Wedging initiation - Museum of Stone Tools

Stone Tool Analysis: Wedging initiation

Rhyolite Flake, wedging initiation, Australia

A wedging initiation occurs when a microscopic particle is driven into a pre-existing micro-crack in the stone. This wedges open the micro-crack and initiates the main fracture, which then runs down the face of the core.

Wedging initiations tend to occur when a stone is struck with a super-hard blow, or when the core is supported on a hard anvil. The strength of the forces involved, relative to the size of the core—or the compression effects of a core resting on a hard anvil—result in a highly unstable crack path and exceptionally prominent and closely-spaced undulations. A wedge-initiated scar is flat rather than convex, lacks a bulb of force, and has a ‘sheared’ appearance.

Wedging initiations are strongly associated with hard-hammer percussion. Wedging initiations rarely, if ever, occur in soft-hammer percussion or in pressure flaking. Wedge-initiated flake scars are a diagnostic feature of bipolar percussion because the core is secured on a hard anvil, preventing the force from readily dissipating and thereby magnifying the compression loading. Wedging initiations are common at quarry sites where large stones are broken into more manageable pieces, or where large flake blanks are produced by hard hammer percussion.

Wedging initiations occur on stone cores used to split wood by hammering them into a crack, and wedge-initiated high-velocity impact fractures are sometimes seen on weapon armatures made from fine-grained stone. Modern geologists and lapidary enthusiasts use steel rock hammers to split apart stones, frequently inducing wedging initiations through the combination of indentor hardness, striking near a stone’s centre, and the use of excessive force. Wedging initiations are deliberately produced by stonemasons splitting rock into pavers or facing stones using heavy steel hammers.

Face Shear

The proximal end of the wedging-initiated flake is often a sharp edge, and frequently crushed. The crack path in wedging initiations is often highly unstable and exceptionally prominent and closely-spaced undulations will radiate from the point of force initiation, or PFA. Linear hackles and splits are often present, also radiating out from the PFA. Eraillures, ring cracks, umboes, and platform lips are not present on wedge-initiated scars and flakes.