The mineralogy and structure of use-wear polish on chert

Abstract

Polished edges of archaeological stone tools are commonly investigated to obtain information on the tools’ uses in prehistory. Yet to this day, it remains unclear what exactly such polishes are and how they form. Answering these questions should allow the elaboration of new interpretative methods based on objective measurements. Two major competing hypotheses of polish formation have been proposed: abrasion and the formation of a thin amorphous film on the chert or flint surface. We employ reflectance infrared spectroscopy, a technique particularly sensitive to thin amorphous films, to investigate these two hypotheses. We found no added amorphous layer that would have formed upon friction against bone, antler, ivory or wood. Our observations suggest polish formation by abrasion, notwithstanding previous claims of added amorphous surface structures. This has implications for our understanding of the physical processes taking place during friction of chert and flint against different materials. Our results also open the possibility to propose new pathways for identifying different use-wear processes, based on the degree of abrasion.

The effect of worked material hardness on stone tool wear

“…are softer than stone and cannot, in theory, abrade the stone tools themselves. However, given that stone tools are, in fact, abraded by contact with softer materials [23][24][25], why does this happen, and which factors related to the target material play the most significant role? Our counterintuitive result suggests that the abrasion 'paradox' must be explained by either the presence of hard grits within the worked material, the presence at the interface of stone particles broken off the tool edge, or a combination of both [53].…”

“…As discussed in another paper related to this experimental setup [25], polish is formed by abrasion of the flint surface. Consequently, we expect that harder worked material would be more effective in creating polish.…”

“…To isolate the effect of material hardness while maintaining reasonable costs and effort, we opted for a tightly controlled protocol [30,31]. Flint bits (Baltic/morainic flint from Denmark with nano-crystalline non-oriented chalcedony texture [25]) were rubbed against dry bone, antler, and wood (beechwood and spruce) using a tribometer at room temperature. Five samples of each pair were produced to obtain a suitable number of samples for statistical study.…”

“…While some have claimed that this produces different traces on stone tools, why that might happen remains insufficiently explained. Given that it is now widely believed that abrasion is the main process by which stone tools are polished during use [23][24][25], it makes sense to test first the effect of hardness. Until now, systematic mechanical tests of archaeologically-relevant target materials have not been incorporated into lithic use-wear research.…”

Traceological analysis of lithics from the Camel Site, al-Jawf, Saudi Arabia: an experimental approach to identifying mineral processing activities using silcrete tools

“…Different theories have been postulated as to the nature of these alterations (adsorption theory vs dissolution theory), incidentally, however, as Rottländer (1975) pointed out, the difficulties in experimentally replicating these alterations and the lack of a unified terminology of 'patination', hamper understanding the origins of these alterations. Recently, Schmidt et al (2020) demonstrated using reflectance infrared microscopy that usewear polish on chert has its origins in a physical process (abrasion), making it possible to infer a similar origin for the formation of taphonomic surface modifications; nonetheless, additional research into the subject remains to be undertaken.…”