Archive for the ‘Material Culture’ Category

Rio Grande from Coronado State Monument, Bernalillo, New Mexico

Rio Grande from Coronado State Monument, Bernalillo, New Mexico

Chapter seven of Crucible of Pueblos brings us to the final geographical region covered by the volume: the Rio Grande Valley, at the eastern edge of Pueblo settlement for the period in question. As it happens, I’m currently visiting my mom in Albuquerque, so I’m actually in this region as I write this. (Today also happens to be my birthday; I’m 31.) The chapter is by Steven Lakatos and C. Dean Wilson, and in a lot of ways it echoes an earlier paper by Lakatos about the Rio Grande Developmental Period that I have discussed before. This chapter, however, discusses only the Early Developmental Period, defined as AD 600 to 900, and primarily focuses on the part of the region that the authors called the Middle Rio Grande Valley, defined as lying between the Rio Puerco of the East on the west, the Sandia and Manzano Mountains on the east, the Isleta area on the south, and the La Bajada escarpment on the north. This is because agricultural populations only occupied this restricted area of the region during the Early Developmental, expanding north of La Bajada only after AD 900 when there was a huge increase in regional population at the beginning of the Late Developmental Period.

The key point Lakatos and Wilson make about the Rio Grande is that the Early Developmental period was a time of low population density and gradual growth, with little change in material culture over hundreds of years. This is in striking contrast to the “boom-and-bust” pattern now richly documented for the Mesa Verde region during the contemporaneous Pueblo I period there. The picture of continuity is reminiscent of that proposed by the authors of the previous chapter for the Little Colorado region, but it’s worth noting that the major data gaps that plague the study of that region are less of an issue for the Rio Grande, which has a long history of intensive archaeological research continuing to the present day. Furthermore, Lakatos and Wilson present several lines of evidence supporting their conclusions, which seem pretty solid to me. Based on this evidence, it really does seem like the Early Developmental was a time of low population, slow growth, and cultural continuity.

As Lakatos and Wilson note, this is actually a rather surprising conclusion in the context of many theories about early agricultural societies. Most strikingly, there is no evidence here for a “Neolithic Demographic Transition,” in which the increased productivity of agricultural societies compared to hunter-gatherers leads to massive growth among early agriculturalists, with all sorts of ecological and social consequences. Some have argued that the Mesa Verde boom-and-bust cycle is a result of this process. In the Rio Grande, however, the adoption of agriculture does not seem to have resulted in this sort of population growth. This is definitely not for lack of arable land, as the Rio Grande Valley is one of the richest agricultural areas in the northern Southwest, and it was intensively farmed later in prehistory and into historic times. Rather, Lakatos and Wilson argue that the richness of the Rio Grande environment allowed for a mixed farming-foraging economic pattern with high residential mobility, in contrast to the more agriculture-dependent societies further west. The greater importance of foraging versus farming is supported by evidence from faunal assemblages and wear patterns on human remains, and the mobility by the fact that residential pit structures were rarely remodeled.

In keeping with low density and high mobility, the settlement pattern consisted of scattered hamlets, with only occasional evidence for “communities” of hamlets loosely grouped together with possible communal architecture such as “protokivas” or oversized pit structures. Sites were mainly located along the major rivers of the region: the Rio Grande itself, the Rio Puerco of the East, the Jemez. Architecture consisted of residential pit structures and surrounding activity areas, generally oriented toward the east or southeast (perhaps oriented to the winter solstice).

Rio Grande people also appear to have been in closer contact with remaining hunter-gatherers than populations further west. It’s not clear if Early Developmental populations resulted from the adoption of agriculture by existing hunter-gatherers in the Middle Rio Grande Valley or if there was some migration of already agricultural populations involved, but in any case the areas north of La Bajada and east of the Sandias/Manzanos were definitely still occupied by hunter-gatherers during this period, and it’s clear that there was a lot of contact between the two groups. This may have contributed to the greater importance of foraging to Early Developmental people and their differences from other Pueblo populations.

Sandia Mountains from Tent Rocks National Monument

Sandia Mountains from Tent Rocks National Monument

All that said, the Early Developmental people definitely were part of the Pueblo cultural tradition, and their material culture shows a lot of connections to populations to both the west and south. This is particularly true of pottery, which was dominated by plain gray ware similar to that of late Basketmaker groups on the Colorado Plateau, but with small amounts of a decorated white ware, San Marcial Black-on-white, which shows stylistic influence from Mogollon populations to the south but with technological characteristics more like those of early white wares to the west. Lakatos and Wilson mention one model of Southwestern prehistory under which early “strong patterns” of material culture originated in the San Juan Basin (ancestral to the Chaco system) and in the river valleys of the Mogollon region, with the Middle Rio Grande forming a “weak pattern” with influences from both but in varying combinations.

