Stone Tool Analysis: Conchoidal initiation
Conchoidal initiations are caused when force is applied away from the edge of the core. This causes a cone-shaped crack to form, called a Hertzian cone. If the cone is initiated near the edge of the core, and the angle between the struck platform and the exterior face of the core is less than about 90 degrees, the crack induced by Hertzian cone can change shape and run down the core face.
Hard hammers are typically used to apply force away from the edge of the core. Hard hammers concentrate the force in one small spot, initiating the tiny circular crack that grows into the Hertzian cone. Conchoidal initiations are most common with the used of hard hammers, but can also occur with soft hammers if the blow is struck away from the edge of the core and the force is sufficiently concentrated to cause a tiny circular crack to start. Conchoidal initiations are typical of pressure flaking because of the way the force is concentrated by the small end of the pressure tool. Conchoidal initiations also occur when soft punches are used in indirect percussion.
Point of Force Application
Flakes are initiated conchoidally through the formation of a Hertzian cone. In Hertzian cone formation, 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 tiny circular crack is sometimes visible in fine-grained stone, and is referred to as a ‘ring crack’. Often only half of the circle is visible where it is exposed when the flake detaches from the core. This semi-circular feature is referred to as the ‘umbo’.
The presence of the ring crack or umbo defines the PFA. This attribute is particularly important because certain metrical attributes are measured in reference to it. For instance, the distance from the PFA to the platform edge records how far back from the core edge the stoneworker delivered the blow, which in turn provides information on technique and precision. Bend-initiated flakes do not have a Hertzian cone, and they therefore do not have ring cracks or umboes. By convention, the PFA on bend-initiated flakes is usually taken as the centre of the platform edge.
Hertzian Cone
If the blow is struck near the edge of the core, the radiating cone-shaped force encounters the stone’s outside or ‘free’ face; this causes the crack to change shape and run down the core’s face. The actual Hertzian cone is normally very short because the reconfiguration of the crack to the core face occurs so quickly. The cone is sometimes visible without magnification, and the ring crack and umbo at the top of the cone are often seen in fine-grained stones like chert, flint, and obsidian.
Bulb of Force
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.
Umbo
The presence of the ring crack or umbo define the PFA. An umbo is sometimes confused with the platform lip seen in bending initiations, but an umbo’s curvature in plan view is smaller in diameter and more pronounced than a platform lip. Also, the profile of an umbo is nearly vertical relative to the platform surface, whereas a platform lip curves outward slightly towards the flake’s ventral surface. Umbos are diagnostic of conchoidally-initiated flakes, although not all conchoidal initiations will display a recognisable umbo.