Bending initiation - Museum of Stone Tools

Stone Tool Analysis: Bending initiation

Bend initiations are formed when the force applied to the edge of the core causes the edge to bend until it breaks. This starts the crack which then runs down the face of the core.

A Hertzian cone is not formed in bending initiations, and diagnostic features of conchoidal fracture—such as eraillures, umboes, and points of force application—are not found on bend-initiated flakes. Bend-initiated flakes lack a true bulb of force, although a subtle bulb-like swelling can occasionally be present. The mechanics of how this swelling forms in the absence of a Hertzian cone is poorly understood, but the swelling is likely caused by a reorientation of the propagating crack to the core face similar to that seen in conchoidally-initiated flakes.

Bend-initiated fractures can also occur when the maximum stress caused by the blow is located well-in from the edge of the stone, and a crack initiates in that stress zone. This sort of core failure normally occurs on thin stones such as tabular material or during biface manufacture. Experienced stoneworkers prevent this by firmly supporting the thin core when force is applied, thus restricting the build-up of stress to the intended location on the core edge. Flakes and blades often break laterally by bending as they detach from the core because of a similar build-up of a zone of stress within the detaching flake.

Soft hammers—antler, bone, wood, or soft stone—were usually chosen when the stoneworker delivered percussion or pressure force directly onto the edge of the core. The softer material allows the stone to ‘bite’ into the indentor, prolonging the contact time and slowly increasing the force loaded onto the platform. Since bending initiations are more likely to occur under these conditions, they are most common with soft-hammer percussion flaking. However, bending initiations can occasionally occur when hard-hammers are used, and soft hammers can also result in conchoidal initiations, so the correlation is not absolute. Bend initiations can also occur in pressure flaking.

Bend initiations are common at a micro-scale when the edge of a stone tool is damaged during use, detaching tiny flakes. Use-wear analysts study the initiation type of these microscopic scars and compare these to the wear on experimental tools of known use. Bend-initiated use-wear scars are more common than conchoidally-initiated scars because of the way the edge of the tool is tweaked during use. The orientation of these tiny scars gives insights into the tool-using gesture, such as the direction it was drawn across the material being worked. Bend-initiated scars are commonly produced in scraping, adzing, and sawing wood and bone. Bend initiations are also common in high-velocity impact fractures seen on stone weapon armatures.

Platform Lip

A bend initiation creates a lip on the flake where the platform edge is wrenched away from the core edge. The lip is usually excurvate in plan view, extending the width of the platform, and often curves outward slightly in profile, towards the flake’s ventral surface. Since a bend-initiated flake lacks a Hertzian cone, the platform will lack a ring crack or umbo, and an eraillure will not be present on the flake’s ventral surface. It is important not to confuse an umbo with a platform lip.