Stone Tool Analysis: Propagation
Once a crack is initiated, it advances through the stone subject to compression force down the length of the flake and shearing force tearing the flake away from the core. These forces control the way the crack expands, or ‘propagates’, through the stone. Attributes of flake propagation can be seen on the ventral surfaces of flakes and their corresponding negative scars on cores.
Undulations
Compression force operates down the length of the flake, stiffening it, and shearing force separates the flake from the core; these two forces operate at right angles to each other, and when approximately balanced, the crack travels parallel to the core face. The balance shifts back-and-forth as the crack grows however, favouring one force until pulled back by the other force, and this results in an undulating crack path.
We see this as concentric ripple-like features that are concave in the direction of flake propagation, like waves radiating from a stone thrown into a pond. As the crack continues to grow and the energy available begins to drop, the crack becomes more unstable, and the undulations tend to become more pronounced. This is why undulations are usually more pronounced towards the distal end of a flake or flake scar and are rarely visible on a bulb of force. Undulations can be very pronounced and dramatic on unstable cracks, as is often seen in wedging-initiated flakes.
Hackles
Hackles are linear features found at the edge of a flake’s ventral surface or at the edges of flake scars. They radiate outward from the point of force application. Hackles are also known as striations, lances, or marginal fissures in the archaeological literature, and forensic scientists and fractographic engineers refer to them as hachures.
The primary crack in flake-making actually progresses through the stone in minute ‘tongues’ emanating from the fracture front. These tongues join up as the fracture front progresses. Towards the centre of the front, where the tensile stress is closest to a right angles to the fracture surface, the space between the tongues is microscopic and the fracture plane appears continuous and smooth to the naked eye. At the edges of the crack, however, the tensile stress is more oblique, and the tongues do not join up as cleanly because of greater displacement between them.
Sometimes the tongues travel short distances parallel to each other after the main fracture front passes. When these displaced tongues are joined, a needle-like sliver is either broken away as knapping dust, or part of it can remain attached to the scar on the core or the ventral surface of the flake. We see these partly-attached tongues as hackles. They are usually less than a millimetre long (as measured in the direction of propagation), but they can be many millimetres wide. An eraillure is a special type of hackle that propagates across the bulb of force on conchoidally-initiated flakes.
Eraillure
Once a crack is initiated, it advances through the stone subject to compression force down the length of the flake and shearing force tearing the flake away from the core. These forces control the way the crack expands, or ‘propagates’, through the stone. Attributes of flake propagation can be seen on the ventral surfaces of flakes and their corresponding negative scars on cores.
Spontaneously-formed flakelets like eraillures are sometimes called ‘spin-off’ flakes. The primary crack in flake-making actually progresses through the stone in minute ‘tongues’ (hackles or lances) emanating from the fracture front. These tongues join up as the fracture front progresses. Towards the centre of the fracture front, where the tensile stress is closest to a right angles to the fracture surface, the space between the tongues is microscopic and the fracture plane appears smooth and continuous to the naked eye. At the edges of the crack, however, the tensile stress is more oblique, and the tongues do not join up as cleanly because of greater displacement between them, and this is how hackles are created. One place where this oblique stress occurs is around the periphery of the bulb of force. The bulb presents a zone of high mass ideal for flake propagation, and an eraillure is formed when a hackle propagates across it.
Usually only one eraillure is formed when a flake is detached, but sometimes two or more may overlap. The eraillure flake either pops off along with the flake or remains stuck to the negative flake scar on the core. Eraillure flakes were selected in Mesoamerica for use as beads, or were used as eye insets in sculptures, but mostly eraillures are part of the discarded debris from toolmaking. Eraillures are only formed in conchoidal initiations and are common on conchoidal flakes struck by hard-hammer percussion or detached by pressure, although not all conchoidally-initiated flakes have eraillure scars. Eraillures never occur on bend-initiated flakes.
Siret Fracture
A siret fracture, named after the famous French archaeologist Louis Siret, is a secondary crack that splits the flake as it detaches from the core.
Due to the speed of the propagating crack, the indentor is usually still in contact with the flake’s platform before it exits the core. As the indentor moves obliquely away from the core edge, pulling the flake away with it, a zone of transverse stress can develop across the flake, forcing a bending crack to open along the flake’s length. The crack initiates at the flake’s dorsal surface and travels towards the ventral surface. Since the flake is only partly formed at this point, the attached portion ahead of the primary fracture front inhibits the bending at the distal end of the forming flake. The interaction of the bending force with the propagating crack causes the primary fracture front to divide and propagate in two mostly independent parts.
A ridge or arris starts at the point where this division occurs, resulting in the emergence of a distinctive ‘siret arris’ or ‘split mark’ towards the distal end of the negative flake scar. Siret fractures are more frequent on flakes struck behind a concavity on the core face because this morphology enhances the bending stress operating on the flake as it forms. Siret fractures usually occur in hard-hammer percussion flaking but they sometimes occur in soft-hammer percussion and indirect percussion. They rarely occur in pressure flaking.
Flakes split by siret fractures are generally considered evidence of stone-flaking mistakes by archaeologists, although siret-fractured flakes were used in prehistory as naturally-backed knives; the right-angle siret fracture surface protected the fingers from cuts. An example was recovered from the Lower Palaeolithic site of Marathousa 1 in Greece, dating to ca. 400,000-500,000 BP. The split siret surface can also serve as a platform for striking elongated blanks from the edges of the flake, as seen in Australian microlith assemblages.
