The trace elements to be measured

LA-ICP-MS is a suitable analytical technique which can determine the concentrations of tens of elements in different order of magnitude in different matrices simultaneously. By LA-ICP-MS technique, the contents of 35 trace elements were measured in this study, including Li, B, Ti, V, Cr, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Th, U, Zn, As, Sn, Sb, Co, Ni and 14 rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu).

Every colour in each glaze sample was measured on three sampling positions in order to calculate the average values. These average element concentrations are given as ppm in different colours of the glaze samples used for the data discussion. These data are listed in the Appendix I Tables A5-A7.

3.3.4 Analysis and standards

In LA-ICP-MS, the fully-prepared sample discs were placed inside the sample chamber which had a window. The laser, NewWave UP193FX excimer (193nm) laser system had a built-in microscope imaging system and used to ablate the sample. The chamber is flushed by a stream of helium gas ( 0.80 L × min⁻¹), mixed with an argon carrier gas ( 0.85 L × min⁻¹), to the Agilent 7500 series ICP-MS. Laser ablation craters were set at 70μm in diameter, and approximately 30μm deep, the laser was fired for 45 seconds on

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sample at 10 Hz and a typical fluence of 2.8 Jcm⁻². The integrated signal from each ablation was blank subtracted using a ‘gas background’ acquired with the laser not firing on the sample for a period of 30s. Data was recorded in a time-resolved analysis (TRA) mode.

To obtain accurate quantitative results, some suitable standard samples were selected to analyse before and after the analyses of the glaze samples. Calibration of the system was performed using a standard glass of known concentration (NIST SRM610). Moreover, the standard glass NIST SRM612 was used for quality control. The bulk composition of the selected standard should be as similar as possible to that of the target samples. However, the target glaze samples in this study are lead-based glazes, and the suitable matrix-matched standard for this kind glaze is not available in the Analytical Geochemical Laboratories of British Geological Survey. In this case, an internal reference element is imperative to be used to compensate for any variation in the measuring conditions.

Specifically, by using the internal reference element (the signal intensity for one selected element), any differences in ablation rates and transport efficiency between samples and sample-standards can be corrected (Resano et al. 2010). In this study, the known Si concentrations of glaze samples determined by EPMA-WDS were used as the internal standard. Calibration and quality control glass standard samples were run with each glaze sample block. Quality control data demonstrated across six analytical sessions and n=96 analyses. Both the average analytical results and the quoted values of NIST SRM612 are presented in Table 3.2. The calculated standard deviations and relative standard deviation (RSD%) of the measurements are also shown in Table 3.2 to check the precision and accuracy of the measurements. As shown in Table 3.2, the precision of the measurements is sound. The precisions of all the elements are as good as can be expected (2-5 RSD%) except Sn and Ti (11.45 and 13.38 RSD% respectively). Moreover, the accuracies for all

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the elements ranged between -5 and +3 % except Sn (+13.57%). Although the element Sn has a relatively high percentage error, it is still within the acceptable range as a

Table 3.2 Measured and expected values (mg/kg), calculated St.Dev values, RSD and Bias values (%) of trace elements for standard reference glass NIST612 by LA-ICP-MS (The

expected values for SRM612 were published by GeoRem (Jochum et al. 2011)

3.3.5

Relevant literature review

LA-ICP-MS is a highly sensitive analytical technique for trace element determination.

During the past two decades, this technique has been used for the chemical characterisation of many kinds of archaeological materials such as bones and teeth, obsidians and glasses, ceramics and their glazed or painted layers, cherts and other miscellaneous stone materials, ancient documents and metal objects. By these studies, the

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use of LA-ICP-MS has been proven as an essential technique for chemistry-based archaeological studies.

Several reviews of the LA-ICP-MS applications in archaeological materials have been published (Gratuze et al. 2001; Neff 2012; Speakman et al. 2002b; Giussani et al. 2009;

Resano et al. 2010). These reviews give brief descriptions of the LA-ICP-MS technique especially its features, its basic operating principles, its analytic parameters and calibrations. These reviews also demonstrate an overview of the contributions of LA-ICP-MS in the investigation of various archaeological materials as aforementioned. Moreover, advantages and limitations of the LA-ICP-MS technique also are discussed critically.

