What can we learn from flakes?

Guidelines for reading flake attributes

Initiation: Fracture starts when an external load stresses the rock beyond its strength.

• How the load is distributed determines how quickly critical stress is achieved. Small platforms dictate high load per square inch. Hard hammers also restrict contact to small areas, while soft hammers deform to spread their load over a larger area.

• The separation of flake from parent rock is guided by the distribution of the impacting load. Hard impactors start with a small ring crack that connects to the core edge in a sweeping recurve that looks like a bird in flight, hence alar shaped. Soft impactors distribute the load so the separation looks like a compass arc, hence arcuate shaped.

• It is possible to force a perpendicular blow with such intensity that the initial cone is collapsed and split to cause a wedge detachment. Wedging presumably drives debris beneath the hammer to split the core in two.

• The vector direction of load application effects the way a crack starts. Loads perpendicular to the core surface cave a small cone into the surface, the same as a pebble pits a windshield. The remaining flake surface propagates from that initial Hertzian cone. If the load is applied at an angle to the edge, it may actually start a crack by bending a portion away from the core edge to cause a lip.

• A bulbous swelling just after the first crack formation is known as the bulb of force. Computer modeling predicts the most noticeable bulbs from impacts perpendicular to the core face. Knapping experience suggests that high rates of loading contribute to bulbar swelling, but an inclined blow can be expected to deliver a lower rate of loading. Gentle rates of loading, such as pressure or soft hammers can leave flakes nearly bulb free.

• Prominent bulbing can leave overhangs at the edge of a flake scar. Platform isolation can emphasize bulbing.

• Tool hardness effects local stress levels and can cause secondary fracture planes to develop. The effect is particularly noticeable with brittle materials like glass, where small tear lines, perpendicular to the flake face, are found around the edge of a flake. Tear lines point back to the point of impact, even when the platform is missing. Compression waves can also initiate cracks deep inside the rock when they encounter an inclusion. Soft knapping tools lessen the problem. Note that hardness of hammer is not so important as intensity of load.

• Offset of the load in regard to the face being flaked determines how deep a path the flake wants to follow. Soft baton blows on sharp edges tend to produce shallow, thin flakes. Thick flakes are usually caused by hard hammers impacting far from the core margin.

• Width of detachment from a core is usually a function of the area contacted by the knapping tool. Platforms can be used to isolate where the blow lands or soft tools can be used to distribute the load broadly.

• The strength of a platform greatly effects when critical stress is reached. Consequently, knappers often prepare platforms very carefully to control flake formation. Grinding strengthens a platform, isolation concentrates stress, and faceting controls contact placement. Battering serves to introduce microcracks that allow cracks to start at a lower minimum stress.

Travel: Fracture progresses as long as critical stress of the rock is exceeded.

• Bending stress in the parent rock promotes curved flake paths. Straight flakes indicate deliberate support of the parent rock to avoid bending.

• Stability of the fracture path rests somewhat on the depth of the fracture, but depends on the load being constant over time. Unsteady support or vibrating knapping tools can force the fracture path to undulate.

• When radial stress at one edge of a flake initiation penetrates to the opposite face, a perverse fracture may split a biface in two. Clovis knappers depended on the action to separate large platter-like bifaces into usable portions.

• Strength of the stone is often indicated by the texture of the fractured surface and tear lines that reveal when the rock does not separate easily. The stronger the rock, the more durable the hammer will have to be.

• A flake can only extend while tool and core are pushing against each other, so short flakes indicate loss of contact with the tool.

• Surface morphology is responsible for most of flake morphology. Flakes progress most readily when surface contours give the flake stiffness. Lacking ridges, a flake will naturally be circular. However, maintaining the load helps drive the flake forward, as in leverage pressure. Restraining the core against an outward blow also promotes spreading flake geometry.

• Ripples are caused by slight disturbances in the load, and arc radially away from the point of impact.

• A special sort of disturbance, called a Wallner wake, shows up in brittle materials like obsidian when the compression wave interacts with the developing fracture surface to leave short lines, sometime known as gull wings, that diverge from a central imperfection. The speed of fracture can be deduced by measuring the angle between the Wallner wake lines.

• Adding stiffness to the system promotes relatively flat flakes. Stiffness comes from added support, harder hammers, or faster blows. Increasing support of the core causes the core to behave as if it possessed greater mass.

• Shock introduced by impactor can manifest as concentric rings with low amplitude.

Termination: Fracture ceases when stress levels fall below the critical level.

• Feathered flakes indicate that contact was maintained between tool and core for the entire duration of the fracture. Hinge flakes indicate that contact was interrupted when the tool and core bounced away from each other. Step flakes may indicate that the blow lost force before the flake could finish, but too much outward force in relation to the forward push can cause a step.

• The termination of a flake often shows how the core was supported against the knapping load. Severe overshots occur when too much bending is present, but light overshots may be deliberately introduced by pinching the far edge to provide a soft anvil support. Overly severe support can cause flakes to undulate in increasingly severe waves as they reach the end of their travel. Ideally, a stable support and blow will allow the flake to feather as it finishes.

• Flake thickness impacts the evidence from flake termination. Thick flakes tend to be stable, but thin flakes are highly sensitive to external interruptions of the knapping load. Indirect percussion tends to take very thin, far-reaching flakes that waver as they terminate. The effect is caused by flake stiffness reflecting the core surface morphology.

• Hard mineral hammers tend to leave upturned terminations where flakes meet at the biface midline.