Workshop 2  - Flake Initiations and Propagation

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In Workshop 1 we looked at 3D models of cores and flakes, viewing them through the eyes of a lithic analyst. 

In Workshop 2 we will look at 3D models of cores and flakes that display the features of conchoidal, bending, and wedging initiations.  We will also look at umbos, eraillures, and features of flake propagation.

These various technical terms are introduced in the Analysis pages.

If you are having trouble locating the attributes mentioned in the descriptions, open up the Annotations in the model's sidebar.  Pins indicating the relevant features can be found there.

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    Conchoidal initiations

    A conchoidal initiation results when a blow to the stone starts a Hertzian cone crack.

    The location of the blow is marked by a discrete Point of Force Application (PFA), often indicated by the roughly semi-circular umbo.  The umbo is the top of the Hertizan cone.

    The PFA is at the apex of the swollen area at the proximal (platform) end of the flake, called the bulb of force or bulb of percussion.  The bulb of force is formed when the crack reorients from the initial cone-shaped crack to a flatter shape, to travel down the face of the core.

    This model has a clearly defined umbo.  The size of this umbo is truly exceptional; usually they are much smaller than this, and a clearly-defined umbo is often absent altogether.  Note how the umbo projects out in a semicircular shape from the platform end of the flake.

    An umbo is diagnostic of conchoidal initiation—a bend-initiated flake will not have an umbo.  Other models of stone flakes with well-defined umbos can be found here, here, and here.

    Eraillure scars and flakes

    An eraillure is a spin-off flakelet that forms when a hackle or lance travels sideways across the bulb of percussion.  Eraillure formation is not controlled by the stoneworker—they are created spontaneously as a result of the physics controlling fracture.  Eraillures are diagnostic of conchoidal initiations, but they do not always form.

    The eraillure flake resembles a fish scale and usually detaches from the core along with the parent flake.  Eraillure scars and flakes occur in various shapes and sizes, and more than one can be formed on a flake, usually on opposite sites of the bulb of force.  In that case, one eraillure scar may intrude into the other eraillure scar.

    The artefact in this model has a clearly-defined eraillure scar.  Have a look at this artefact to view two eraillure scars on one bulb of force.

    Sometimes the eraillure flake does not detach and sticks to the negative scar on the core.

    This artefact has two examples of eraillure flakes stuck to negative scars; refer to the annotations if you have trouble finding them.

    Note that while an eraillure may sometimes remain attached to a negative scar on a core, it will never remain attached to the ventral surface of the flake.

    Bending initiations

    A bending initiation occurs when the blow causes the platform to ‘tear away’ from the edge of the stone, initiating the flake down the core face.

    A Hertzian cone is not formed in a bending initiation, so an umbo will not be present on the flake, and a discrete PFA is normally not identifiable.

    The ventral platform edge of a bend-initiated flake appears as a broad curve and lacks the point-features characteristic of conchoidal initiation. A slight lip is sometimes present, marking the transition from the platform surface to the flake's ventral surface.

    A true bulb of force (a featured induced by a Hertzian cone) is not present on a bend-initiated flake.  However, a bulb-like swelling may sometimes be present. Bend-initiated flakes do not have eraillure scars.

    The flint blade in this model was detached by bending and was probably struck by indirect percussion using a soft punch.  Bending initiations are often associated with soft flaking tools, such as antler, bone, or soft stone, although bend-initiated flakes can sometimes be produced by harder flaking tools, particularly soft hammerstones.  The late stages of bifacial thinning was often accomplished by a soft hammer, and this and this biface thinning flakes were initiated by bending.

    The broad curve of the ventral platform edge appears in reverse on the scar it leaves behind.  A bend-initiated flake scar can be clearly seen in this model.


    A flintknapper controls downward and outward force as a way of manipulating the size and shape of the resulting flake.  The downward and outward force vectors operate together in detaching the flake.

    Downward force is delivered down the length of the flake.  In flintknapping vernacular, this is the force that ‘pushes’ the crack through the stone.  In fracture mechanics terms, this is the compression force.

    Outward force is the vector directed away from the core.  Outward force ‘tears’ the flake away from the core.  In fracture mechanics terms, this is the tensile stress or force.

    Downward and outward forces work against each other.  First one vector will gain the upper hand until pulled back by the other vector.

    This counterbalancing is preserved in stone as wave-like undulations that radiate away from the point where the force was applied.  Well-balanced forces result in relatively smooth surfaces, and poorly balanced forces result in prominent undulations.

    Downward and outward forces tend to be more poorly balanced as the force decreases, just prior to the crack terminating—and undulations tend to be most prominent near the distal end of a flake or flake scar (although this is not always the case).

    The artefact in this model is a flint axe made by the notorious forger and flintknapper Edward Simpson ('Flint Jack') in about 1863.  Eyewitness accounts say that Flint Jack used an iron hammer or rod as his flintknapping tool.  Hard hammers like this can result in exaggerated undulations, particularly in the hands of less-than-proficient flintknappers who have yet to master the techniques for balancing inward and outward force vectors.

    It would appear that Flint Jack tended to strike quite steeply into the stone which, combined with the use of the hard iron hammer, resulted in relatively unbalanced forces and exaggerated or 'stacked' undulations.  Stacked undulations are clearly visible on many of the scars on this artefact.

    Undulations can be seen on many of the 3D models, but these artefacts—here, here, here, and here—capture much of the range of variation on both cores and flakes, from nearly non-existent to quite prominent.  As you explore the undulations on flake scars, note how they radiate from the core’s edge and show the direction the crack propagated.

    Now have a look at the flake made by the bonobo Kanzi.  Stone-tool researchers attempted to teach Kanzi how to make stone tools as part of their research into Oldowan technology.

    The ventral surface of Kanzi’s flake is marked by stacked undulations starting right at the PFA (point of force application). Kanzi used far more force than necessary to detach this flake, and the force vectors were not well-balanced.

    This indicates that Kanzi was a novice stoneworker with a poor understanding of the balance of forces for initiating and maintaining fracture.  The flake also split down the middle as the result of a siret fracture.

    Wedging initiations

    A wedging initiation occurs when a microscopic particle is driven into a micro-crack, wedging it open and initiating the fracture.  In flintknapping, this tends to occur when a stone is crushed or struck with a super-hard blow.  The strength of the forces involved, relative to the size of the core, results in highly unstable crack path and exceptionally prominent and closely-stacked undulations.

    Wedging initiations are typically associated with bipolar flaking—when a stone is smashed while resting on an anvil—but they frequently occur when flaking materials that require very strong blows to initiate fracture, such as volcanic rocks.  Wedging initiations are also common in impact fractures associated with tool use.

    A flake initiated by wedging lacks a well-defined bulb of force and usually has a very flat ventral surface.  The corresponding flake scar is also flat.  Because of this, it can be difficult to determine which products of bipolar flaking are the flakes and which are the cores.  Many archaeologists avoid this problem by calling them ‘bipolar artefacts'.

    On this model, the large scar has the stacked undulations typical for wedge-initiated flakes.  The smaller scar with stacked undulations on the opposite face was also initiated by wedging, perhaps at the same time as the larger scar.

    This model is of a chert projectile point from Texas with an impact fracture at distal end.  Note the stacked undulations and relatively flat profile of the impact scars—these features suggest wedging initiations.