The Ancient Ceramics of West Mexico

Robert B Pickering & Ephraim Cuevas. American Scientist. Volume 91, Issue 3. May/Jun 2003.

If you look through an auction catalogue from Sotheby’s or Christie’s, you’ll invariably come across listings for 2,000-year-old ceramic figurines from West Mexico. Often beautifully made, the figures typically depict human forms, playing music or games, or engaging in what appear to be rituals. A desirable figurine in pristine condition can command $8,000 or more at an auction. It’s enough money to inspire desperate people to turn to crime: looting tombs and creating forgeries of the ancient artifacts. The problem has gotten so bad that it’s interfering with scholarly attempts to understand the pre-Hispanic cultures that lived in the region.

Rampant looting diminishes the value of the original artifacts, even if they somehow find their way into the hands of an archaeologist. A systematic excavation allows an archaeologist to see the objects in context with other remains, and it virtually guarantees that the objects are authentic. Taken out of their archaeological context, the cultural artifacts are often reduced to being little more than objets d’art. But the message doesn’t seem to make it through to people in the art world. More than one collector or dealer has said that he is “saving” the objects for future generations by buying antiquities.

Unfortunately, museums around the world find themselves in the middle of this dilemma. For more than a century, they have accepted collections from the private sector or sponsored archaeological expeditions for the purpose of acquiring objects for display Because the provenance of objects from private collections is uncertain, they are often merely placed on display as beautiful artifacts. Yet, if museums did not accept these collections, where would they go?

The situation is further complicated by the introduction of fakery. Looters have not been able to satisfy the antiquities market for a long time now, and some dishonest or merely opportunistic artisans contribute to the scholar’s dilemma by producing modern copies. Many of these forgeries are easy to identify, but others are made by people who actually are quite talented. These artisans read the professional literature, own extensive libraries and know the current values set by the auction houses. They also understand the psychology of their customers, and they know the folklore used to differentiate between ancient and modern figurines. They may even be creating part of the folklore!

This disagreeable state of affairs is the primary motivation for the study described in this article. The story of how this exploratory research unfolded is best told as a first-person account, so it will be described from the perspective of author Pickering: I am a forensic anthropologist and, until recently, a museum curator who inherited the responsibility of caring for a collection of ancient ceramics. Most of these objects were donated by private collectors, and I know that some of them are forgeries. Nearly every day I am faced with the question of how to make the collection useful to scholars, knowing that there are problems with the data. My collaboration with author Cuevas has resulted in a new, nondestructive way of recognizing authentic artifacts, and it holds the further promise of telling us more about the little-known people who made the objects many centuries ago.

Descending into the Tomb

In 1995, I was asked to examine a small group of human skeletal remains recovered at a site called Huitzilapa, just west of Guadalajara, which was being salvaged as part of a construction project. The human remains were lying in a tomb, 8 meters below the floor of what had once been a house or, at least, a house-like structure. One entered the tomb by way of a vertical shaft which opened on the structure’s floor and led to two rooms carved out of weathered stone. The rooms contained the remains of five adults and one female in her late teens, their personal adornments and some grave goods.

Although I was there ostensibly to examine the human remains, I couldn’t help but notice some of the other items in the tomb. Among them were some large, impressive ceramic figurines. This part of West Mexico is historically famous for the production of sophisticated hollow figures of people, animals and life scenes. The human figures wear a variety of clothing and headgear, and often carry objects from everyday life or from ritual life. Creatures such as conchs and crustaceans, as well as dogs and other mammals, are also represented. The ancient ceramists also created detailed models of village scenes, houses, ceremonies and even ballgames. Over the course of centuries, these subjects were depicted in a variety of ceran-tic styles, which archaeologists now use to differentiate between the various cultures that have lived in the region. Although the styles have attracted a great deal of attention, primarily from art historians, the purpose and meaning of these figures are still unclear.

