Heat treatment of Polish flints

Abstract

In Poland three types of flint (chocolate, spotted and banded) were intensively mined from the Terminal Palaeolithic to the Early Bronze Age. Although heating of flint to improve its flaking properties was practised across the world from ∼110,000 years ago to the recent, particularly in southwestern Europe, heat treatment of flint in Poland is known from only two sites. Experimental studies showed that controlled heat treatment of Polish flints causes subtle, variable changes in colour but a consistent marked increase in lustre on fractured surfaces, which should make heat-treated artefacts easy to recognise in the Polish archaeological record. Heat treatment also causes a substantial reduction in the fracture toughness of all tested Polish flint types and a concomitant increase in flakeability. The effect is greatest for banded flint, due to its initially higher fracture toughness. The decrease in fracture toughness is caused by recrystallisation, allowing fractures to propagate more easily. In banded flint the average crystal size increases, and within chocolate and spotted flint the fibrous chalcedony patches become more granular. Although the flaking properties of banded flint are improved markedly by heat treatment, the procedure was applied only occasionally by prehistoric flintknappers to this material. Banded flint was preferentially used for completely polished axes, and only rarely for blades, because chocolate and spotted flints are available close to the banded flint mines; these flint types have far superior flaking properties and were extensively used for blade manufacture. Spotted flint is generally less suitable for heat treatment; it is subject to explosive thermal spalling due to its abundant calcite inclusions. Although heat treatment improves the flaking properties of chocolate flint, heated chocolate flint is rare in the Polish archaeological record, probably because high quality flint was so abundant in Poland.