PUMACEOUS POTTERY: AN INSIGHT INTO THE ANCIENT GUANGALAN PEOPLE VIA CERAMIC TEMPER ANALYSIS
Niki Apollonia Buechel, Sabrina Rose Casavechia, Matthew Christopher Guido, Gabrielle Hoang, Abhiram Karuppur, Sangho Andrew Lee, Heather Amelia Newman, Dorothy Yingtao Qu
Advisor: Dr. Maria Masucci Assistant: Kaushaly Patel
ABSTRACT
The Guangala people of southwest Ecuador have a relatively unknown culture. However, recent research has shown that the Guangala had a profound knowledge of ceramic technology. Analysis of the Guangala ceramics reveals an extensive use of pumice as an inclusion to make finely decorated pottery, though the pumice was not available locally to the Guangalan potters. There are different hypotheses that describe the reasoning and source behind the presence of pumice in the ceramics. One hypothesis contends that the pumice was derived from ashfall from explosions of volcanoes in the Andes mountains. Another hypothesis details that the pumice was obtained through trade from people inhabiting the Highland areas surrounding the volcanoes. Evidence from analysis of Guangala ceramics using optical petrography, a technique used to analyze the mineralogical contents of ceramics, supports trade in pumice. These results will help in determining the relationship between the Guangala and other contemporaneous cultures and will shed light on the activities and technology of a relatively unknown society.
INTRODUCTION
The Guangala were an indigenous people who inhabited southwest Ecuador from 200 BC- AD 800 (1). While little information is known about the Guangala, studying their unique ceramics can lend insights into technology, culture, and their interactions with other ancient peoples. Archaeologists often pursue provenance studies in order to reach these ends, and in this study, a particularly puzzling question of provenance with respect to ancient ceramics is encountered. Namely, the ceramics of the Guangala people (Figures 1, 2) appear to be tempered with pumaceous ash, and yet geological studies of the area have not identified pumice in the region where the Guangala lived. This material is present in the volcanically active Ecuadorian Highlands 249 km from the Guangala region (1). Identifying the source of the pumice found in Guangala ceramics is the motivation of this provenance study, in which three possibilities for acquiring the pumice temper will be investigated: trade, direct procurement, and ashfall. Using optical petrography, the project analyzes Guangala ceramic samples, to be compared with pumice and ash samples and experimental samples of clay with pumice and ash.
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Figure 2. Guangala polychrome bowl Figure 1. Guangala masculine figurine with geometric decoration. (Private whistle with engraved body decoration and collection; photograph from Valdez and hammered gold earrings. From the Veintimilla 1992: Figure 70). (4) collection of the Museo del Banco Central
del Ecuado r, Guayaquil. (MBCG No. GA 2-2164 -82; photograph from Valdez and Veintimilla 1992: Figure 65). (4)
By attempting to determine the source of the pumice found in ancient Guangala pottery new information can be offered on the ancient ceramic technology of coastal Ecuadorian people. In addition, the findings could offer information on trade, economy, and cultural practices of the ancient Guangala as well as possible relationships between the Guangala and groups from other geographical regions. The research could also contribute to archaeological methods by demonstrating the utility of petrographic analysis for studying artifacts and materials. From a global standpoint, the results of the study could add new comparative information to research done on other ancient peoples such as the work of Anabel Ford of the University of California at Santa Barbara on the Mayans in El Pilar, Belize. Ford contends that volcanic ash, a volcaniclastic material related to pumaceous ash, is present in Mayan pottery and was derived from ashfall from volcanic eruptions, rather than trade or direct procurement (2). In the present study, Ford’s hypothesis is tested as one potential explanation for the source of the material in Guangala ceramics. This case is tested only on a comparative basis, as the case of the Maya and Guangala are not identical. The research done on the Guangala ceramics could either serve to reinforce Ford’s hypothesis or serve as a contrasting case if a difference in type of source is found.
