Monday, April 13, 2009

Maps, part 2: Archaeometry

People often ask me if I can tell where a pot was made by studying it closely and reading the clues of form, fabric, style, and glaze.  I still have to answer them, "Not yet, not yet."  We are just starting the detailed study of each pottery shop. 

In addition, the problem is really pretty complicated. Right now there seem to be some elements of style that can be attributed to individuals, but we must have caution.  A decorative pattern or shape of form may appear made by one potter in one location, but could be imitated or shared by another potter somewhere else.  Take this example of a decorative pattern which archaeological research showed to be made by Frederick Petersen in Salt Lake City.

 This pattern appears on two antique pots we will exhibit in our show at the Iron Mission  Museum this summer.  Can we conclude that these pots were made by Frederick Petersen?  Not yet.  Even if we assume that all the other potters in Utah refrained from crassly or simply copying each other's decorations and styles, consider these possibilities:

1. Frederick Petersen learned to make pots as an apprentice of his uncle, Niels Jensen.  Frederick, along with Mr. Jensen and his two other apprentices, Jacob Hansen and Frederick Hansen, all immigrated to the Salt Lake Valley in 1852.  The vessel forms the three young apprentices made would have been informed by common elements of style derived from their Danish heritage and their common experience under their master's tutelage.  As the other apprentices grew up and established their own shops, they later worked in Brigham City and Hyrum, Utah. Could they not have shared some common decorative strategies?

2. While not as important in this specific case, we know that clay workers moved around between potteries.  Different potters collaborated at different times.  This means that one person could pick up a decorative technique, vessel form, or glaze type while collaborating and then carry those techniques forward to later work.

3. Of course, we also have many examples of parent-to-child transition of craft practices.  Some families established 'craft dynasties' with multiple generations of potters.  The Roberts family serves as the best example of this pattern, since the generations of this family spread out over the entire state, from Vernal to Panguitch.

So style remains a problematic tool right now for attributing vessels to specific makers.  There will be several examples of these puzzles in the show, including the Petersen example above.  Only a few potters stamped their ware.  Of those those that used maker's marks like on the previous post, they only stamped a fraction of what they made.  

After further study and excavation, we do expect to be able to attribute specific styles to specific individuals or at least pottery shops, but we have another tool to approach this problem: archaeometry.

Archaeometry is an area of scholarship that involves measuring the characteristics of archaeological artifacts, generally using materials science.  In this case, we are using two well-established tools to examine Utah-made pottery, Instrumental Neutron Activation Analysis (INAA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).  The details of the technique aren't important to review here, but those curious can find more detail at sites like the Archaeometry Lab of University of Missouri's Research Reactor (INAA here and LA-ICP-MS here).

Both techniques involve measuring the quantity of different isotopes in an object and statically  comparing the ratios of those isotopes.  Each technique measures 33 or 34 different elements and then the lab staff subject the data to multivariate statistics. In the case of INAA for example, we pound small fragments ceramic kiln wasters from each site into a powder, seal them in vials, and drop them into the core of a nuclear reactor (pictured below).  After irradiating them, the samples are put into a detector that counts the unique decay of each isotope in the ceramic fabric.  These counts are very accurate and are measured in parts-per-million (ppm) or parts-per-billion (ppb).

When comparing all that data using computer statistical programs, the lab researchers plot each sample onto a graph in multi-dimensional space, one axis for each isotope.  It is easy to imagine the three-dimensional version.  I put a picture below of a bi-plot using Aluminum and Sodium.  This graph appeared in an article my colleagues and I published in Historical Archaeology in 2007.  As the analyst adds data about more isotopes, they add dimensions and the data forms into 'clouds' where samples with similar ratios of isotopes tend to group together.

The analysts produce these multi-dimensional clouds that result in something like a statistical 'fingerprint' for each pottery.  When doing this type of study, one hopes that the clouds don't overlap very much so that some specific isotopes, usually the rare ones, can be used to argue that a sample goes with one group or another.  If things work out well, when one begins to compare unknown samples (such as an unmarked antique pot) to the known samples (from archaeologically excavated kiln wasters), one can make a statistical argument like, "We are 90% confident that Pot X was made at Pottery Y."

We've been fortunate because our data shows very clear patterns, and with the caution that our sample size is small for multivariate statistics, the results are very promising.  We have completed two studies where we compared archaeological fragments from consumption sites in  Utah and Nevada and the results have been very provocative.  As we map these patterns, we can consider a set of sites like the Muddy Mission, near modern-day Las Vegas.  Most of the ceramic fragments we studied matched those we knew were made in Salt Lake City and another set we think had been made in Provo.  But some of the ceramics that Muddy Mission residents used had been made by Thomas Davenport in Parowan, for example.  Other pieces matched J. J. Hansen's pottery from Hyrum, Utah, 500 miles away on the other end of the Mormon Domain.  (This study is forthcoming in a book called Archaeologies of the American West, edited by Margaret Purser and Mark Warner).

We are very excited by these results.  We expect that as we add more samples to the database, we'll be able to start mapping the extent of distribution of each potteries products in time and space.  In addition, we will start trying to sort out the different routes through which people circulated pottery in the economy: direct market exchange from stores, peddling, neighboring, family gift giving, tithing and poverty relief, and even population migration. 

This is a long term goal for our studies.  It will help us to really understand how individuals and families like the Davenport family managed to make their businesses succeed over their lives in Utah.

No comments:

Post a Comment