ratio of the 400–300◦ C peaks was strongly di=erent in the arti;cial and natural signals, being always ¿ 1 in the natural glow-curve. This fact could be explained by the presence of charge transfer processes, phenomena that should not be detected after the laboratory irradiation, that is, several orders of magnitude faster than the natural one. In fact, this charge transfer could be somehow related to a fading phenomenon that needs long time to be relevant. The preliminary results of the fading test, reported in Fig. 2, showed an appreciable regeneration of the glow-curve in the temperature region around 400◦ C after a few days storage at 100◦ C. At the same time, a decrease of the 300◦ C peak is evident. The TSL growth vs. dose of the 300◦ C peak was reconstructed up to about 80 Gy, and was linear, as shown in Fig. 3. Wavelength resolved TSL measurements were performed on nine samples, X-ray irradiated, the natural signal being too low to be detected. No information about the recombination centres responsible for the natural emission could therefore be achieved. The maximum temperature of our experimental set up being 350◦ C, neither information about high-temperature emissions could be obtained. The main observed emissions are reported in Table 2. An intense UV emission (370±10 nm) was present in all samples, and is ascribed to the SiO2 glass matrix. The emission at 440±10 nm, associated to calcium antimoniate (Galli et al., 2003) was found only in a few cases, despite the similar concentrations of Sb measured in all samples but two (Table 1). The very di=erent intensities of the 440 nm emission con;rmed that the overall concentration of Sb did not account for the TSL emission spectrum, suggesting the presence of other Sb compounds like lead antimoniate. Analogously, the emission at 580 ± 10 nm was not associated to the overall Mn elemental concentration in the sample, being instead attributed

The samples analysed are 12 blue-green coloured mosaic tesserae, taken from an early-medieval glass deposit found during the archaeological excavations in the S. Lorenzo Church in Milan. They were cut into slices 500–800
m thick, using a diamond saw (Bueheler, Isomet type 11/1180 Low-speed Saw, Diamond Wafering Blade). The slices were submitted to di=erent analyses: those obtained from the inner core, once powdered to 80–100
m grains, were used for the luminescence measurements, after deposition on aluminium discs. The external slices were used for energy dispersive X-rays :uorescence (EDXRF) measurements to obtain preliminary information on the elemental composition of the materials. Analyses were performed using a secondary target EDXRF set-up, measuring the Compton scattering intensity to evaluate the self-absorption corrections (Bonizzoni et al., 2000). The conventional TSL measurements were acquired by home-made system, consisting of an oven for controlled heating in ultra-pure N2 atmosphere (available heating rates: from 1◦ C to 15◦ C=s; where not di=erently speci;ed, the heating rate was 15◦ C=s), using a photon counting technique with an EMI9635QB photomultiplier coupled to Corning BG12 blue ;lters. To study the wavelengths of TSL emission, a highsensitivity home-made apparatus was used, exploiting a two-stage micro-channel plate (MCP) with a 512-photodiode array. The system, cooled by a Peltier control unit, reached a signal-to-noise ratio and sensitivity comparable with that of a photomultiplier. The spectra, recorded at 2◦ C=s, were corrected for the wavelength response (Martini et al., 1996). Isothermal OSL measurements were performed using a home made modi;ed TSL system. The excitation wavebands were selected with an automated imaging spectrometer monochromator (TRIAX 180) coupled to a 100 W tungsten lamp. The emitted luminescence was detected with an EMI 9235QB photomultiplier coupled to an interchangeable system of ;lters. OSL measurements were always performed illuminating at 460 nm, wavelength corresponding, for these materials, to the maximum eIciency in trap depleting. Arti;cial irradiations were performed using a 1400 MBq 90 Sr-90 Y beta source (dose rate 1:4 Gy=min) and an X-ray tube (operating conditions: 20 kV, 10 mA. Dose rate: 16:6 Gy=min). 2.1. EDXRF measurements The results of the quantitative analyses performed on the mosaic tesserae for the main detectable elements (atomic

2.2. TSL measurements

Table 1 Quantitative EDXRF analysis results Colour

Ca (%)

Ti (%)

Mn (%)

Fe (%)

Co (%)

Cu (%)

Zn (%)

Sr (%)

Sb (%)

Pb (%)

SL16/A SL17/A SL18/A SL19/A SL20/A SL21/B SL22/A SL23/B SL24/B SL25/A SL26/B

Blue Green Light green Green Blue Green Light green Green Azure Green Azure

2:10 ± 0:20 2:60 ± 0:30 2:63 ± 0:30 3:43 ± 0:30 4:68 ± 0:50 3:70 ± 0:30 3:25 ± 0:30 2:45 ± 0:20 2:36 ± 0:20 2:64 ± 0:30 3:24 ± 0:30

0:03 ± 0:01 0:05 ± 0:01 0:05 ± 0:01 0:17 ± 0:03 0:11 ± 0:01 0:05 ± 0:01 0:04 ± 0:01 0:07 ± 0:01 ¡0.02 ¡0.02 ¡0.02

0:11 ± 0:01 0:38 ± 0:03 0:39 ± 0:03 0:82 ± 0:07 0:28 ± 0:03 0:45 ± 0:04 0:39 ± 0:03 0:38 ± 0:04 0:29 ± 0:03 0:32 ± 0:03 0:22 ± 0:02

0:31 ± 0:03 0:52 ± 0:04 0:51 ± 0:04 1:21 ± 0:10 0:67 ± 0:07 0:55 ± 0:05 0:52 ± 0:04 0:45 ± 0:04 0:43 ± 0:04 0:47 ± 0:04 0:36 ± 0:03

