Use Wear:
Ground Stones - Quartzite
Use Wear Type - Adhesive
Adhesive wear can be observed when substances introduced between the surfaces of the upper and lower grinding stones go through a tribological process and become attached to one of the surfaces. Materials that come in contact with tools and become adhesives include substances such as clay and pigments (Figure 1), skin oils mixed with products and air particles such as dust and moisture and food oils (Figures 2-6). Heat caused by friction creates a tribological reaction adhering the oils, substances, and intermediate products to the surface grains. Strong adhesion can be explained by electron transfer between contacting surfaces as electrons are not bound by a rigid structure so can move from one body to another if the distance between the two bodies is small. Electrons can bond two solids, e.g., clays or oils to stone, despite different atomic structures.
One form of adhesive wear accumulates from the grinder’s hand pressed and rubbing against the madit handstone. The oils from the hands mix with the loose grit from the stone and some of the product being processed, as well as particles in the air such as dust and moisture droplets. Oil from the hands provides a lubrication and as the hand exerts pressure on the stone, the soft pliable skin works against the quartz crystals smoothing and rounding the asperities and rounding the edges of the crystals into the interstices (fatigue wear). No fracturing is evident, and the surface appears levelled and smooth all over as the oils mixed with the other substances eventually adhere to the stone. Through the heat of friction and tribochemical reactions, the resultant use-wear observable is a sheen over the surface of the stone where the hand was in contact. The sheen presents as a ‘washed over’ appearance on the surface. Within the sheen are embedded particles of processed product showing as flecks of white in Figures 2-6.
Figure 2. Ethnographic Brizaf5 Madit – adhesive hand wear Rounding of quartz crystals (examples indicated with arrows) (71X)
Figure 3. Ethnographic Brizaf4 Madit – adhesive hand wear Product embedded in sheen (small white specks) (70X)
Figure 4. Ethnographic Brizaf6 Madit – adhesive hand wear Sheen has dried out and is cracking (examples of cracks indicated by arrows) (69X)
Figure 5. Ethnographic Kashi8 Wedimadqos – adhesive hand wear Rounding of quartz crystals (arrows) and product embedded in sheen (examples in circles) (71X)
Figure 6. SN 0024 Ona Adi Wedimadqos A1:2:3 Post Aksumite – adhesive, possible hand wear Surface sheen and rounded crystals on surface (~40X)
Figure 7. SN 3915 Mezber Wedimadqos C1:45:87 mixed phase context – adhesive wear Small white reflective dots/specks are likely small crystals grains broken away through abrasion and then adhering to the surface (shown by arrows), the larger reflective areas are asperity tips broken away (circled) (39X)
When stone tool surfaces move, for example during processing of grains, animal skins, or other substances, chemical bonds are broken, and energy is released in the form of frictional heat. Surface wear causing deformation and added friction create conditions for adhesive wear. The friction and heat can also loosen crystal grains from the stone which remain between surfaces and can float and/or eventually adhere to one of the interacting surfaces. In the microscopic images taken from the artifacts, these are observable as very minute reflective ‘dots’ or ‘specks’ on the surface (Figure 7).