Chemical Alteration of Lithic Artefacts: an Experimental Case Study on the effect of Guano on Stone Flakes and Its Contextualization in the Archaeological Assemblage of Azokh Cave (Southern Caucasus)

Abstract

Chemical weathering of archaeological material is well known; however, while there have been a number of experimental studies demonstrating different types and degrees of chemical alteration on faunal remains, little attention has been paid to stone tools, and the few studies that exist relate mainly to siliceous materials and water‐induced chemical alteration. Azokh Cave, located in the South Caucasus, contains a Middle Pleistocene to Holocene infill, and detailed macro‐ and microscopic examination of the lithic assemblages recovered there indicates potential chemical weathering of the stone artefacts. The cave is also currently home to one of the largest bat colonies in the region, and their guano forms a significant component of the infill of the inner galleries. Based on these two factors, an experimental pilot study was set up to artificially chemically alter a range of stone flakes produced specifically for the task, in order to determine the nature and likely cause of weathering. The experimental flakes, produced from different raw materials, were buried in fresh bat guano for up to two years. The results reported herein demonstrate that in a relatively short time, the highly acidic composition of bat guano strongly affects calcium‐bearing rocks (e.g., limestone, basalt) altering their entire surface. Similar comparisons may be made with chemical alteration evident on archaeological lithics from Azokh Cave, suggesting that bat guano has played a significant role in diagenetic alteration.

A late Middle Pleistocene Middle Stone Age sequence identified at Wadi Lazalim in southern Tunisia

“…Nevertheless, the effects of weathering on lithic artefacts are not easy to predict. They vary by both artefact mineralogy and environmental conditions, including the geochemical properties of the sedimentary deposits 18 – 21 , the combination of which can result in highly selective and differentiated modes of attack during the taphonomic processes. The basic assumption that fine grained flint resist weathering better than coarse grained one 20 , and the recognition that most of the strongly weathered artefacts (Group 1) are made on medium-coarse grained flint, whereas fine and very fine grained flint artefacts show a good preservation of edges and ridges, lighter surface alteration and little to no patination (Groups 2 and 3), significantly narrows down the seeming heterogeneity and suggests the likelihood that the observed differential alteration largely depended on raw material type.…”

Rock Features and Alteration of Stone Materials Used for the Built Environment: A Review of Recent Publications on Ageing Tests

“…Some studies have also found etching of calcite crystals resulting from limestone heating [22]. Experiments with burying in guano, which can have saline and acid effects, have described reactivity of lithologies with calcium phases in limestone and basalt [9]. According to Deng et al [36], clay minerals can react with acid solutions, enhancing the microporosity and susceptibility to freeze-thaw of sandstones.…”

A closer look at an eroded dune landscape: first functional insights into the Federmessergruppen site of Lommel-Maatheide

“…Asryan et al 2014;Burroni et al 2002;Michel et al 2019;Newcomer 1976;Sala 1986;Thiébaut 2007). Chemical processes, such as those related to soil chemistry or weathering, may alter or obliterate wear traces, as these processes may lead to surface patination, gloss or polish formation (Asryan et al 2017;Plisson and Mauger 1988;Rottländer 1975). Also, thermal modifications, being the result of (unintentional) exposure to heat or freeze-thaw actions, have to be taken into account when evaluating post-depositional wear and other wear (e.g.…”