The clear picture that emerges from this is of a small population of forager-farmers moving around within the Middle Rio Grande area but maintaining their basic cultural features with little to no change for about 300 years, from AD 600 to 900. Then, in a development that is likely very important but poorly understood, there was a massive increase in population at the same time that agricultural groups for the first time began to occupy the higher areas about La Bajada. Lakatos and Wilson note that the timing of this change, while not as precise as might be ideal, seems to correspond closely to the collapse of the late Pueblo I villages in the Mesa Verde region and the major population movements involved with the depopulation of that area, including the apparent influx of people into the Chaco Basin that likely laid the groundwork for the Chaco Phenomenon.

It seems very plausible that the increase in population in the Rio Grande was linked to these developments, though exactly how is unclear. Material culture actually remained fairly stable and consistent with Early Developmental patterns across this transition, although architecture did become more standardized and San Marcial Black-on-white was replaced by Red Mesa Black-on-white as the main decorated ceramic type. The latter change, especially, suggests influence from the west, as Red Mesa is the main decorated type in the Chaco area and other parts of the southern Colorado Plateau during this same period. It’s possible, as Lakatos and Wilson suggest, that the increased population in the Chaco Basin directly spurred Middle Rio Grande populations to move northward, although it’s not clear how exactly this would have worked. Other possibilities are that populations from the intermediate areas, such as the Puerco of the East, began to move eastward in the Rio Grande Valley as a result of the population movements immediately to the west of them, perhaps pushing existing Rio Grande populations north, or that western populations were moving directly to the Northern Rio Grande area above La Bajada, “leap-frogging” existing populations in the Middle Rio Grande.

The fact that material culture continued to show local Rio Grande features throughout the region, however, suggests that some level of assimilation or cultural accommodation between the locals and immigrants was involved, rather than a more directly confrontational situation. It’s noteworthy that Lakatos and Wilson don’t discuss evidence for warfare or defensive features at all, which of course doesn’t mean those things didn’t exist but does suggest that they may have been less prevalent than in some other regions.

Turquoise-Encrusted Cow Skull, Santa Fe, New Mexico

Turquoise-Encrusted Cow Skull, Santa Fe, New Mexico

Another thing Lakatos and Wilson don’t discuss, but which seems particularly important to understanding these relationships, is turquoise, specifically that from the well-known mines in the Cerrillos Hills east of the Sandias. Turquoise is of course strongly associated with Chaco, and while not all of the turquoise there has turned out to be from Cerrillos, a substantial portion of it definitely was. Evidence for increased connections between the San Juan Basin and the Rio Grande area at the same time as the rise of Chaco as a regional center is very intriguing in this light. Could increasing demand for turquoise at Chaco have led to the Cerrillos mines being a “pull” factor leading western groups into the Rio Grande Valley? Could the mines have even led local Rio Grande groups, or mixed groups of locals and immigrants, to move further east, across the mountains and even up over La Bajada into the Santa Fe area, which may have become more attractive as increased immigration reduced the supply of land in the Middle Rio Grande? And what about those remnant hunter-gatherer groups east of the Sandias and north of La Bajada? What happened to them? Were they attacked and defeated by the encroaching farmers? Pushed out into areas further north and east? Assimilated into agricultural society, which even in the Late Developmental period had a strong foraging component? There are a lot of questions about this period in this area, and very little evidence on which to base any answers. Lakatos and Wilson recognize this and suggest some research directions that would be helpful in answering the remaining questions, although they don’t point out as many as I have here.

Overall, this is a very informative chapter that brings into the discussion of Pueblo I societies an area that is often left out of these discussions. It’s an area of crucial importance for understanding regional dynamics throughout the northern Southwest, however, so I’m glad it was included in this volume. This chapter concludes the geographical summaries in the book; the remaining chapters cover various thematic topics of interest in understanding the Early Pueblo period as a whole.

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Mercer Museum, Doylestown, Pennsylvania

Mercer Museum, Doylestown, Pennsylvania

I was in Philadelphia for Thanksgiving, and the next day I went with my family to the quaint nearby town of Doylestown, where we visited two local museums: the Michener Museum (named after, yes, that Michener, who grew up in Doylestown and spent most of his life in the area) and the Mercer Museum. The Michener is basically a local art museum, and we went there to see an exhibit about Grace Kelly, who is a big deal in the Philadelphia area. Not really my kind of thing, but it was fine.

The Mercer, on the other hand, is a really unusual sort of museum. It was established by Henry Mercer, a Doylestown native who had a variety of interests and a good deal of money with which to pursue them. He studied law but never practiced it, instead going into archaeology in the 1890s. I haven’t found much information about his specific contributions to American archaeology, which was in its infancy at that time, except that he apparently supported the authenticity of the obviously forged Lenape Stone that allegedly contains an image of a mammoth and is now part of the Mercer Museum collections (though not on display).

In the late 1890s, however, Mercer came to the realization that the advancement of industrialization meant that most aspects of traditional life in the US were likely to disappear forever, and he began to collect what were then considered mundane objects for the museum of the Bucks County Historical Society. He collected huge numbers of things from all aspects of pre-industrial life, over time branching out to the US as a whole and eventually other parts of the world as well. His collection got so big that he built a new building to house it, using an innovative design and construction approach using poured concrete. He organized the collection thematically by the sorts of societal needs that objects served, and put together display cases by category.

