Luminescence determination of firing temperature of archaeological pure sand related to ancient Dian bronze casting, China

Abstract

In the present paper, we have investigated the thermal history of an archaeological ‘core sand’ from a cow-shaped bronze ornament attached to a cowrie container. It was unearthed at the Lijiashan site, located in southwest China, in the area of central Yunnan, and dates from the Han period (3 c. BC–2 c. AD). We compared the archaeological sample and a control sample of the modern quartz, examining the sensitivity of 110 °C TL and 210° C TL glow peaks, and sensitization characteristics of 110° C TL and OSL. Large differences between the ‘core sand’ samples and the control sand samples have been observed in our work. The firing temperature of the ‘core sand’ was determined to be 550–700 °C. The results offer a key to understanding the ‘core sand’ as a unique casting technique in Bronze Age Yunnan. These luminesence techniques form a new method for determining the historical firing temperature of archaeological material.

Introduction

Thermoluminescence (TL) testing of ceramic cores of bronzes from museum collections can be applied to detect ancient artifacts and modern forgeries, as reported by Doreen Stoneham (1995). But when dealing with excavated bronzes, researchers may be interested in topics other than authenticity. One example is the firing temperature of ceramic cores or moulds during original casting processes, which relate to ancient casting techniques.

According to the literature the phenomenon of luminescence sensitivity variations has often been observed in quartz from unheated sand and heated sand. For example, Aitken and Smith, Botter-Jensen and Duller reported that there are large differences in OSL (optically stimulated luminescence) sensitivity between unheated quartz and quartz from heated archaeological samples (Aitken and Smith, 1988; Botter-Jensen and Duller, 1992). Yang and McKeever (1990) reported that an increase in sensitivity can be related to the absorbed radiation dose, and sensitization by heat treatment alone also can be observed. They explained the enhanced OSL sensitivity and phototransferred TL sensitivity as a function of annealing temperature. Lima et al. (2002) reported an investigation on the influence of different thermal treatments on the glow curves of natural quartz samples, and showed that sensitization can be achieved. The main effect is related to the thermal treatment by itself, and annealing increases the sensitivity of the natural quartz samples, especially around the 110° C TL peak. Lai et al. (2008) reported a determination of the effects of thermal treatment on the growth curve shape for OSL of quartz. They claimed that a significant difference in the growth curve shape before and after heating to >370 °C at a heating rate of 1 °C/s (TL was measured) might be used as an indicator to detect the thermal history. In addition, many authors reported an enhanced 210° C TL sensitivity of quartz from pre-exposing the sample to gamma radiation followed by annealing at a high temperature, above 400 °C (David et al., 1976; Bailiff and Haskell, 1983; Haskell et al., 1985; Kaipa and Haskell, 1985; Benny and Bhatt, 1997). Some authors reported that the thermal history of the archaeological artefact and/or an appropriate annealing treatment can be approached by the comparison of the shape of the regenerated TL glow curves obtained after annealing a series of aliquots at different temperatures, with the shape of the natural glow curve or by the use of the dose-plateau test (Spencer and Sanderson, 1994; Roque et al., 2004).

In the present paper, we perform TL and OSL measurements on an archaeological ‘core sand’ and a standard sample after heat treatments at different temperatures in order to investigate the thermal history of the ‘core sand’ by comparing quartz with different thermal historical backgrounds.