An analysis of archaeological and experimental samples was undertaken to consider the likelihood of the three explanations. By comparing type samples of volcanic materials, experimental ceramic samples made by mixing clay with various pumaceous tempers, and a sample of archaeological ceramics spanning the Guangala time period, it may be possible to determine the specific origin of the pumice found in the Guangala ceramics as well as the means by which it was procured over several centuries. This study leads to the postulation that the pumice was obtained through trade from the highlands. It is unlikely that the Guangala would travel to the Highlands to mine the pumice through direct procurement; the mining regions in the Highlands were inhabited, and these people likewise have been linked to trade with the Lowlands already, through obsidian. Moreover, the volcanic expulsion model is unlikely as
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pumice would have to reach El Azúcar Valley (Figure 3), where the Guangala inhabited, regularly enough to account for the abundance of pumice found in ceramics spanning the entirety of the Guangala time period. Also, this ashfall is unlikely, because of the generally northward wind patterns in Ecuador (3). In order to test the trade hypothesis, ceramic petrography was used to compare the experimental samples, whose sources of temper are known, with the archaeological samples of the Guangala people. By obtaining this information, it will be possible to further characterize a relatively unknown people who demonstrated a profound knowledge of ceramic technology. These results will help in uncovering the process used by the Guangala people to create their pottery and will also help in detailing the environment of El Azúcar Valley and the trading networks of the region. In addition, knowledge of the ceramic production methods of the Guangala people may aid other provenance studies in pre-Columbian ceramics, such as Ford’s investigation of ancient Maya pottery.
Figure 3. Map of Ecuador adapted from Reitz and Masucci, showing the Guangala area of interest, El Azúcar, and northwestern wind patterns (5).
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BACKGROUND
Geology The geology of Ecuador itself is the result of a Mesozoic-Tertiary age (250-2.6 mya) volcanic belt, which formed at the convergence of the oceanic Nazca tectonic plate and the continental South American plate forming the Andean mountain chain (7). Ecuador contains one of the most volcanically active zones of Latin America and is an ideal location for finding volcanic materials. Pyroclastic materials like pumice, however, are concentrated only near the volcanic zones in the highlands. The geology of El Azúcar Valley within the Santa Elena Peninsula region, which is the source of the Guangala ceramics in the study, is characterized by sedimentary materials including chert and sandstone, among which the minerals include varying quartz, feldspar, and muscovite; neither ash nor pumice is found (7).
El Azúcar Valley is in the lowlands, less than 30 km from the ocean and 25 km south of the coastal hill range referred to as the Chongon-Colonche hills. The hills expose igneous rocks of the Pinon and Cayo Formations. The Pinon contains basaltic lavas and phenocrysts such as plagioclase, augite and Fe-Ti oxides, and the Cayo contains volcanic breccias, tuffs and basaltic lavas. The Chongon-Colonche hills are composed of volcanic and volcaniclastic rocks, and these sediments do reach the lower coastal valleys but they are intrusive igneous materials and not pyroclastic materials such as pumice (8). The presence of these volcaniclastic rocks affects the clays of El Azúcar Valley, but would not result in the presence of pumice or ash naturally in the clays. This distinction indicates that pumice would have to be intentionally added from a separate source. Samples of El Azúcar clays were tested in previous research to confirm the absence of natural pumice (7).
It is important to make the distinction between the igneous rocks available to the Guangala people only kilometers away at the Chongon-Colonche hills, and the pumice and ash in the ceramics. The latter materials would only be available through active volcanic activity, i.e. ash and phenocrysts ejected and transported by wind, and rock formations found at the base of the volcano. For example, volcanoes such as Tungurahua have the proper igneous rocks that would be added to fine pottery, however Tungurahua is greater than 250 km from El Azúcar Valley (Figure 4). Figure 5 illustrates the ash and pumice deposits of Ecuador in the Qc region.
El Azúcar
Figure 4. A map showing the major volcanoes in Ecuador, the closest of which is Tungurahua (6).
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El Azúcar Valley
Figure 5. A geological map of Ecuador. The yellow-brown region labelled Qc represents the highland region which is a possible locus of trade from which the Guangala may have obtained the pumice that is widespread throughout their ceramics. The map also shows that pumice deposits are not local to El Azúcar (8).
Culture During the Guangala era, the people of Ecuador were going through a phase of regional development, where each region obtained a unique identity. One way people marked or showed this identity was through fine paste decorated pottery. The Guangala made some of the most beautiful and colorfully decorated pottery in Ecuador, creating black “sombreware” and two and three colored “bichrome” and “polychrome” pottery (1, 9). This pottery was an innovation of the
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Guangala, although little is known of how they created their ceramics. Accordingly, this innovative pottery feature unique to the Guangala disproves the theory that the Guangalan pottery could have be traded from other sources, e.g. the Highlands, as the ceramic style of the pottery being studied is unique to the Azúcar region.
The Guangala people inhabited a stretch of the coastal plain of Ecuador which included El Azúcar Valley, which lies southwest of the present day city of Guayaquil. Numerous Guangala settlements have been found and excavated in this valley (1) as well as neighboring valleys (10). Research into Guangala ceramic technology identified possible pumice or pumaceous ash in the decorated ceramics, but was unable to determine its source (9).