The museum is still much as he designed it, although there have been various changes over the years. It’s a fascinating place, idiosyncratic and full of extremely detailed information. What I found especially interesting, however, was the way the museum’s own self-descriptions explicitly tied Mercer’s collecting of what most people considered “junk” to his earlier interest in archaeology. That is, one way to see what Mercer was doing was taking an archaeological approach to studying and preserving the material culture of the present and recent past, to ensure it would be understood in the future. This approach was quite ahead of its time for both history and archaeology, and the museum that resulted is fascinating and well worth a visit.

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Utah Welcome Sign

Utah Welcome Sign

The initial discovery of chemical markers for chocolate on potsherds from Chaco Canyon in 2009 was a hugely significant development in understanding Chaco. The evidence for the presence of chocolate, a Mesoamerican product that couldn’t possibly have been locally grown and is very unlikely to have been gradually traded northward through a series of intermediaries, gave a huge boost to the “Mexicanist” school of thought about Chaco, which holds that many of the unusual aspects of the Chaco system are due to influence from Mesoamerica.

The initial study only involved a few sherds, though, and understanding the exact role of chocolate at Chaco and its implications for Mexican contact needs a much deeper understanding of where and when cacao was present in the ancient Southwest. Thus, soon after the initial discovery further research by a different set of researchers (using somewhat different methods) began to test other pots from Chaco and elsewhere. They did find further evidence that at least some of the famous cylinder jars from Chaco were used in the consumption of chocolate, but they also found traces of cacao in vessels of similar form from the later Classic Hohokam period in southern Arizona, and, most surprisingly, also in vessels from the “small-house sites” at Chaco and elsewhere that are thought to have housed the lower classes of Chacoan society. The previous evidence for chocolate came from distinctive vessels at the “great houses” that are the hallmark of the Chaco system and seem to have been used by elites (though exactly what they used them for remains unclear and controversial). This is exactly the kind of setting where it would be unsurprising to find unusual, exotic things, and indeed the great houses clearly contained many such things in addition to the chocolate. Finding this sort of exotic foodstuff in more mundane pots at the small houses implies that it may have been more widely accessible than previously thought, which has important implications for understanding the nature of the Chaco system.

Well, now things have become even more complicated. The same researchers who did that follow-up study have done another, this time looking at a much earlier period and a different part of the Southwest. They used their same techniques to test for the presence of chocolate in pottery at Alkali Ridge Site 13 in southeastern Utah, a very important early village site dating to the eighth century AD. Site 13 was one of the earliest large villages established in the northern Southwest during the Pueblo I period, and its architecture shows some striking parallels to later Pueblo I villages such as McPhee Village in the Dolores, Colorado area, as well as to some of the early great houses at Chaco and elsewhere that developed even later. The early Pueblo I period in southern Utah is also associated with the introduction of a new type of pottery, San Juan Red Ware, which was widely traded from an apparently rather restricted production area and probably used for ceremonial purposes of some sort. In addition to being a different color from the more common gray and white pottery of the area, San Juan Red Ware also featured a distinctive design system in its decoration, one without obvious local antecedents. Combined with the distinctive architecture, this has led some archaeologists to posit that there was a migration into southern Utah during early Pueblo I from somewhere to the south, bringing these distinctive traits.

In that context, looking for cacao makes sense, as that would be a clear sign of ties to the south and cultural distinctiveness. Dorothy Washburn, who was the lead author on both this and the previous study,  has actually written mainly on design style in ceramics and other handicrafts, focusing on symmetry patterns. Based on the changes she has found in these patterns, she has argued for a very strong Mexicanist interpretation of Chaco, involving actual migration of people from far to the south bringing a distinctive pottery decoration style. She seems to have a similar view about Alkali Ridge, for similar reasons.

In any case, the study found that there was in fact evidence for cacao on several of the vessels found at Site 13, including some (but not all) of the redware ones. The conclusions, understandably, focus on the association between the new ceramic design system and the use of chocolate, but in fact the redware vessels don’t seem to be much more likely to have evidence of chocolate use than the other ones that were tested. It’s quite possible that San Juan Red Ware was associated with consumption of chocolate specifically, but it seems that other types of pottery were also used for chocolate-related purposes.

This is all very interesting, but it’s also confusing and hard to interpret, in a way that the authors of this paper don’t really address. Back when it seemed like chocolate was limited to cylinder vessels at Chaco great houses, that was easy to interpret: chocolate, like many other exotic goods found at these sites, was part of an extensive trading systems for elite goods, probably used for ritual purposes, which the elites of Chaco participated in (and perhaps dominated and directed). Finding it in the Hohokam vessels implied a similar system operating among elites at Classic Hohokam sites, which is consistent with some interpretations of Classic Hohokam society, plus the Hohokam in general show lots of evidence of contact with Mesoamerica in general so the presence of chocolate is much less surprising there than it was at Chaco. Finding it in the small houses at Chaco complicated the story somewhat and implied that the chocolate imported to Chaco wasn’t as restricted as had been thought, but since it was already known to be present at the great houses it’s not too surprising that the contemporaneous small houses had it too.

