Wednesday, August 10, 2011

Bipolar Lithic Technology at Post-Classic Calixtlahuaca?!

By Dr. Bradford Andrews

P1: Bipolar technique

Analysis of the Calixtlahuaca stone tools has revealed that bipolar percussion was used to make stone tools in the city. So, what exactly is the bipolar technique (BP)? BP involves striking the top of a piece flakable stone placed on an anvil stone (see picture P1). As such, it enables the knapper to transmit force from both “poles” of a “focal” piece (Flenniken 1981). It is also effective for making implements out of small pieces of stone that are difficult to flake because the knapper is able to securely support them allowing for efficient flaking.
BP as a common technique for making stone tools is usually associated with the early Formative period (2000 B.C. to A.D. 250). By the late Formative, prismatic blade technology began to emerge, becoming the most important source of slicing, cutting and scraping tools in Mesoamerica in the centuries that followed (Clark 1981; Parry 1987). Hence, many people tend to think BP was largely absent in Mesoamerica after the Formative. So, the simple fact that the Calixtlahuacans were using it is somewhat of a surprise.
Of more than 6000 technologically diagnostic artifacts (those with features that indicate how they were produced), 11 percent were flaked with BP technology. This figure is not a majority, but it is significant. The BP artifacts we have identified include BP-flakes, “Scalar cores,” and bipolared blade sections. The BP-flakes (see picture P2) are variously shaped and have flake scars with ripples of force trending in opposite directions. Some were presumably flaked to make smaller tools - some were also used because they have use-wear on one or more edges.

P2: Bipolar flakes
The scalar cores are an odd, somewhat controversial artifact
because of the debate over their function (see picture P3). The term we use for this artifact is barrowed from Clark’s (1981) work in Formative period Chiapas. Artifacts similar to this one include the “scaled flake” (Parry 1987) and the piéce esquillées (Hayden 1980). One question is whether these three items were functionally different or had overlapping uses. They could have been used as 1) “bipolar cores” that yielded flakes, 2) and/or “chisels” or “wedges” used for some specific task, perhaps woodworking. My guess is that they had both functions.

P3: Scalar core, front and
back views
The bipolared blades are sections of prismatic blades (made with the blade technology), which were later “smacked” with bipolar percussion (see picture P4). Note the ripples of force coming from both ends towards the center of the segments. Also, some have a “burinated” fracture surfaces – what Clark (1981) calls bipolar corner flakes - along their lateral edges. These fractures are distinct in form. Similar features are visible on segments of experimentally bipolared blades (see pictures P5, P6). Archaeologists sometimes try to replicate the technological characteristics of archaeological artifacts, something called experimental archaeology. In this way, archaeologists are better able to interpret how tools were made. Other “bipolared” blade sections have been reported from Formative San Lorenzo (De León 2008) and the Valley of Oaxaca (Parry 1987:figure 22).

P4: Bipolared blades -top
dorsal, bottom ventral view
So why was bipolar technology used at Calixtlahuaca? The general perspective is that bipolar technology was a simple “non-specialized” way of making tools, and was primarily used in places where obsidian was relatively scarce (Clark 1981; Parry 1987). Therefore, it has been viewed as a means by which knappers worked or recycled tool stone (nodules, “used-up cores,” or used-up implements like bifacial knives, etc.) to maximize the amount of cutting edge available. Recent research, however, has shows that bipolar technology was present in areas of Formative Mesoamerica where obsidian was relatively abundant (De León 2008). Its use probably had less to do with scarcity, and more to do with the size of raw material available; in particular, it seems to have been used a lot when tool stone was available in small pieces. This interpretation may be consistent with its use at Calixtlahuaca.

P5: Experimental
bipolar flaking
As my previous post on stone tools indicates, 75% of the obsidian was gray. We still have yet to chemically source the gray obsidian, but we think much of it came from West Mexico. It is also the case that most of the obsidian artifacts are not large pieces (<10 cm maximum dimension, most are much smaller) and we have no evidence of blade production in the city; we think ready-made blades and blanks used to make bifaces arrived in the city, perhaps via the market system. As such, many of these imports were relatively small – if the inhabitants of the city wanted to flake any of this material further still, bipolar technology would have provided an efficient means for doing so.

P6: Experimental blade
sections - top ventral,
bottom dorsal
What is particularly interesting is that after Calixtlahuaca became part of the Aztec Empire in 1474, the amount of green obsidian from Pachuca rose significantly. Preliminary figures from dated houses indicate that from the Dongu (A.D. 1100-1300) to the Yata (A.D. 1470-1530) phases green obsidian increased from 11% to 48%. This shift is coincident with an increase in blade-core artifacts, more than 60% of which were green. This shift is also coincident with a decrease in the amount of bipolar artifacts, which dropped from a high of 17% in Dongu to a low of 7.6% by the Yata phase.
Calixtlahuaca’s incorporation into the Aztec Empire meant more Pachuca green obsidian reached the city. Moreover, if prismatic blades were preferred and were more readily available in the Yata phase, then perhaps there was less pressure to recycle using bipolar technology. Ongoing research, including a chemical study of the gray obsidian artifacts will be evaluating these trends and what we think they indicate about life at Post-classic Calixtlahuaca.

Clark, John E.
1981 The Early Preclassic Obsidian Industry of Paso de al Amada, Chiapas, Mexico. Estudios de Cultura Maya 13:265-283.

De León, Jason
2008 The Lithic Industries of San Lorenzo-Tenochtitlán: An Economic and Technological Study of Olmec Obsidian, Penn State University.

Flenniken, J. Jeffrey
1981 Replicative Systems Analysis: A Model Applied to the Vein Quartz Artifacts from the Hoko River Site. Washington State University Laboratory of Anthropology. Submitted to Reports of Investigations. Copies available from 59.

Hayden, Brian
1980 Confusion in the Bipolar World: Bashed Pebbles and Splintered Pieces. Lithic Technology 9(1):2-7.

Parry, William
1987 Chipped Stone Tools in Formative Period Oaxaca, Mexico: Their Procurement, Production, and Use. Memoirs of the Museum of Anthropology. University of Michigan, Ann Arbor.

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