Silcrete heat treatment in the South African Middle Stone Age has been understood as a proxy for several key concepts in the evolution of early anatomically modern humans, although relatively little is yet known about the techniques and procedures used for it. Tempering-residue on heat-treated artefacts is one of the few known proxies for such a heating technique. It was reported to indicate heat treatment in open-air fires rather than in a sand-bath. There are however, several questions that need to be answered before tempering-residue can be used as an unambiguous proxy for heat treatment in open-air fires. These questions included: does it exclusively form in contact with embers? Is it only produced by green wood? We investigate the conditions under which tempering-residue forms using an experimental approach and wood from the zone of natural silcrete occurrence in South Africa. Our results confirm earlier publications by highlighting that tempering-residue only forms during heat treatment in direct contact with embers. Moreover, it indicates the use of green (not dried) wood in these fires. We also propose a potentially non-destructive analysis protocol for identifying tempering-residue on silcrete artefacts using infrared and Raman spectroscopy. The implications of these results for our understanding of early stone heat treatment and Middle Stone Age archaeology in general are highlighted by the ongoing debate about technical complexity, required investment and modern behaviours of early Homo sapiens.