AN EXPERIMENT TO ASSESS PETROGRAPHIC AND PHYSICAL CHANGES INDUCED BY THERMALLY TREATING LITHIC ARTIFACTS

Thermal “heat” treating was a well-established technique employed by indigenous peoples to improve knappability of cryptocrystalline silica materials. Our ancestors discovered that chert exposed to heat from wood fires improved fracturing during flake removal and resulted in greater efficiency in tool manufacturing. Past research has found that thermal heating induces changes in color and/or luster, as well as cracking intensity. Some studies have also suggested that the process involves recrystallization and changes in silica crystal size. This undergraduate study tests these suppositions. Our research focuses on a Middle Archaic to Mississippian archaeological site (c. 3000 BC to 1000 AD) on the Homochitto River in Mississippi. We selected three unique samples of bulk (not heat-treated) chert from the site. Nine thin-section sized billets (2 x 2 x 4 cm) were made from each of the samples (27 total). All were polished on one side. We used a diamond wafer blade and a diamond drill bit to engrave each polished surface with consistent grooves and pit marks. Each surface was then photographed under a binocular microscope. Samples were divided into 3 groups of 3 triplicate samples: 1) no treatment, 2) heat treatment at 600C in a controlled kiln, and 3) heat treatment at 600C+ in a wood fire. Heat-exposed specimens were re-examined and re-photographed in order to resolve any physical/surface changes (e.g., thermal polishing). Following this, thin-sections of all 27 billets were made to assess microscopic/petrographic alteration(s) induced by thermal heating. This presentation presents preliminary results from the study.