Density measurements as a non-destructive approach to investigate the heat treatment of siliceous lithic artefacts

The heat treatment of siliceous raw materials is one of the most ancient evidence of pyrotechnology in human history. In fact, the controlled heating of rocks to facilitate the production of lithic artefact has been proved in several contexts, even back to the Palaeolithic. The use of the heat treatment can be preliminarily assessed by eye inspection of the stone tools. However, the rock variability and possible surface alterations make uncertain the result of this simple macroscopic approach. Therefore, more reliable methods, based on archaeometric techniques, have been developed, in order to characterize structural modifications of rocks due to the heating process. These techniques may involve some limitations, as concerns the size of the sample and/or its preparation. In this paper, we show how density measurement, using the Archimedes balance, may provide a clear indication on the presence of heat-treated archaeological lithic artifacts. The method involves the construction of a reference master curve: density vs heat treatment temperature, of geological chert samples with the same provenance of the archaeological artifacts under investigation. The novel procedure has successfully been tested with some stone tools from La Vela Neolithic site (Trento - Italy), which had been previously proved to be heat-treated by means of infrared spectroscopy and scanning electron microscopy. In this regard, the “density method” turns out to be a cost-effective and fast non-destructive approach, particularly suited for a preliminary assessment of the heat treatment of large lithic assemblages.