cultural attraction in pottery practice: group-specific shape transformations by potters from three communities

Pottery is a quintessential indicator of human cultural dynamics. Cultural alignment of behavioral repertoires and artifacts has been considered to rest upon two distinct dynamics: selective transmission of information and culture-specific biased transformation. In a cross-cultural field experiment, we tested whether community-specific morphological features of ceramic vessels would arise when the same unfamiliar shapes were reproduced by professional potters from three different communities who threw vessels using wheels. We analyzed the details of the underlying morphogenesis development of vessels in wheel throwing. When expert potters from three different communities of practice were instructed to faithfully reproduce common unfamiliar model shapes that were not parts of the daily repertoires, the morphometric variation in the final shape was not random; rather, different potters produced vessels with more morphometric variation among than within communities, indicating the presence of community-specific deviations of morphological features of vessels. Furthermore, this was found both in the final shape and in the underlying process of morphogenesis; there was more variation in the morphogenetic path among than within communities. These results suggest that the morphological features of ceramic vessels produced by potters reliably and nonrandomly diverge among different communities.

Methods
Potters from three communities were instructed to faithfully reproduce four different model shapes with the thinnest walls possible using two different quantities of clay (0.75 kg or 2.25 kg), giving a total of eight experimental conditions. The four shapes (referred to as Cylinder, Bowl, Sphere, and Vase, respectively) were presented as pictures without providing any indication of the absolute dimensions to be produced. The four shapes were not part of any of the potters’ daily repertoire. Potters produced five specimens for each of the eight vessel types. The experimental sessions were video-recorded under standardized conditions using a Panasonic NV-GS320 camcorder. The camera was fixed on a tripod with lens orientation centered on the vertical rotation axis of the wheel. The camera was positioned at a height of 30 cm above the level of the wheel at a horizontal distance of 4-6 m. The lower edge of the video scene was aligned with the center of the wheel. The zoom was adapted to fully cover a 36-cm high by 42-cm wide calibration object (inverted T-shape) placed on the wheel at the start of each recording. For each trial, the images of the clay body profile after each fashioning gesture were extracted from the video frames (image resolution: 720 x 576 pixels; video sampling frequency: 25 fps). The first image captured the profile immediately following the (centering and opening) pre-forming phase and the last image captured the final profile; the intervening images captured the intermediate profiles during form development. This succession of profiles captured the vessel’s morphogenesis. The overall duration of the forming process was also analyzed. From the images, we extracted the 2D coordinates of the right half of the cross-sectional profiles by tracing them out on a Cintiq 21UX Wacom (Kazo, Japan) tablet with an integrated screen. The profile coordinates were converted from pixels to centimeters using a calibration factor obtained from the digitized dimensions of the calibration object. The profiles were re-sampled to generate an equal number of points at regular height intervals along the vertical (Y) axis and the resulting coordinates were smoothed with a low-pass filter. Because wheel-thrown vessels are typically axisymmetric, profiles were subsequently converted to full pot outlines by multiplying the horizontal (X) coordinates by -1 to create the corresponding left edge. Of the total 840 vessels thrown (twenty-one potters, each throwing five specimens of eight different vessel types), 12 vessels were not included due to problems with the video recording.