0:09 ± 0:04 — — — 0:07 ± 0:01 — — — — 0:015 ± 0:003 —

0:050 ± 0:005 ¡0.010 ¡0.010 0:45 ± 0:04 0:14 ± 0:02 0:039 ± 0:003 0:034 ± 0:003 0:023 ± 0:002 0:34 ± 0:03 0:37 ± 0:03 0:60 ± 0:05

¡0.010 0:018 ± 0:002 0:018 ± 0:002 — ¡0.010 0:017 ± 0:002 0:015 ± 0:001 ¡0.010 ¡0.010 ¡0.010 0:010 ± 0:002

0:051 ± 0:004 0:055 ± 0:004 0:057 ± 0:004 0:034 ± 0:003 0:030 ± 0:003 0:050 ± 0:004 0:061 ± 0:004 ¡0.020 0:031 ± 0:002 0:11 ± 0:01 0:12 ± 0:001

1:14 ± 0:20 0:88 ± 0:20 0:89 ± 0:20 0:84 ± 0:40 1:46 ± 0:60 1:45 ± 0:40 1:28 ± 0:30 0:80 ± 0:30 0:84 ± 0:30 ¡0.20 ¡0.20

0:06 ± 0:01 ¡0.020 ¡ 0.020 0:30 ± 0:02 0:06 ± 0:01 0:47 ± 0:04 0:04 ± 0:01 0:12 ± 0:02 0:02 ± 0:01 0:06 ± 0:01 0:04 ± 0:01

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Fig. 1. Natural and arti;cially induced TSL glow-curves representative of the two observed behaviours (a and b, respectively). (1) natural TSL; (2) 2:8 Gy; (3) 5:6 Gy; (4) 11:2 Gy.

Fig. 2. Fading test. TSL measured immediately after irradiation (dotted) and after 144 h storage at 100◦ C (continuous). Irradiation dose: 44 Gy.

A. Galli et al. / Radiation Measurements 38 (2004) 799 – 803

Sample and class

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A. Galli et al. / Radiation Measurements 38 (2004) 799 – 803

Fig. 3. TSL growth vs. beta dose.

Table 2 TSL emission wavelengths Sample and class Colour

Emission wavelength (± 10 nm)

SL16/A SL17/A SL18/A SL20/A SL21/B SL23/B SL24/B SL26/B SL27/A

370, 370 380, 370, 370 370, 370, 370, 370,

Blue Green Light green Blue Green Green Azure Azure Blue

440, 580 580 440

Fig. 4. Sample SL21, TSL emission spectrum. Irradiation dose: 20 Gy.

580 580, 600 440, 480 440, 480

to the presence of the element in the 2+ oxidation state. Two samples also showed a weaker band at 480 ± 10 nm (Fig. 4), whose assignment is uncertain. For one of the B class samples, SL26, the evaluation of traps depth was attempted with the initial rise method combined with the partial cleaning method (Martini and Meinardi, 1997). The results are reported in Fig. 5. At low temperatures a continuous of levels accounts for the complexity of the TSL spectrum, while at higher temperatures the presence of two traps levels at about 1.9 and 2:0 eV is evident. The trap depths reported for the high-temperature peaks of quartz are lower (Spooner and Questiaux, 2000; Franklin et al., 2000).

Fig. 5. Sample SL26, evaluation of tap depth using the initial rise and the partial cleaning methods (see text).

2.3. OSL measurements The OSL characteristics of the mosaic glasses were similar for all the measured samples, and can be summarised as follows: • No detectable natural OSL signal was recorded, while it always appeared after an arti;cial irradiation of a few Gy. • The OSL decay was not a simple exponential (Fig. 6), as reported for quartz (McKeever et al., 1996; Wintle and Murray, 2000), excluding the presence of a single electron trap.

Fig. 6. Sample SL19, OSL decay. Irradiation dose: 10 Gy.

A. Galli et al. / Radiation Measurements 38 (2004) 799 – 803

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the emission spectra characteristics. EPR analyses appeared to be much useful than the elemental ones to understand the TSL processes in glasses (Azzoni et al., 2002), that are often associated to the oxidation states of the ions, and not to their total concentration in the glassy matrix. References

Fig. 7. Sample Sl19. TSL without illumination (dotted) and after illumination for 900 s at 460 nm (continuous). Irradiation dose: 15 Gy.

• The OSL growth vs. dose was linear in the same range where TSL growth was also linear. The absence of the natural signal obviously indicated that the OSL emission is associated to easy to bleach traps, the samples having been surely exposed to daylight before entering the laboratory. Combined OSL and TSL experiments, described in detail elsewhere (Galli et al., in press), showed that the illumination at RT (460 nm) of an irradiated sample induced an emptying of the 300◦ C TSL peak (Fig. 7), the residual OSL being strictly correlated to the extent of TSL traps emptying. A strong relationship between OSL and TSL traps could therefore be hypothesised, similarly to what evidenced in quartz (Wintle and Murray, 1997). 3. Conclusions The luminescence characteristics of the medieval mosaic investigated glasses were close to that of quartz, showing the presence of an easy to bleach trap (300◦ C, 1:95 eV) and of a hard to bleach trap (at 350◦ C, 2:20 eV); charge transfer phenomena, involving the low-temperature peaks have been observed. There is a strong indication that the easy to bleach traps are responsible for both OSL and TSL emission at 300◦ C. The maximum eIciency in OSL trap depleting was obtained with exposure to 460 nm light. An appreciable regeneration of the 350◦ C peak was observed after short storage of arti;cially irradiated samples at relatively low temperatures (a few days at 50◦ C and 100◦ C). The wavelength resolved TSL measurements showed that high concentrations of colouring and opacifying elements did not necessarily result in a TSL emission associated to the corresponding recombination centres: in the cases of Sb and Mn, the simple elemental concentration did not account for

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