Alkali Ridge, though, is much earlier and much further north than any of these other sites. Getting chocolate there in significant quantities would have required a pretty elaborate and robust supply chain over a very long distance, much of which was inhabited by societies that are not generally considered to have been capable of this kind of long-distance coordination. Checking some of those intermediate areas (especially the Hohokam region) to see if they too had chocolate this early is necessary to understand the logistics of this.

There’s also the question of time. We now have evidence of chocolate from Utah in the eighth century, New Mexico (and to a lesser extent Colorado and Arizona) in the eleventh, and Arizona in the fourteenth. There are some big gaps there that need to be filled in to determine if these are three snapshots of a long-term and continuous tradition of chocolate consumption in the Southwest (which would have important implications about trade networks and relations with Mexico) or three separate episodes of chocolate being introduced from the south, possibly through population movement (which would have important implications for regional culture history in general). I think the most important place to look for evidence of continuity between Alkali Ridge and Chaco is in the large late Pueblo I villages in southwestern Colorado, especially the Dolores-area ones like McPhee Village. These sites have apparent connections to both earlier villages like Site 13 and later developments at Chaco. If they also reveal evidence for chocolate use, that would be a strong indication of continuity. The most important places to check for continuity between Chaco and the Classic Hohokam would probably be the Pueblo III communities in east-central Arizona, which again show connections in both directions. Both of these sets of sites are among the best-studied in the Southwest and there should be plenty of pots available for these analyses.

Finally, there is a methodological issue here. It’s possible that these tests aren’t actually detecting chocolate at all, but something else. The authors of the recent paper noted this possibility and looked into whether there are any plants native to the Southwest that might have chemical profiles similar to cacao that would throw off the analysis. They didn’t find any, but they note that many plants have not been analyzed in this way and it’s possible there is a different plant that is showing up in these analyses instead. Another possibility is that there is something about their method itself that is leading to false positives. It’s noteworthy that they have been finding much more extensive evidence of chocolate than the team, led by Patricia Crown and Jeffrey Hurst, that did the initial Chaco study found. That team hasn’t published any more about chocolate at Chaco since then, but I hear Crown was able to do some re-excavation in Room 28 at Pueblo Bonito this summer so there may be more from her on this in the future. Ideally I’d like to see a test of both methods on the same vessels to see how they match up.

The ultimate message here is that even important discoveries, like chocolate at Chaco, require many further studies and refinements to interpret properly. We’re nowhere near a full understanding of the true role of chocolate at Chaco or any other site in the prehistoric Southwest, but every study gets us closer.
Washburn DK, Washburn WN, & Shipkova PA (2013). Cacao consumption during the 8th century at Alkali Ridge, southeastern Utah Journal of Archaeological Science, 40, 2007-2013 DOI: 10.1016/j.jas.2012.12.017

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Black-on-white Sherd at Pueblo Alto

Black-on-white Sherd at Pueblo Alto

In keeping with what seems to be the recent pattern around here, my previous post led to some interesting discussion in the comments. Frequent commenter marcaeolog provided a link to a pdf version of Anna Shepard’s Ceramics for the Archaeologist, a classic work that challenges some of Florence Hawley’s theories that I discussed in the post. Andy Ward, who has done extensive experimental work attempting to recreate prehistoric pottery-making techniques, then chimed in with some information on his own efforts, focusing specifically on the question of what keeps carbon-based pigments from burning off during firing. Here’s part of his comment:

I have been doing research in just this area by collecting clays and firing them in open mesquite fires. While it is easier to keep carbon based paint on a pot in a reducing atmosphere this doesn’t account for polychromes. To make polychrome with a carbon based paint, like the ubiquitous Gila Polychrome, requires an oxidizing atmosphere. Certain kinds of clays have the ability to hold these carbon paints through a firing, bentonite clay in particular. Modern pueblo potters have used a slip called Cochiti slip which has these properties, ancient potters in cultures such as the Salado had access to similar slips.

As it turns out, Shepard had the same theory, based on her own experiments, which also provided solid evidence against Hawley’s theory, based on experiments with hydrofluoric acid, that it was compounds in the ash of the organic material that reacted with silica in the clay to protect the pigment from burning away. She acknowledged that this was a reasonable conclusion based on the information available to Hawley at the time, but Shepard’s own experiments comparing organic material with and without this sort of ash showed no substantial difference. Instead the biggest factor was clay type:

The most striking fact brought out by these tests is the effect of the type of clay on the permanence of the carbon. Bentonitic and other highly adsorptive clays retained a good black under firing conditions that completely oxidized the carbon on unnadsorptive, sedimentary clays. All of the clays that held the paint well required a high percentage of wafer to bring them to the plastic state and they had high shrinkage. These clays take up much more of the paint solution than the less adsorptive clays, and on drying, the clay plates are drawn tightly together, protecting the film of paint that surrounds them. The effectiveness of hydrofluoric acid in facilitating oxidation of carbon paints is explained by the fact that it attacks the clay, opening the surface and loosening the bond and thus exposing the carbon to air.

