Technical innovations during the recent Solutrean in the southwest of France: Recognition of heat treatment of chert and estimation of heating temperatures based on the example of Le Piage (Lot, France)

Abstract

The Solutrean technocomplex is characterized by the use of innovative techniques for stone tool knapping during the Upper Palaeolithic. These processes include the development of pressure retouch and heat treatment of silica rocks. These innovations remain unprecedented at the scale of the recent European Paleolithic. The aim of our analysis is to acquire new information on the application of heat treatment, by investigating the heating technique and the lithic chaîne opératoire in which this treatment occurs. Macroscopic observations of the material combined with an infrared spectroscopic analysis of 69 archaeological samples provide new data on heat treatment in the Solutrean lithic series of Le Piage (Fajoles, Lot). Raw materials were heated in controlled conditions and to temperatures of 250-300°C. We also found that only leaf-shaped pieces were heat-treated at Le Piage. Part of the results obtained by infrared spectroscopy reveal discrepancies with macroscopic observations. These data raise questions concerning the validity of common macroscopic criteria used for recognising heat treatment and shed light on some of the choices made by Solutrean groups.

Recognizing heat‐treated chert: A comparison between infrared spectroscopy and fracture surface analysis

“…IR and roughness analyses gave similar results on the heating temperatures used by Solutreans. They also concur with previous studies on translucent Tertiary chert artefacts from Laugerie‐Haute West (Schmidt & Morala, 2018), and also from the site of Le Piage (Lot) (Bachellerie et al, 2019). On these two types of chert (Bergerac chert and Tertiary chert), Solutrean knappers would have heat treated at temperatures between 250 and 350°C.…”

“…The heating technique used by Solutrean groups IR and roughness analyses gave similar results on the heating temperatures used by Solutreans. They also concur with previous studies on translucent Tertiary chert artefacts from Laugerie-Haute West (Schmidt & Morala, 2018), and also from the site of Le Piage (Lot) (Bachellerie et al, 2019). On these two types of chert (Bergerac chert and Tertiary chert), Solutrean knappers would have heat treated at temperatures between 250 and 350 C. This consistent temperature signal observed at several Solutrean sites suggests a specific management of the heating environment and strongly pleads against opportunistic heating in open-air fires (such as in Schmidt et al, 2017).…”

“…Several analytical techniques have been developed to do this (see for example: Melcher and Zimmermann 1977;Toyoda et al 1993;Borraldie et al 1993, Santaniello et al 2021. Near infrared spectroscopy (IR) is today the most often used method to investigate the heat treatment of artefacts (Schmidt et al 2013;Santaniello et al 2015;Schmidt et al 2017;Bachellerie and Schmidt 2019; for a similar related approach see: Weiner et al 2015). The technique is based on the structural changes of silica rocks that lead to progressive reduction of intergranular pores.…”

“…Generally, the signal-to-noise ratio is problematic for thick artefacts and artefacts made from opaque chert varieties. The most important caveat is that this method cannot distinguish between intentional and post-depositional heating when it is not associated with other macroscopic analyses of surface gloss (Bachellerie et al, 2019).…”

“…The first recognition of intentional heating, in the context of Solutrean bifacial shaping sequences, was during the 1960–70s (Bordes, 1967, 1969; Collins, 1973; Inizan et al, 1976). Recent studies have refined our knowledge of Solutrean heat treatment (Bachellerie et al, 2019; Schmidt et al, 2018; Schmidt & Morala, 2018; Schmidt & Morala, 2020). These new data suggest that Solutrean knappers used a standardized heating technique that aimed to create a heating environment allowing temperature control.…”