Heat treatment and mechanics of Moravian Jurassic cherts

• Published in: Archaeological and anthropological sciences Vol. 13; no. 10
• Main Authors: Moník, Martin, Hadraba, Hynek, Milde, David, Chlup, Zdeněk, Nerudová, Zdeňka, Schnabl, Petr
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-10-2021; Springer Nature B.V
• Subjects: Ambiguity; Anthropology; Archaeology; Change agents; Chemistry/Food Science; Chert; Crack propagation; Earth and Environmental Science; Earth Sciences; Geography; Heat treating; Heat treatment; Heating; Imports; Jurassic; Life Sciences; Original Paper; Prehistoric era; Propagation; Water loss; Infrared spectroscopy; Elastic modulus; Chert; Heat treatment; Fracture toughness
• ISSN: 1866-9557; 1866-9565
• DOI: 10.1007/s12520-021-01388-z

Experimental heat treatment was applied to Moravian (Czech Republic) Jurassic cherts of Olomučany type to find whether it leads to better flaking properties. Indentation tests and other tests were used and compared with the changes in infrared (NIR) spectra which are usually indicative of mechanical changes linked with water loss from the material. The indentation tests provided ambiguous results due to irregular propagation of cracks from indent corners. On the other hand, three-point-load fracturing and resonance tests were usable and indicated that the cherts are best flakeable after heating to 300 °C when fracture toughness is reduced whereas elastic modulus increases. These two changes complement each other: The first causes easier flakeability and the other more stable crack propagation and predictability. The simultaneous formation of fluid inclusions probably contributes to swifter crack propagation. As shown by our sequence statistics analysis, the drop in fracture toughness seems to be a more gradual process, whereas the increase in stiffness at 300 °C is rather sudden. The fact that the Olomučany chert or other fine-grained lithologies were not heat-treated in the prehistory of eastern Central Europe indicates that the improvement by heat treatment was not practised in this area in prehistory, unlike in more westerly regions. This may be due to long-distance imports of other, finer-grained lithologies, a strategy frequented in the region since 40,000 BCE.