The upshot of this in cultural terms is that, contrary to what I said in an earlier post, it seems like the difference between mineral- and carbon-based black pigments is actually technological rather than cultural. Here’s Shepard again:

The relation of the type of clay to the depth of color of carbon paint from a plant extract is not a matter of merely theoretic interest to the technologist. It has a bearing on the distribution of this class of paint because it could not be produced in regions lacking adsorptive clays unless such clays could be obtained in trade. The use of plant extracts is not open to free choice or established merely by custom; it is limited by ceramic resources.

This is particularly interesting given the well-documented expansion of carbon-based pigments at the expense of mineral ones over the course of the late prehistoric period. If special clays were required to make this work, and those clays had restricted distributions, what accounts for the change? In areas that switched from mineral to carbon paint, such as Chaco, was this the result of a change in clay sources? The discovery, perhaps due to increased contact with other areas that had long used carbon, of which local clay sources worked for this? Importation of clay? Importation of finished pots? This raises a lot of questions, which to my knowledge have not received much attention in the archaeology of these areas, despite the fact that it’s now been several decades since Shepard’s research.

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Black-on-white Potsherd at Kin Bineola

Black-on-white Potsherd at Kin Bineola

“Never read the comments” is generally sage advice, so it’s likely that many of my readers have missed the interesting comment thread in response to my previous post. I won’t try to summarize it all here, but the gist is that a potter showed up and took issue with my use of the word “paint” to describe the pigments applied to prehistoric Pueblo pottery. Apparently, while archaeologists routinely refer to these pigments as paints, that’s not actually an accurate characterization and they are instead best described as types of “slip,” a term that archaeologists usually use only for the underlying layer of thin clay that the “paints” are often applied to. There’s a long history of archaeologists appropriating and misusing terminology from other fields, so it’s not surprising that they would have done this in this case too, but I’m not sure how best to proceed with the terminology, as it is pretty useful in practice to be able to distinguish the underlying “slip” from the “painted” designs on top of it. For now I’ll try using the (hopefully) more neutral term “pigment” for the “paint” and continue to use “slip” for the underlying clay.

Another issue that came up in the discussion, however, is separate from the terminology. This is the question of how the carbon-based pigments that are so well known from many parts of the Southwest could have adhered to pots after firing at all. After all, at the temperatures necessary to create usable ceramics any purely carbon pigment would burn right off. As I started to think about it, I realized that this was indeed a puzzle, and I went looking back into the literature to see if it had been addressed anywhere.

As it turned out, it had, and in the very first research on this subject, that of Florence Hawley in the 1920s. Hawley was an important figure in Southwestern archaeology throughout much of the twentieth century, and her early chemical studies of pottery pigments were fundamental in establishing the classification system by which they have been interpreted ever since. The most important aspect of this system was the division between “carbon-based” and “mineral-based” black pigments, although looking back at her initial publication on this topic from 1929, it’s clear that the dichotomy is more subtle than the terms imply. In fact both types of pigment contain both carbon and mineral elements, the difference being the relative proportions and the specific minerals involved. Both involve the boiling of certain plants, usually Tansy mustard or Rocky Mountain beeweed, to create a thick, black substance that can be applied to an unfired pot with a brush. The carbon type consists only of this substance, while the mineral type involves a subsequent step of adding ground iron oxide before applying it to the pot. Hawley also identifies a third type using manganese, which she speculates was used for certain types of pottery made of clay to which the usual carbon pigment wouldn’t adhere properly. (She also identifies a fourth type, so-called “smudged” pottery, that resulted from adding organic material after firing and letting it sink into the clay. This type is generally associated with the Mogollon culture in the highlands of east-central Arizona and west-central New Mexico and is quite different from the others in appearance.)

The chemical tests Hawley suggests to distinguish among these types of pigment are complicated and involve the use of dangerous acids, so they have not caught on as routine methods for archaeological analysis in more recent years. The tests appear to have been worked out and conducted mainly by Hawley’s father Fred Hawley, who was a chemist at a smelter in one of the mining areas of Arizona. Instead of these tests, archaeologists have generally tried to learn to distinguish the more common types by sight. As one of the papers I discussed earlier found in a test of this method, it works pretty well except in more marginal cases that are difficult to interpret using the standard visual criteria. The methods that have been proposed more recently using modern imaging technology have also not yet become very popular. In general the visual criteria seem to work fine.

As I noted in the previous post, the fact that both organic and mineral pigments seem to work to create black designs, but the mineral type is basically the same as the carbon one, raises the question of why there were two types at all. I suggested that cultural tradition rather than technological considerations probably was the main factor. Hawley doesn’t address the question in those terms, but she does argue that the different pigment types correspond to different cultural traditions, and the second half of the paper consists of her conclusions about cultural provinces and their spheres of influence based on pigment types through time. Her basic scheme of carbon pigments predominating in western areas and mineral pigments further east has generally held up, but otherwise her interpretations show how little was known at the time about Southwestern prehistory. She was clearly writing before tree-ring dating had established a firm foundation for the absolute chronology of cultural developments and pottery types, and the assumptions she makes about which sites were contemporary are strikingly wrong in light of that chronology. This makes her conclusions about changing patterns of cultural influence through time basically useless.