The figures in the Huitzilapa tomb were those of human males and females, created in a style that thrived nearly 2,000 years ago. Because they had been found in situ, they were especially valuable-to archaeologists as clues to an ancient people’s way of life and to collectors as authentic antiquities. As I was looking at the details of how the figurines were constructed, I found something that proved to be much more interesting. Looking closely, I could see that the surface of each figurine was encrusted with small, ovoid forms, less than 5 millimeters long. Tiny lines, perpendicular to the long axis of each “ovoid,” divided the shape into segments. After a few seconds, I realized that these small ovoids were probably the remnants of insect puparia-the “casks” that house pupating insects between the larval and adult stages.

Because of my work as a forensic anthropologist, I was well aware that necrophilous insects often left evidence of their presence in or near human remains. I also knew they could provide clues about the conditions of the tomb before and after it was sealed. At the same time, I couldn’t believe that such delicate organic structures could survive for nearly 2,000 years. My colleagues in Mexico and the United States were quite skeptical about my interpretation-no one had ever identified these ovoids in this context.

When I returned to the Denver Museum of Nature and Science, I decided to examine the West Mexican ceramic figurines in the museum’s collections. It didn’t take long to find the same small ovoids adhering to a few of the figurines. At about the same time, the museum received a donation of West Mexican ceramic vessels from a private collection. Among the ceramic vessels was a large olla (a widemouthed jar) that had been broken while still in the private collection. The olla had many puparial remnants, with a very clear pattern of distribution inside and outside the vessel. Although puparia were located at various places on the exterior, they could only be found on the upper parts within the vessel, just inside the mouth. It suggested to me that the vessel was nearly full (of liquid?) when it was placed in the tomb and that this must have kept the insect larvae from going farther in. This was a clue to the timing of the insects’ activities. They were present before the food and drink offerings became dry or decomposed– perhaps days or at most a few weeks after the tomb was closed, rather than months or years.

The investigation took another turn when author Cuevas, a retired chemist, expressed his interest in volunteering for me in the anthropology department. We soon developed a collaboration to document the presence of the ovoids and other characteristics of the figurines. As a chemist, Cuevas became intrigued by the presence of dense, black stains on the ceramics. Archaeologists have long recognized similar stains as concentrations of manganese dioxide that develop on various materials (not just pottery) found in West Mexico. The region has a history of earthquakes and volcanism that unearth or produce mineral deposits in the area. Were the black stains actually mineral deposits produced by some geologic process? We began an object-by-object examination to find all ovoid structures and black stains on the figures and vessels in the museum’s West Mexican collection.

A Matter of Identification

Within a year, two forensic entomologists, Neal Haskell of St. Joseph’s College, Indiana, and Robert Hall of the University of Missouri, Columbia, confirmed that the ovoid structures are the puparial remnants of necrophilous insects. Unfortunately the remnants are too incomplete to precisely identify the species that made them. All the insect remnants are about the same size and shape, suggesting that they represent one or just a few species. Flies (perhaps phorids, piophilids or sphaerocerids) are the most likely suspects. But a mineralized impression of a chitinous wing cover suggests that dermestid beetles were also involved. Dermestids are commonly used by museum curators to remove the flesh from specimens for osteological collections. The beetle larvae can pick a skeleton so clean that only minor preparation is needed before the specimen is placed in a collection. To some extent the life cycles of the flies and beetles are similar, and both need to find a protected site for pupation.

We also explored various scenarios that could explain how the insects got inside the tomb. From my examination, I knew that the human remains from Huitzilapa had been intact but that five of the six bodies were desiccated before they were lowered into the tomb. The sixth person probably died just before the tomb was closed. Given the hot, humid environment, it’s reasonable to assume that an unprotected body could be infested with insect eggs within minutes (or at most a few hours) of death. So when the sixth body was lowered into the tomb, the necrophilous passengers also went inside. After the eggs hatched, the newly emerged larvae fed on the decaying corpse until they were sated, then migrated to a suitable place to pupate-in this instance, the ceramic objects in the tomb.

The tomb environment is critical not only to the preservation of the corpses but also to the survival of necrophilous insects. A defining characteristic of a tomb is that it is an open, room-like facility that may have openings to the outside (or, in this case, to the structure above) that allow air to circulate within. In contrast, a grave is a much smaller space, filled with dirt, and containing no more than a small pocket of air. Although flying insects may enter a tomb through the small openings to the outside, the process of interring human remains provides the greatest opportunity for infestation. The “shaft tombs” of West Mexico can be far below the surface, with vertical shafts often 3 to 20 meters long. Each tomb provides a relatively stable, warm and humid environment. From the perspective of a necrophilous insect, a tomb can be quite hospitable: It has a food source (the corpse), oxygen and a sultry climate. As long as these conditions prevail, the insect life cycle can continue. However, the necessities won’t last forever, and it seems likely that the food source-the body-will be the first to disappear.

