Rock-magnetic and color characteristics of archaeological samples from burnt clay from destructions and ceramics in relation to their firing temperature

Abstract

Determination of ancient firing temperatures of archaeological pottery is a widely discussed topic in archaeometry. Here, a set of magnetic characteristics (magnetic susceptibility, isothermal and anhysteretic remanences, hysteresis parameters) and color parameters were studied for a collection of pottery fragments and burnt clay from house destructions. The results show that magnetite and hematite of superparamagnetic to single domain grain size are the main iron oxides produced during heating. Hematite fraction is more important and frequently detected in pottery sherds than in burnt clay from destructions. An inverse linear regression was obtained between the estimated firing temperature and the ratio value/chroma, which is shown to be site specific for pottery samples. For burnt house destructions, the regression is less well constrained and most probably reflects differences in the raw material. Consideration of rock-magnetic parameters against firing temperature estimates reveals a direct link between saturation magnetization and ancient firing temperature for burnt clay from house destructions. In contrast, this link is inverse and worse defined for pottery materials. This different behavior is attributed to different prevailing processes of iron oxide transformations in burnt clay and pottery, related to the specific firing conditions.