All that said, what about the question I mentioned earlier? How does the carbon pigment survive firing? Hawley’s answer, backed up by the chemical tests, was that it actually contains some mineral inclusions in addition to the carbon, especially silica and carbonates that melt in the process of firing and form a very thin film over the carbon that protects it from burning away. She considers this layer too thin to be considered a glaze, but it appears to serve the same basic function. It only works, however, when firing is in a reducing atmosphere, i.e., with oxygen prevented from reaching the surface of the pot. It’s not totally clear from her explanation where these mineral compounds come from, but the implication seems to be that some are from the plants from which the pigment is made and others are perhaps from the clay of the underlying pot. In the case of the mineral pigments, these potential sources are complemented by the trace silica in the iron oxide. As the more recent papers note, this interpretation has been questioned by others, most significantly Anna Shepard (another pioneering ceramic analyst at the same time as Hawley). It seems clear, however, that getting these pigments to survive firing requires some sort of chemical reaction along these lines, involving compounds in the pigment and/or the clay, along with a reducing atmosphere during the firing process itself.

It turns out pottery is pretty complicated! Archaeologists usually use pottery types primarily as a means to date sites and establish trade relationships, but studying potsherds intently with an eye to technological issues can reveal a lot of additional and potentially useful information.
Hawley, FM (1929). Prehistoric Pottery Pigments in the Southwest American Anthropologist, 31 (4), 731-754 DOI: 10.1525/aa.1929.31.4.02a00110

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Hachured Potsherds at Casamero Pueblo

Hachured Potsherds at Casamero Pueblo

When I was working at Chaco, people would ask me a lot of questions. I usually knew the answers, but when I didn’t I was quite upfront about saying so. I would often try to find out the answers to questions that had stumped me, but I didn’t always succeed, and many of those questions are the ones that I remember best, even now.

One of those questions had to do with the black-on-white pottery that is a hallmark of Chaco. A guy once asked me what they had used as a fixative to get the paint to adhere to the pottery, and I didn’t know. I didn’t really make much of an effort to find out then, as I didn’t really have any idea where to look. Recently, though, I’ve read two papers that shed interesting light on this question. Understanding them, though, requires some background on Pueblo pottery in general.

Hachured Potsherd at Pueblo Alto

Hachured Potsherd at Pueblo Alto

Basically, there are three main types of paint that were used in the prehistoric Pueblo Southwest. Two of these were in use during the period when Chaco was occupied: mineral and carbon. (The third, lead glaze, wasn’t used until later.) Both were used to produce a black color. Other colors were occasionally produced by a variety of means, but for this period black was the overwhelmingly dominant color of painted ceramics. It’s generally possible for experienced archaeologists to tell mineral and carbon paint apart by sight. Although they both tend to look black on a well-fired pot, there are various subtle differences between the two.

Carbon paint is basically just plant material, usually Rocky Mountain beeweed or tansy mustard, boiled down into an extract and applied to the pot. Mineral paint is made of ground iron-rich minerals, usually hematite, mixed with either water or an organic extract similar to those used alone as carbon paint, and sometimes with clay as well. Both types have been used both in prehistory and among the modern Pueblos. However, there were some marked patterns to usage of the different types of paint both geographically and temporally. Overall, the trend was of increasing use of carbon rather than mineral paint over time, spreading from west to east. Chaco ceramics at the height of the Chaco phenomenon in the eleventh century AD were overwhelmingly mineral-painted, while Chuska and Mesa Verde ceramics, while very similar in decoration and imported to Chaco in large numbers, were generally carbon-painted. Around AD 1100 there was a marked shift, and even Chaco pottery began to be primarily carbon-painted. By the 1200s use of mineral paint only continued in areas on the eastern and southern fringes of the Pueblo world. Distinguishing between carbon and mineral paint is thus quite useful for making inferences about culture history during this period. (In subsequent periods things became much more complicated with extensive long-distance migration and the introduction of glaze paints.)

Sherd with Checkered Pattern, Kin Klizhin

Sherd with Checkered Pattern, Kin Klizhin

Both of the papers I mentioned above are about using more formal means than visual criteria to tell mineral and carbon paint apart. One, by Joe Stewart and Karen Adams, used a scanning electron microscope-energy dispersive X-ray spectrometer (SEM-EDS) system to differentiate the two based on the amount of iron detected in the pigment. The other, by Robert Speakman and Hector Neff, used laser ablaction-inductively coupled plasma-mass spectrometry (LA-ICP-MS), a simpler and potentially less expensive method to distinguish between the two types of paint based on the concentrations of various elements. Both found that the methods they used came up with similar results to traditional visual methods, and were superior for certain sherds with unusual mixes of characteristics that made them visually ambiguous. Speakman and Neff also found a surprising amount of variety in elemental composition even within the “carbon” and “mineral” groups, suggesting that there were multiple “recipes” used to mix paint in prehistory, not just the two broad categories. This isn’t all that surprising given that this is clearly the case in modern times based on ethnographic data, but it’s still interesting.

