Discovery through Ceramic Analysis!
by David Q. Bustoz, Senior Ceramic Analyst
Hi readers! It’s your friendly neighborhood ceramic analyst David here to
share a little about what ceramic analysts do, and what broken bits of pottery can tell us about Phoenix’s First Farmers, whom archaeologists call the Hohokam. First, it is necessary to set the stage by giving a brief history of pottery in the Phoenix Basin.
Ceramic technology reached the Phoenix Basin, probably from Mexico, around 2,000 years ago, although there is evidence from the Tucson area that it reached the greater region much earlier. Archaeologists who study ceramics believe that the increased use of ceramic containers over time was connected to an increased reliance on farming and the more sedentary lifestyle that resulted from it. The first ceramic containers in the Phoenix Basin were what archaeologists call “seed jars,” which ceramicists surmise were created to provide a rodent-proof container in which to store the next year’s planting seed. Over time the repertoire of container shapes increased in lockstep with the rise of villages and a proportionately increased reliance on irrigation-based agriculture. The broken pieces of these containers are known as potsherds, or just sherds, and their form can convey what kind of container they were a part of, such as a bowl or a jar. This information provides clues about what kind of activities took place within a site because container form implies container function. This is derived through the idea that people didn’t eat their dinner out of huge storage jars, and likewise, a shallow bowl wasn’t a sufficient container to store a field’s harvest.
Some of these villages lasted for a thousand years, and this resulted in the presence of huge deposits of potsherds. These deposits can be so large that often when archaeologists excavate portions of these villages, potsherds are the most prolific artifacts recovered and can number in the hundreds of thousands. So what can we say about these numerous bits of ancient pots, and what conclusions can we draw about the people who created them?
Most importantly, ceramics can convey information about chronology, which refers to the timeframe in which the ceramics were used. This is most often related to the style of the pottery. Around A.D. 750, potters in villages near where the I-10 Freeway crosses the Gila River south of the Phoenix Metro Area, perfected the recipe for a pottery that fired a pink buff color, which they complimented with maroon painted designs. Archaeologists generally call these pots red-on-buff ceramics. There had been pots decorated similarly for centuries at that point, but these pots were special. The images that were painted upon what archaeologists call red-on-buff pottery conveyed ideas that hinted at the things and ideas that were important to the people who created them. The well-documented series of developments leading up to the creation and the changes in the designs after, provide a baseline for archaeologists to reckon changes in the archaeological record. Archaeologists figured out that the designs in some cases are so specific, that the time period for these designs can be reconciled to within a 50 year period! They did this by refining the chronology over time. Initially the earliest archaeologists knew which design styles came before which, based on the fundamental archaeological principal that artifacts found in the lowest levels of a deposit were deposited before the items which were recovered above them. This is known as the Law of Superimposition. Since a particular style would be associated with certain levels within a given deposit, archaeologists were able to date that deposit relatively accurately using a highly advanced techniques such as radiocarbon dating. Researchers were then able to look at the individual motifs present within the designs that were in use during these different time periods, and track how they changed over the entire sequence and region. Looking at the manufacturing techniques and painted designs of red-on-buff ceramics, archaeologists can estimate when that pot was created and through that information I can surmise the time period of the deposit from which it was recovered.
This is a group shot of red-on-buff sherds that were recovered from a 2012 project in the vicinity of 16th Street and Roosevelt St. in south-central Phoenix. These particular sherds are really neat because they depict people, can you make them out? These were likely manufactured near where the I-10 crossed the Gila River south of Phoenix around A.D. 850-950.
In the 1990s, researchers pieced together that the sand within the ceramics can convey where the artifacts were produced. In any given potsherd there is the clay, but there is also another component, called temper, which is added to keep the pot from exploding when it is fired. The composition of the temper was associated with specific geologic locations in the Phoenix Basin and throughout the region. In the 2000s, researchers figured out that for a 550-year period from about the time of the Dark Ages in Europe, circa A.D. 600 through about A.D. 1150, about 90% of the pots in Phoenix were being produced in just five major regions around the valley. Those pots were being produced in forms that complimented each other. This meant that people in the villages could, through a combination of the pots from different areas, have all the containers they needed for daily life.
By looking at the temper in the potsherds, archaeologists can tell where they were produced, and through that they can surmise who was interacting with whom. Proportions of temper present within these collections and their composition can be compared to documented trends in interaction between regions over time, which can also potentially provide limited information regarding chronology.
Here is a photo of a sherd in section, which I took through our lab’s microscope. This sherd has a clean break so that we can see what’s inside. In this case, we can see the individual sand or rock grains as the lighter inclusions. The sand tells us that this undecorated sherd was manufactured in Phoenix on the south side of the Salt River in the vicinity of the South Mountains.
