Identification and evaluation of post-depositional mechanical traces on quartz assemblages. An experimental investigation

The evaluation of the state of preservation of archaeological lithic artefacts is the first step before starting a functional analysis. If lithic artefacts show a variety of damage from many contact materials, they are also subject to modifications from “natural” causes, such as water transport, soil abrasion, trampling etc. Studying alteration features gives us information to reconstruct the life story of lithic tools after their abandonment and helps us to better understand formation processes of archaeological sites. Prolonged movements under soil could be responsible for post depositional surface modifications that sometimes look like wear traces due to use. These alterations could have chemical and/or mechanical origin. Recognizing them is of paramount importance especially on quartz assemblages where usewear analysis requires a long and complex methodology that differs to a certain extent from the protocol applied to other artefacts produced with different raw materials such as flint, obsidian etc … For this reason, the use wear and post-depositional surface modification analysis on quartz and quartzite tools have been undertaken by very few researchers until now. The aim of this paper is to increase the experimental quartz field through the attempt to recreate mechanical contact between quartz flakes and sediments such as in archaeological contexts. Through the use of a Polishing Machine MECAPOL P320 equipped with two counter-rotating platens, it was possible to wear out two experimental flakes in a sandy quartzite sediment coming from Sai Island (Sudan) for 240 h. After the experimentation, the two flakes showed no edge damage or other visible modifications, but some micro traces on the crystal surface were present. Although long and demanding, this step sharply reduces possible errors in the recognition of use-wear during the experimental stage. The micro traces definable as abrasions on some parts of the crystal surface, have been analysed by a metallographic microscope and compared with the same crystal portion taken before the experiment. Even if it is difficult to reconstruct post depositional processes in a laboratory, this attempt shows that the contact between sediment and stone tools in a continuous movement can randomly abrade the crystal surface.