Symmetry With Certainty

When you look at prehistoric clay pots, how can you tell which were made by experienced craftspeople and which were made by, well, amateurs? One good indicator of skilled pottery-making is symmetry.

“Most of these pots were made by hand; not on a wheel and not made with molds,” explains Arleyn Simon. The ASU archaeologist studies ancient pots found throughout Arizona and the Southwest. Without the help of a wheel or mold, it takes a skilled hand to create a highly symmetrical pot.

Until now, however, there hasn’t been a good way of measuring symmetry, beyond the common but highly subjective “eyeballing” technique.

Using PRISM 3D capture technology, and specially designed software, archaeologists can now detect symmetry with certainty.

“This is the first time we’ve been able to mathematically analyze symmetry,” explains Simon. “The symmetry algorithm is one that Anshuman [Razdan] and Gerald [Farin] came up with. It takes a curvature plot of the two halves of the vessel and folds them over on top of one another. It then calculates how much they differ.”

The software can even include three-dimensional measures of symmetry for more comprehensive analysis.

“We’ve done some studies comparing the symmetry of Hohokam pots from Central Arizona,” Simon explains. “It’s evident that some potters were very expert and others were perhaps learning the craft. These measurements are crucial in assessing the rise of craft specialization and division of labor and economic pursuits.”

The curve analysis technology may also help the researchers trace pots from the same potter.

“Some of the larger pots may have used a base as support. We can isolate the bottom of the pots and compare curvatures,” Simon says. “We could recognize pots coming from one potter, or at least from the same workshop. A lot of these vessels were traded over hundreds of miles. This, in combination with chemical and mineral studies, will help us identify where they were made and who was making them.”

Working with Simon, graduate assistant David Van Alfen and others have scanned nearly 300 vessels from ASU’s Archeological Research Institute. Recently, PRISM acquired two portable scanners, each about the size of a laptop computer. In the future, researchers will travel to other museum collections and scan their contents as well, creating a vast, easily accessible library of artifacts.

Using this digital library and museum, researchers could easily access pots in other countries or even pots that have been reburied on tribal lands. And no matter how many times a vessel was studied, it would suffer no wear and tear.—Diane Boudreau