Ok, now that you have some background, I’d next like to show how all of this is put into practice with a recent study, and of how these concepts guided my investigation of the ceramics recovered during our recent excavations in the middle of Downtown Phoenix, and led to a pretty cool discovery. My initial impression of this collection was that there were no red-on-buff or other decorated sherds with which to gauge chronology. I also observed that these undecorated sherds had a color and composition which bore a striking resemblance to an unusual type of pottery that was being produced near the Salt River to mimic the special red-on-buff pottery being produced along the Gila River. We call this local “knock-off” pottery, which are brown instead of pink or buff in color, brown-paste-variants or BPV for short. These BPVs were being made locally, in Phoenix, on a very limited basis in at least one, and probably more, of the major pottery production areas.
Here is a group shot of some BPV sherds. These sherds are from the same site in Phoenix as the red-on-buff in the first photo above. They look very similar to the red-on-buff, don’t they?
However, these mystery sherds also seemed to be from a much later period after the production of BPVs and most red-on-buff had ceased, based on my observation that many of the artifacts included broken pieces of sherds known as grog within their temper. This was unexpected because we know that the addition of grog in pottery recipes was a practice which became very popular in the Phoenix basin AFTER A.D. 1150, and it is very rare to non-existent before that time. None of the five major pottery production areas have been found to include grog within their pottery repertoire, until that later time. However, other members of the Logan Simpson archaeological team were sure that the deposits investigated during the project were very early and on the other end of the pottery timeline. And when I say early, I mean early, they were adamant that these deposits were from before the 550-year pottery production period, when pottery was somewhat new to Phoenix. Often these sort of disagreements lead to the coolest discoveries. This turn of events led to my suggesting that we send some of these sherds to a lab for a special test where they look at several individual grains of sand in the sherd and subject them to a special analysis where they can figure out when those grains were last subjected to intense heat. This provided a date for the sherds. I also sent some of these mystery sherds to a special scientist called a petrographer who could tell us whether or not the little bits of grog were actually pieces of pots and chemically confirm the source of the sand temper.
These special analyses resulted in the mystery pottery dating to much earlier than I expected, and proved I was wrong about my initial time assessment. The petrography proved that the pottery was manufactured with grog. This is the earliest documented use of grog in the ceramic making process within the Phoenix Basin! This was a result which caused me to wonder about my initial thoughts about how this pottery was reminiscent of the BPVs and whether these two types of ceramics may be related to one another. I decided to delve deeper and look to see if the BPVs with similar sand also included grog. Now I think it’s important for you to note that other researchers have looked at BPVs before, and also note that spotting a few innocuous inclusions within a sherd can be tricky. Ceramic studies build upon the awesome work of the people who came before us, and if we don’t know to look for something, sometimes the small details fall between the cracks. At the time the Logan Simpson Lab had 5 examples of BPV pottery from a collection of 36,000 sherds from Queen Creek. Keep in mind this shows how rare it is – 5 out of 36,000! When I looked at them 4 had only the same sand from the Phoenix area as our mystery sherds, and the fifth had the same sand and very distinct pieces of grog! I was very excited but knew that I needed more BPV examples with grog before I would feel comfortable with concluding that the earlier undecorated grog-tempered pottery was the unpainted platform for the latter BPVs. We started reviewing other research and found a few more examples hiding in the literature including three BPVs that had their grog temper proved through petrography. These last examples were the most important because the presence of the grog within them was independently verified through the scientifically rigorous petrographic process, and not just with my eyes.
This result led me to a couple of really interesting conclusions, the first of which is that I now felt that it was within reason to conclude that the mystery pottery is the earlier unpainted platform for the latter BPV’s. This was based on a couple of key points, first was that in the zone of production for both the BPV’s and the mystery pottery, none of the other pots were being produced with grog as a production technique. We know this because, those other pots have been intensively studied over the years because the zone of production near South Mountain is one of the top two largest producers for most of the Phoenix Basin Hohokam Sequence, and this pottery is commonly one of the most prolific utilitarian types recovered in local excavations. Second, since both the BPV’s and the mystery pottery include sand from the same production area, then they are related, with the only visible or compositional difference between them is that the latter BPVs included painted designs. These findings are entirely and are really exciting because from a Hohokam-big-picture-perspective it means that some of the only pottery being produced and painted with Hohokam ideological imagery within the Hohokam core, but away from the Red-on-buff production area on Gila River, was being produced with a production technique employed by some of the earliest potters in the basin and which was subsequently, moving forward from the early time period, utilized only for the production of the BPVs. This is entirely new and unexpected!
If you’ve gotten this far let me say Thank You, and I’m sorry if I lost you a little along the way because it’s difficult to consolidate complex research into such a small post.
Let me leave you with a final thought – all of these points can provide a picture of daily village life for Phoenix’s first farmers and how their lives changed over time, but these conclusions can only be reached for sites such as these if the materials at those sites remain in place. This means that when visiting sites, especially outside of the city, one should practice a “take only pictures, and leave only footprints” philosophy. This ensures that the information is there for the archaeologists and the Native American descendants of these amazing people to interpret, and so the sites remain intact for future generations to learn from and enjoy.