While he was examining the ceramics, Haskell noticed that the puparial remnants were often densely mineralized. Was there a relationship between the puparia and the black stains? An examination of the black stains under a scanning electron microscope confirmed their identity as mineralizations, predominantly of manganese and iron. More answers started to fall into place after a search in the literature and some conversations with bacteriologist Roy Cullimore of the University of Regina, Saskatchewan. It appears that such stains result from the actions of bacteria-a process that was first suggested for the West Mexican artifacts by Meredith Aronson at the University of Arizona-rather than from geological phenomena. Some types of bacteria, such as Metallogenium and Leptothrix discophora, use the electrons of certain metals (especially iron and manganese) to produce ATP and then deposit the oxidized metal at the site of the colony’s growth-producing a black stain. Similar processes lead to the formation of manganese nodules in the ocean and manganese scale in water pipes. So the same microenvironment in the tomb that allowed the insects to go through their life cycles made it possible for the manganese-fixing bacteria to thrive. The puparia on the vessels may also serve as a source of energy for the bacteria, and this may explain why some puparia were mineralized. In these instances the mineralizations almost certainly helped preserve the puparia for 2,000 years.

The manganese stains do indeed have characteristics that are consistent with the residue expected of a bacterial colony. After it dies, each bacterium leaves behind a cell-sized insoluble metal particle, and collectively the individual deposits have the physical characteristics of a bacterial colony, but one that is made of metal. Iron and manganese are the most common metals oxidized by bacteria, and these predominate in the black stains we examined in the scanning electron microscope. In the light microscope, the stains are uniformly lustrous and black, with an irregular shape, but often with a ringed structure and fine “dendritic” patterns on their margins. The deposits also have three-dimensional depth, sometimes with layers of varying concentrations of metals. Such variations could be related to changes in a colony’s growth in response to the microenvironment of the tomb.

So where did the bacteria get the metals they needed? Although the ceramic figurines contain trace amounts of metals, the iron and manganese in them would already have been oxidized in the firing process of baking the clay, so they are an unlikely source. We believe the metals were probably supplied by surface water that dripped into the tomb and onto the artifacts over time. The water would have been enriched by minerals as it percolated through the soil, providing both moisture and oxidizable metals to sustain the colony’s growth, possibly for years. Such a mechanism might yield metal deposits that reflect the cyclic conditions of climate over a long span of time, and the layered stains we observed may be an indication of such changes.

And what about the insect puparia? In general it appears that the mineralization of the remnants depended on their location on the ceramic object: The more exposed the remnants were, the more likely they were to be mineralized. This is consistent with the idea that the remnants were mineralized by bacteria. Consider this scenario: The bacteria might have been attracted by the carbon and liquid content of a recently deceased insect, but became dormant once these provisions were gone. Some time later, water percolating into the tomb, with a dissolved metal content in the parts-per-million range, would have sufficed to induce the colony’s growth, ultimately resulting in the deposition of the oxidized metal in the shape of the insect puparium. Because dripping water could only reach the most exposed surfaces of the ceramics, the puparial remnants that did not come in contact with water were unlikely to be heavily mineralized.

Of course, the black stains are evidence that mineralization is not limited to the site of puparial remnants. The stains probably occur where other conditions of moisture and source material combined. Although the insect remnants are not directly related to the manganese deposition, the bacterial colony may have first thrived where the puparia were present. Manganese deposits have also been found related to other organic substances-such as wood splinters, grass and plant-fiber matting-on the surfaces of various artifacts. So although we can’t identify an organic substance at the site of every manganese deposit, food debris or other organic remains on the artifact may have once served to initiate growth of the bacterial colony.

