The Organization of Chipped-Stone Economies at Piedras Negras ...

100% accurate, the technique has been widely accepted as a reliable way to identify
geologic source for obsidian (Bettinger et al. 1984; Clark and Lee 1984:241-243, 1990).
Although some scholars have rejected visual sourcing as a viable method (Cann et al.
1970:581-582; Jackson and Love 1991:51; Rovner 1989:428; Zeitlin 1979:183-190),
Aoyama (1991:80-82; also see Aoyama et al. [1999]) and others have achieved a high
degree of success with a combination of visual and chemical sourcing. A similar method
was used by John Clark and the author (Hruby, Clark, and Nelson ms.). We have
connected the chemical types to visual types, and have succeeded in producing a sourcing
method for obsidian from Piedras Negras that has a 95% success rate (cf. Aoyama 1991).
The obsidian sourcing began in 1997 when obsidian artifacts recovered from
test-pit excavations were visually sourced. John Clark and the author initially separated
the artifacts into 12 categories based on distinct suites of visual characteristics, and
surmised a possible source for each. These artifact groups were then tested by Nelson
using X-ray fluorescence spectroscopy (XRF). The first round of visual sourcing proved
to be a failure because the extreme variability in the El Chayal source was not accounted
for. The test revealed that all the categories but three represented the El Chayal source.
The other sources represented at Piedras Negras were Ixtepeque and San Martín
Jilotepeque in Guatemala, and Zaragoza in Mexico.
For the second test, we accounted for the variations within the El Chayal source
and the number of categories was increased to fourteen. Eleven visually distinct groups of
El Chayal obsidian were created, as well as three others for Ixtepeque, San Martín
Jilotepeque, and Zaragoza. The test revealed that the fine, clear brownish obsidian came
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