Determining firing temperature of clay-based archaeological materials with FTIR absorptivity method

Abstract

FTIR analysis has been proven to be a useful tool in measuring the firing temperature of archaeological sediments and ceramics. A recent development of this method is the finding of a negative correlation between absorptivity of 1030 cm⁻¹ band and the firing temperatures. With potassium ferricyanide (K₃[Fe (CN)₆]) added as an internal standard, the 1030 cm⁻¹ absorptivity can be measured quantitatively to reflect firing temperature of samples. However, the following investigations show that the 1030 cm⁻¹ absorptivity can also be influenced by a series of other factors including homogeneity of sample, the mixing duration between sample, internal standard, and KBr, as well as grinding time of sample and internal standard. This research quantifies the influence of these factors and recommends the best practice for using this method. The heat-induced absorptivity change of characteristic bands of archaeological clay-based materials is then monitored, revealing that 1030 cm⁻¹ band was the most optimal temperature indicator. Based on a series of Kruskal-Wallis one-way analyses, it is determined that for samples fired over 400°C, the absorptivity ratio of 1030 cm⁻¹/778 cm⁻¹ can be used to estimate its original firing temperature. However, for those fired below 400°C, K₃[Fe (CN)₆] has to be added and the ratio of 1030 cm⁻¹/2117 cm⁻¹ is more proper for quantifying its original firing temperature. The sediments from two different sites were then artificially fired to varied temperatures and tested following the analytical protocol established in this research. The results demonstrate the high accuracy of this method and its great potential in future study of ancient pyrotechnologies.