Stone Tool Analysis: Bulb of force

The bulb of force is a feature of conchoidally-initiated flakes that is created when the propagating crack transitions from a cone shape to a planar shape and continues down the face of the core.  


Flake, Biddenham AIA


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Macroblade Core

Thumbnail scraper, Australia

Thumbnail Scraper

A Hertzian cone is created when the area in direct contact with the indentor—usually less than 2-3 mm in diameter—is depressed until the stone fails and a crack starts.  The crack follows the cone-shaped boundary of the molecules compressed under the indentor, but only does so as long as the cone is not displaced from the stone.  However, as the indentor moves obliquely away from the point where force was applied, is displaces the cone laterally and redirects the forces involved in forming it.  New fields of stress are formed which become aligned in the direction of displacement towards the core’s free face.  The realignment of the part of the perimeter of the crack at the bottom edge of the cone creates the pronounced swelling archaeologists refer to as the bulb of force. 

The size and prominence of the bulb is related to the degree of realignment that must occur, which is in turn influence by the type of indentor, the shape of the core face, the diameter of the Hertzian cone, the orientation of the cone relative to the core face, and the amount of applied force.  A bulb of force is characteristic of flakes initiated by percussion flaking—the attribute is sometimes called a bulb of percussion—but bulbs are also found on conchoidally-initiated pressure flakes.  Bend-initiated flakes do not have a true bulb of force because they lack a Hertzian cone, although some may have a diffuse bulb-like swelling at the proximal end.