That’s all well and good, but one thing that occurred to me in reading these papers was that it’s not at all clear why there would be multiple paint recipes, since they all ended up with pretty much the same result and some were definitely more complicated than others. This is particularly the case when considering mineral paint, since in many cases it appears that it was actually mixed with the plant concoctions that were in other places used independently as carbon paint. If boiling down beeweed produced a perfectly suitable black paint, why bother adding hematite? This might well explain why the trend over time was from mineral to carbon paint, but it’s still unclear why mineral paint originated in the first place. It does not appear to have predated carbon paint overall, at least at Chaco, where both carbon and mineral types are present as early as Basketmaker III.

Hachured Potsherds at Kin Ya'a

Hachured Potsherds at Kin Ya’a

The lack of obvious functional reasons for preferring mineral to carbon paint suggests that the choice may have been more a matter of cultural tradition, which is interesting and implies that some sort of cultural differentiation may have been present quite early in the prehistory of the northern Southwest. This isn’t really surprising, as there is plenty of other evidence for this, but teasing out the implications of all these different lines of evidence pointing to differentiation in material culture is a task that archaeologists have only recently begun to take on. It should be interesting to see what more we have to learn from these differences.
Speakman, RJ, & Neff, H (2002). Evaluation of Painted Pottery from the Mesa Verde Region Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) American Antiquity, 67, 137-144 DOI: 10.2307/2694882

Stewart, JD, & Adams, KR (1999). Evaluating Visual Criteria for Identifying Carbon- and Iron-Based Pottery Paints from the Four Corners Region Using SEM-EDS American Antiquity, 64, 675-696 DOI: 10.2307/2694212

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Mt. Taylor from Chaco

As I’ve discussed before, the patterns of use and importation of chipped stone at Chaco are somewhat puzzling. Unlike many other commodities, such as wood, corn, and pottery, which were imported from specific distant locations within the Chacoan sphere of influence in astonishing quantities during the height of Chaco’s regional power, chipped stone seems to have been largely a mundane, utilitarian concern. Throughout all periods of Chaco’s occupation most chipped stone was local. At Chaco’s peak of power and influence between AD 1020 and 1130 there was a slight uptick in imports of stone, particularly a distinctive pink chert from the Narbona Pass area to the west.

As I noted in the earlier post, however, obsidian follows a different pattern from the other imported stones. It is most common not at the height of Chaco’s regional power in the eleventh century but much earlier, in the Basketmaker III period between AD 500 and 750, when it is the most common nonlocal type of chipped stone. This was a time when Chaco may have seen an earlier period of regional importance, although figuring out what was going on at this time is very difficult for several reasons. By the Pueblo I period the amount of obsidian seems to drop precipitously, and it doesn’t start to recover until the very end of Chaco’s period of Pueblo occupation after AD 1120. This pattern puts obsidian decidedly out of phase with most other material culture imports to the canyon, which tend to correlate with the well-known evidence for social complexity and monumental architecture that we associate with the Chaco Phenomenon.

A recent paper by Andrew Duff, Jeremy Moss, Tom Windes, John Kantner, and Steven Shackley tries to put the obsidian evidence on a firmer footing by using geochemical sourcing to identify the source outcrops for a broad sample of obsidian found at Chaco and at various Chacoan outlier communities in the San Juan Basin. As they note, this is the latest chapter in a complicated story. Way back in the 1980s, the Chaco Project did an extensive sourcing study of obsidian found in its excavations in the canyon using X-ray fluorescence (XRF), a non-destructive sourcing technique that was then relatively new in archaeology. Their results, reported by Cathy Cameron in a number of publications, were surprising. They seemed to show that the closest source of obsidian, Mt. Taylor, provided very little of the obsidian found at Chaco (about 4%), while a distant source, Red Hill in Catron County, New Mexico, provided a very high proportion, especially in the assemblages from earlier sites. Also well-represented was obsidian from the Jemez Mountains, the second-closest source, with the proportion of Jemez obsidian increasing over time, a common pattern in the northern Southwest.

This seemed to indicate that there were substantial early ties between Chaco and the Red Hill area, far to the south but still just barely adjacent to some known Chacoan outliers. This result was mentioned in many publications on Chaco over the years, although many people didn’t really seem to know what to think of it. However, it soon began to be questioned. After this initial sourcing study had been done, Tom Windes submitted some samples of obsidian from Pueblo Alto and the Spadefoot Toad site for obsidian hydration dating, which involved a sourcing analysis as an intermediate step in the dating process. These analyses were inconclusive when it came to dating the artifacts (not uncommon in the Southwest, where obsidian hydration has a poor record as a dating technique), but the sourcing portion suggested strongly that the samples that had previously been sourced to Red Hill instead came from Mt. Taylor. Windes mentioned this anomaly in his site reports, as did Cameron in her subsequent publications on the subject, but a full published account didn’t appear until this new study.

