Use Wear:
Ground Stones - Quartzite
Interpretations - Smooth vs. Coarse
Comparing Coarse and Smooth Surfaces
As documented in Nixon-Darcus (2014), in northeastern Tigrai, there is a cultural preference for coarse textured surfaces to grind large cereal grains such as wheat, barley and sorghum while smooth surfaces are preferred for smaller cereal grains such as t’ef and finger millet. The images that follow represent both types of textures and the differences in use-wear are described. Coarse and smooth areas can occur on the same surface, both in modern and artifact samples.
Microscopic Examples of Coarse (Rough) Textured Surfaces
Especially evident on coarse (rough) surfaces are topographic differences with deeper interstices (Figure 1) and/or protruding crystals (Figure 2). Wear on these surfaces began at the highest tips of the asperities. If there was an intermediate substance that is not abrasive, the crystals could become rounded through wear (Figure 2). It is important to differentiate when the crystals have been rounded through natural process (such as long-term metamorphosis) versus through cultural practices. In the case of Figure 2, the protruding grains are rounded along the tops and sides of the crystal, unlike the lower topographic crystals that while showing some rounding through natural processes, have not been exposed to the cultural practices that round the sides of the crystals.
The load of the madit weight against the mațhan creates internal fracturing (cracks) in crystals due to the pressure and frictional heat. The downward and tangential forces of grinding can also lead to stone-on-stone abrasion where the highest tips of the asperities receive the greatest amount of contact. Eventually the tips display fatigue wear, with those tips fracturing away though shearing (Figure 3). In microscopic images these fractured asperity tips appear as irregularly shaped reflected light off individual crystals. Tips of asperities can also be crushed and fractured through hammering applied to roughen the surface during manufacturing or ongoing maintenance. These types of fractures tend to appear in a cluster of crystals where the hammer or hammer stone has made forceful contact with an area of the surface and the multiple crystals fractured create a ‘brighter’ zone. Despite this type of destructive wear, the crystal grains maintain their borders and are distinct.
Coarse surfaces are worn smooth through successive grinding sessions. While the tops of the asperities fracture and become worn flat and the topography starts to level off, the borders of the crystals remain distinct. Figure 3 is an example of a surface with worn crystal grains and an early stage of levelling is apparent in the topography. Some crystals have become rounded, but many are still angular in shape with obvious borders
Microscopic Examples of Smooth Surfaces
Smooth surfaces in this data set exhibit a different type of wear pattern than coarse surfaces. The topographic differences are leveled creating a smooth texture (Figure 4 and 5). A levelling process is occurring as the friction between the upper and lower grinding stones, with and without intermediate substances, eventually reduces the topographic highs through fatigue and abrasive wear. Eventually, through continued grinding, the quartz crystals become less individually distinct and appear to ‘meld’ or blend together. The surface takes on a smoothed, ‘washed out’ appearance. Because quartz is clear or opaque and each crystal in a rock is so similar in ‘colour’, when levelling brings all the crystals to the same plane it is difficult to see the crystal borders (Figure 4). Oils, products processed on the grinding stone, stone particles, and air born dusts fill the small remaining interstices and impressions in the crystals through the many hours of use. This process is occurring due to the heat and friction (tribology) developing on the surface as a result of downward pressure and forward push of the madit handstone against the mațhan.
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