This chert bipolar core from Allen County, Indiana, is made from Upper Mercer chert. The precise age of this artefact is unknown, but Upper Mercer chert is mostly associated with Early Archaic assemblages in this part of Indiana, ca. 8000-10,000 BP.
The bipolar core in this model is relatively thick has a wedging-initiated scar that removed most of one face of the stone. The relatively flat profile and stacked compression rings is typical of a wedging scar produced by the bipolar technique.
Bipolar flaking is a common way that cores were reduced into usable flakes. In bipolar flaking, the core is braced on a hard stone anvil surface and a blow is struck onto the top, splitting it into sharp-edged flakes and fragments. The technique is similar to using a hammer to crack a nut. Bipolar flaking was done by the earliest hominins millions of years ago, and it was still done in the recent past in New Guinea, North America, Africa, and Australia. It is widespread across many times and places world-wide.
Bipolar cores (or, in French, pièces esquillées) are split by a crack initiated by the hammerstone or from the anvil support. Because the core is constrained from moving when struck, the flake is frequently initiated by a process called ‘wedging’ which results in a flat fracture surface without a cone of force. Various features like hackles and compression rings (which tend to be very pronounced in wedging initiations) radiate away from the point of force application. Most bipolar cores were struck many times, creating a large number of scars, some of them initiated by wedging and others initiated conchoidally. Bipolar cores are usually bidirectional, with flakes initiated from both ends of the core face. Since the blows tend to be delivered near the middle of the platform, flakes may initiate down either side of the core. Also, the core can split into pieces that run its full length; this was likely one desirable aspect of the technique, because a stone can be split into nearly equal-length pieces. Both the platform struck with the hammerstone and the end supported by the anvil tend to become crushed, usually to both faces of the platform (bifacial flaking and crushing), and attrition often creates a U-shaped platform edge when viewed straight-on. It is sometimes difficult to determine which is the core and which is the flake, so many archaeologists call them ‘bipolar artefacts’. Bipolar cores were often rotated 90 degrees during reduction, establishing a new pair of platforms at right-angles to the first pair, and a roughly rectangular bifacial core. Bipolar flaking is an effective way to maximise the use of a desirable piece of stone and to exploit stone that comes in very small pieces or pebbles. It is also a low-skill technique for quickly producing expedient, usable tools. Waste flakes from the freehand percussion technique were commonly reduced as bipolar cores, and sometimes tools produced by other techniques were recycled as bipolar cores at the end of their use-lives.
Artefacts that look like bipolar cores can also be produced when a stone (usually a flake) is used as a wedge to split organic materials like wood or bone. In this case, the top edge of the core is struck with a hammer to drive it into the crack and wedge it apart. The hammer blows to the top can detach flakes in a similar way to that seen on bipolar cores. If the wedge is reversed, a bidirectional core is produced. Some archaeologists suggest that wedges are distinctive because the flake scars are non-invasive and they were clearly made on flake blanks. However, experiments show that invasive scars are frequently produced on stone wedges, and use-wear analysis is the best way to determine whether bipolar cores in an assemblage were for making flakes or were tools used as wedges.