The new study also used XRF to do the sourcing analysis, but both analytical techniques and source characterizations have improved considerably since the 1980s, so the results were quite different from the first effort. For some reason this study was unable to do a complete reanalysis of the earlier samples (although it implies that this may be possible in the future), so there was only limited overlap and the focus was mostly on recent samples collected by Windes at Basketmaker III and Pueblo I site in and around Chaco, as well as outlier sites studied by Kantner in the Red Mesa Valley near Mt. Taylor and by Duff at the southern edge of the Colorado Plateau near Red Hill.

The results were not really surprising, in that they have been known in broad outline since Windes submitted his samples for dating and reported on the sourcing anomalies, but it’s nice to see them formalized in a peer-reviewed paper. Basically, this study found that no samples from Chaco came from Red Hill, although a few came from other sources in the same general area, and that the most common source found at Chaco was Mt. Taylor. Over time there was a trend in the Chaco data showing a shift from Mt. Taylor to Jemez sources, accompanied by the well-known trend toward less obsidian in assemblages overall. The sample from the Blue J site near Mt. Taylor, in contrast, showed high proportions of Mt. Taylor obsidian increasing over time, in marked contrast to the Chaco pattern. The southern sites showed assemblages of obsidian almost entirely composed of Red Hill and other nearby sources.

Basically, the overall pattern was a classic distance-decay distribution, where the prevalence of a given source at a given site was mostly predictable by the distance between the source and the site. This is in sharp contrast to the pattern for many other imported goods at Chaco, which are present in high quantities in the source areas and at Chaco but not in between. This suggests strongly that obsidian was not part of any general Chacoan exchange system(s) but was procured by individual communities in accordance with their own needs, mostly using the closest sources. This is in keeping with the general tendency for chipped stone to be a relatively low-priority commodity in these societies.

The paper mentions the decline in overall abundance of obsidian after the Basketmaker III period at Chaco, but doesn’t spend much time discussing it beyond saying this:

The overall decrease in obsidian use noted at Chaco sites may reflect a shift in technological focus away from hunting and a subsequent focus on grinding technology as agriculture becomes the dominant subsistence strategy.

As I’ve noted before, this is almost certainly wrong; the decrease in question occurs at the end of the Basketmaker III period, at which time there is considerable evidence that Southwestern populations were already heavily dependent on agriculture. The decrease in obsidian is still odd, though. One thought I’ve had to explain it is that maybe the obsidian from after this period isn’t actually missing at all, but is at Pueblo Bonito, which had lots of obsidian but was excavated a long time ago using techniques that aren’t really comparable to the modern techniques used by the Chaco Project and later efforts that resulted in the collections being analyzed here. I would suggest that an XRF sourcing analysis of the Bonito obsidian would be interesting. As it is, there’s a huge shift in the proportions of the different sources at Chaco between Basketmaker III and Pueblo I. The earlier samples (dominated by the huge samples from the major villages of Shabik’eschee and 29SJ423) show a predominance of Mt. Taylor obsidian, while the later ones show mostly Jemez sources. The sample size is so much smaller for the later period, however, that I’m skeptical about taking this flip at face value. Including the Bonito assemblage might help to bridge this gap, or at least explain it.

Finally, it’s again noteworthy how unimportant obsidian appears to have been to the Chaco system. Even if the Pueblo Bonito evidence ends up indicating a more important role at Chaco itself, the various outlier communities appear to have used local sources and to have followed their own priorities in acquiring this commodity rather than getting it through any Chaco-controlled or -oriented system. This is one of the ways that Chaco appears to diverge from Mesoamerican societies, despite recent evidence that it may have had more contact with them than was previously believed. Obsidian was hugely important symbolically in Mexico, and control of major sources was a major source of power and wealth for various Mesoamerican polities. In the Southwest, however, nobody seems to have cared that much about controlling major obsidian sources, and obsidian seems to have been distributed as a fairly ordinary commodity without any particular symbolic importance. I think this is one of the strongest pieces of evidence suggesting that whatever influence Mesoamerican societies may have had on Chaco was indirect and mediated by Chacoan elites rather than imposed directly from Mexico, as some have argued.

In any case, while this isn’t really the most exciting paper, it’s still an important one in straightening out a part of Chacoan archaeology that had become pretty confused. Obsidian may not have been all that important at Chaco, but it’s still worth studying in part precisely because of its mundanity.
Duff, Andrew I., Moss, Jeremy M., Windes, Thomas C., Kantner, John, & Shackley, M. Steven (2012). Patterning in procurement of obsidian in Chaco Canyon and in Chaco-era communities in New Mexico as revealed by X-ray fluorescence Journal of Archaeological Science, 39 (9), 2995-3007 DOI: 10.1016/j.jas.2012.04.032

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