The first published application of LA-ICP-MS to ceramics traces back to the paper by Robertson et al. (2002), who carried out a preliminary study on the ceramic standard materials of Ohio Red Clay and NIST SRM612 by both LA-ICP-MS and PIXE techniques. Later papers focused on linking the elemental compositions of ceramics analysed by LA-ICP-MS to the raw material sources and manufacturing techniques used to produce ceramics. Chemical compositional analyses of ceramic bodies and clays have been used widely in provenance studies (Cochrane and Neff 2006; Bartle and Watling 2007; Zhu et al. 2012).

Less compositional analysis by LA-ICP-MS has been used to study glazes and glaze-paints of glazed ceramics. Speakman et al. (2002a) analysed glaze-paints on 37 prehistoric Mesa Verde and Mancos Black-on-white pottery from the Mesa Verde Region of the American Southwest using LA-ICP-MS. The results of paint compositions show that Mancos Black-on-white is characterised by having a mineral-based paint derived from an iron-manganese ore, while the Mesa Verde Black-on-white is usually painted by an

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organic-based (carbon-based) paint. This work confirmed that the LA-ICP-MS was an appropriate method for distinguishing mineral from carbon-based paints.

Hill et al. (2004) studied the major and trace elemental compositions of the glazes of 175 glazed ceramics from Sasanian and Early Islamic period sites located on the Deh Luran Plain in southwestern Iran by LA-ICP-MS, combined with the INNA analysis determining the ceramic pastes. They identified five compositional groups. The glazes of Group 1 are composed of soda/lime/silica glazes. Group 2 glazes contain trace contents of lead in a soda/lime/silica glaze. The glazes of group 3 and group 4 are all lead-based, and the major discriminating element between these two groups is the existence of copper used as a colourant. The glazed ceramics of Group 3 include blue or green monochrome coloured glazes, green coloured press-moulded glazes and splashed glazes. The glazed ceramics of Group 4 include non-decorated tin-glazed, lustreware glazes and yellow-brown lead stannate coloured glazes. The glazed ceramics of group 5 are high nickel glazes, using cobalt as their colourants. The presence of nickel is thought to be an indicative of the utilisation of the similar ore deposits as for the cobalt.

Resano et al. (2005) investigated the trace and major element compositions of blue-coloured (Co-enriched) glazes of ceramics produced in the Aragon area during the 14th to 18th centuries AD. The results inferred that the differences in the contents of Cu, As and Mn allow the classification of these glazes into three different categories, which could be related to the different ceramic production places and the various possible sources of the Co pigments. The classified results of the glazes are in good agreement with the results obtained by the ICP-AES analysis of the ceramic bodies. In this work, the potential of LA-ICP-MS for obtaining spatially resolved information and for analysing glazed layers was confirmed.

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Duwe and Neff (2007) studied the trace and major elemental compositions of glaze-paints of 161 White Mountain Red Ware (WMRW) ceramic samples (AD 1275-1325) from east-central Arizona by the time of flight-laser ablation-inductively coupled plasma-mass spectrometry (TOF-LA-ICP-MS). By PCA analysis of chemical compositions, three different glaze-paint recipes can be determined in the WMRW ceramics by the discriminating elements of iron, copper, antimony, manganese and lead. This work inferred that the TOF-LA-ICP-MS analysis is potential for future studies on glaze-paint of ceramics.

Hao et al. (2013) studied the element concentrations of 13 glazes of Chinese proto-celadon in the Western Han Dynasty (BC202-AD8) excavated in two archaeological sites from Northern China- the Rizhao Cemetery of Shandong Province and the Cuipingshan site of Jiangsu Province, as well as one site from Southern China- the Xiaolongjing Cemetery of Zhejiang Province. The results show that the differences between Northern China proto-celadon glazes and Southern China ones can be found in concentrations of several significant elements of Fe, Ca and Mg. The glazes of Cuipingshan and Rizhao sherds from Northern China have higher content levels of Fe and Ca but lower Mg than Xiaolongjing glazes from Southern China, suggesting a unique manufacture technique in Northern China, differing from the southern ones. Besides, differences of several rare earth elements (Gd, Dy, Ho and Eu) are also remarkable between samples from Northern China and Southern China, implying a distinct feature in the raw material selection of the proto-celadon manufacture between Northern China and Southern China. These results may suggest that Chinese proto-celadon produced in Western Han Dynasty were manufactured both in Southern and Northern China.

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