Use Wear:
Ground Stones - Quartzite
Use Wear Type - Fatigue
Fatigue wear occurs over time where continuing load and friction, such as that created by grinding, weaken the surface. When there are pressures on contact points of surfaces or stress of movements against the contacting surfaces when working the material, the highest surface elevations bear the stress. If it is more than what that surface can bear there is collapse and crushing of the elevations. The observable results include micro-factures such as scars, cracks and step fractures on the quartz crystals and pits on the surface. This type of damage will reflect the light and usually appear as irregular angular shapes. The back-and-forth pressure and force of the madit against the mațhan can also cause quartz crystals to crack internally. Figure 1 provides an example of fatigue wear on an archaeological specimen. The back-and-forth downward pressure of this madit against the mațhan has caused the highest tips of the asperities to come under load pressure, breaking off the tips which then reflect light. Internal fractures are showing as cracks within the crystals.
Another example of an internal fracture (crack) is noted in Figure 2. This microscopic image of madit SN 0704 Ona Adi, Late Aksumite, is from an unused section of the rock where the tool had broken. The internal fracture may have occurred as a result of the pressure applied that originally broke this stone, perhaps when resharpening, or as a result of damage incurred during excavation from digging tools.
Impact fractures are a type of fatigue wear that often occur instantly rather than over a period of time. When an object comes into contact with another object, the force, or impact, can create damage to one or both surfaces. In the case of grinding stones from Tigrai, impact fractures can be the result of resharpening a tool’s surface, where a hammer or hammer stone comes in contact with the grinding stone with some force. In the case of hammering to make the surface coarse, the hammer would fall heavily onto the surface (Figure 3). If the goal was to reduce just the tops of the asperities to create a finer texture or flatten the surface, a less powerful stroke would be used (Figure 4). The impact of the hammer fractures and crushes the quartz crystals where the surface of the hammer comes in contact with the grinding stone. If the hammer stroke is heavy, many crystals can be crushed within the striking zone and under a microscope this can appear as a lighter area as the group of crystal fractures reflect the light and take on a frosted appearance (Figure 3). If the hammer blow is gentler, only the highest asperities will fracture, and the result is individual frosted crystals and appear as points of reflected light on the surface, usually in pattern that resembles the rounded end of a hammer (Figure 4).
Figure 4a. Expert Experimental Madit after 4.75 hours of grinding – impact fracture Image (a) the impact can be seen as the lighter area, especially in the centre
Figure 4b. Expert Experimental Madit after 4.75 hours of grinding – impact fracture Image (b) the impact area fills most of the circle, and the arrows show where the tips of the asperities have been fractured by a less forceful blow than the images above (Figure 3); arrows point to asperity fractures that appear roughly within the shape of a circle. Some asperity fractures are outside the circle to the right but would have been from the same hammer blow, as it caught the highest asperities
Figure 5. SN 0066 Ona Adi Madit B1:2:5 Post Aksumite , fatigue wear Impact fracture (circled) – light reflecting off the crushed crystals (examples indicated with arrows) Dinolite Illumination (~40X)
Figure 6. Ethnographic Nigisti12 Madit – fatigue wear Recent impact fracture in center, the small reflections of light at the tops of the asperities of multiple quartz crystals form a rough circular shape (within the circle) (70X)
Figure 7. Ethnographic Nigisti11 Madit – fatigue wear Recent impact fracture especially noted in upper right (circled) (70X)
Figure 8. Ethnographic Gebreselassie8 Mațhan, home of Heleka Gebreselassie and Waizaro Ziad,– fatigue wear Recently resurfaced (resurfacing communicated by consultant) impact fractures indicated by reflected light off the broken crystal asperities over the surface (70X)
Figure 9. Ethnographic Gebreselassie10 Mațhan – fatigue wear Recently resurfaced (resurfacing communicated by consultant) impact fractures indicated by reflected light off the broken crystal asperities over the surface (70X)
The microscopic image from SN 0066, Ona Adi madit dated to the Post Aksumite (Figure 5) reveals another impact, with light reflecting from the crushed quartz crystals. Often these impacts are situated in a circular shape on the surface, indicative of the rounded hammer or hammer stone used to strike the surface. Hammer stones or modern metal hammers are used when maintenance is being done to resharpen the surface to a coarse texture or to crush and flatten the asperities when a smoother surface is preferred.
Ethnographic examples of microscopic images of impact fractures are presented in Figures 6 - 9. Consultant Waizaro Nigisti explained that she had recently resurfaced this madit through hammering to prepare a coarser surface used for grinding large size cereal grains. In Figure 6 the impact fractures on the tips of asperities form a circular shape (note the small reflective areas on the tops of the quartz crystals in the center of the image). In Figure 7 the impact fracture is noted in the upper right with the various irregular shapes of reflected light at the top of the crystal asperities.