From Blemishes to Knowledge

In some respects the puparial remnants and the manganese deposits deface the beauty of the ancient West Mexican ceramics. But rather than being deplored by the art collector or the archaeologist, these “blemishes” should be appreciated. For one thing, the presence of puparial remnants on a ceramic artifact is a sure indication of its authenticity. There was a period in the 1960s and 1970s when many museums and private collectors did not hesitate to buy ceramic objects looted from West Mexican tombs. This was also about the time that some artisans started to produce sophisticated forgeries. It seems likely that most, if not all, of the collections from that period are tainted by forgeries.

The identification of the “ovoids” as puparial remnants was unknown to active archaeologists in West Mexico, and so the fabricators of modern forgeries wouldn’t have known about them either-even those who are knowledgeable about the professional literature. At this point even the most sophisticated forgers are probably not faking the presence of puparia on their reproductions. Of course, the absence of puparia doesn’t mean that the ceramics are forgeries. There are a number of reasons why puparia may fail to develop, or ways in which these delicate remnants could have been removed-say, by a well-meaning conservator who gave the object an exuberant cleaning.

To a certain extent, the manganese deposits may themselves prove to be an even better indicator of authenticity, since they are more common than the puparial remnants. The task of authentication is slightly harder, however, because the forgers know that black stains are commonly found on these ancient ceramics, and so they often apply such blemishes to their fabrications. However, a microscopic examination of a modern reproduction that had been painted with black spots to resemble the manganese stains revealed obvious differences: The painted stains tend to be unevenly colored, dull and not completely black. They also tend to be round with abrupt, smooth edges. Occasionally, when the edges do appear to have the dendritic pattern of true deposits, higher magnification reveals no dendritic pattern on a fine scale. The true manganese deposits are also difficult to scratch (Mohs hardness 6-6.5), whereas the painted stains are easily scratched. Once these distinctions are recognized, it becomes relatively easy to distinguish between genuine artifacts and reproductions under the microscope.

At the moment, there are relatively few methods for determining the authenticity of a ceramic artifact. Most curators rely on aesthetics, stylistics and subject matter-highly subjective judgments-and compare the object to some idealized type. The best tool for determining the age, and hence the authenticity, of fired ceramics is thermoluminescence dating. Although the technique can provide a relatively accurate age for the artifact, it does have some drawbacks: It involves some destruction of the artifact, specialized equipment, as well as time and money if one needs to send a sample of the artifact to another laboratory. In contrast, most archaeology departments and museums have ready access to low-power microscopes, and the puparial remnants and the manganese deposits are relatively easy to recognize with little training.

When we used puparial remnants and manganese stains as authentication criteria for a selection of 271 artifacts at the Denver Museum of Nature and Science, we were able to identify a rather large number of forgeries. In total, 68 artifacts were judged to be modern reproductions, and 35 were of questionable authenticity. On the other hand, 38 were assessed to be authentic on the basis of insect remains, whereas another 105 were judged to be authentic based on the presence of genuine manganese stains. An additional 25 were probably authentic, but the stains were too faint to make a positive identification.

Beyond aiding the authentication of the ceramic artifacts, the puparia and the manganese deposits may also provide some insights into the West Mexican people, and so actually add to the value of the artifacts-at least for scholars. Because so many tombs were looted, and so few were systematically excavated by archaeologists, much is not known about them. For example, identifying the species of insects present in the tomb would tell us something about the people’s mortuary practices. Some species of necrophilous flies are more common during certain seasons of the year, and this might tell us when the bodies were placed in the tomb. Also, some insects prefer recently deceased remains, whereas others, such as dermestid beetles, prefer desiccated bodies. Precise identification of the species would indicate the condition of the remains when they were interred. DNA testing will be required to definitively identify the insects involved.

Currently, this research has focused only on West Mexico. However, its implications go far beyond that region. Cultures in other parts of the world that buried their dead in tombs under similar conditions could also have insect infestation and manganese staining. Indeed, Doug Bradley of the Snite Museum of Art at the University of Notre Dame, recently found puparia and manganese stains on Olmec figurines (about 3,400 years old) that were unearthed near Mexico City Some tombs of the Mayan region should be promising for this kind of research as well. Continued efforts should refine our ability to reconstruct the lives and deaths of the peoples in these ancient cultures.