In order to gain a complete sense of the options available to the Guangala in procuring the pumaceous ash used as a tempering agent in their pottery, it is necessary to detail the characteristics of their economy. The economy of the Lowlands region was agricultural-based, but complemented by fishing and hunting practices (5). Crops produced included maize, squash, and beans. Fisherman were known for their shallow and deepwater shellfish, marine fish and shark, while hunters commonly brought home deer, armadillos, turtles, and various monkeys. The Guangala also had domesticated guinea pig. For much of Guangala history, there was a marked division of labor, with occupational specialization and personal identity based on the distinctions between being a farmer, fishermen, etc. However, ties were formed between these different occupational circles through the exchange of foodstuffs. As the different groups went through natural cycles of production and struggle, they became increasingly dependent on one another, offering early examples of trade and collaboration in Southwestern Ecuador (5).
During the early years of Guangala society, trade was typically a private affair between select and prestigious people. Soon, a number of new products and practices made more complex trade possible, including wooden canoe building and the development of tools such as chisels, axes, scrapers, and knives. The economy of ancient Ecuador evolved, becoming more complex and organized, with trading routes linking many different areas (10). These trade routes led to greater interaction and a flow of ideas between different areas. Evidence exists that obsidian (a hard, dark, glasslike volcanic rock formed by the rapid solidification of lava without crystallization) was traded from the Highlands to the Lowlands, including to the Guangala region (10). It may seem dubious that two societies 250km away would trade, especially without amenities such as draft animals, however chemical sourcing of obsidian artifacts found in Guangala sites with obsidian sources in the Highlands prove that trade between these peoples did happen. Given the obsidian trade demonstrated to have occurred, it seems possible that pumice could also have accompanied the trade of obsidian from the highlands to the lowlands, especially since obsidian weighs more than pumice, thereby giving no reason to doubt the viability of trade of pumice. In addition, pumice is found near sources of obsidian, as they are both the result of volcanic eruptions. In another instance, it is evident that salt, which can be much heavier than both pumice and obsidian, was traded by way of canoes and by foot from the Lowlands to the Highlands (11). This trade opens the possibility that pumice and volcanic ash could have made the trip down from the Highlands in a similar manner as other products that were transported to the Highlands. Besides salt, the Lowlands had flint, cotton, cocoa, marine fish, shellfish and marine shell ornaments to provide the Highlands in exchange for the pumice (11). Based on this evidence of Guangala trade with the Highlands and the nature and sophistication of the Guangala
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economy, it is possible that pumice was traded from the Highlands to the Lowlands to the Guangala potters.
Ceramic Technology Clay contains a very fine lateral crystalline structure which takes on plasticity when mixed with water and can then be pressed into shapes. In addition, incorporating grains of rock, salt, organic matter, and other non plastic materials into the clay can serve various purposes, from color to plasticity to help the forming and firing process of pottery. Thin-walled, fine paste pottery is particularly difficult to form and fire as, without non-plastics, the objects often shrink or break in the firing. This occurs because the clay undergoes stages of change after it is put into the kiln for a firing. First, the water vaporizes and exits the clay, decreasing the plasticity and making the matrix more porous. Then, the various minerals in the clay chemically react and oxidize. In the vitrification stage, the matrix hardens, tightens, and the clay partially converts into glass. These stages occur at different temperature ranges and can occur simultaneously. In this process a plastic object becomes a hard ceramic (12).
Artificial inclusions, or temper, in the clay matrix prevent excessive shrinking of the matrix during drying and firing. By making the matrix more porous, it facilitates even drying and resists cracking in the clay. Volcanic ash materials such as pumice are fine, naturally angular shaped-materials that provide all the advantages of rock temper to strengthen clay for forming and firing, but are lightweight and occur naturally in small particle sizes (12).
The use of pumice as a tempering agent is innovative and sophisticated on a technical level. Its introduction to fine paste pottery by the Guangala appears to have allowed the potters to create the thin walled decorated pottery, which identifies these people and their culture. Pumice is advantageous as an added inclusion in the clay matrix because, as a light and airy material, it will not add weight or thickness to vessels and it is not volatile in low firing temperatures, thereby enabling the thin-walled pottery characteristic of the Guangala (12).
Comparative Case The Guangala were not the only ancient culture to discover the properties and advantages of volcanic ash materials for ceramic technology. The Late Classic Maya of El Salvador traded volcanic ash for use as a ceramics temper, and were able to transport large quantities of ash through sea and river routes (13). This provides evidence of long distance trade involving volcanic ash by civilizations with similar capabilities. Other research on the Lowland Classic Maya by Anabel Ford has reached a different conclusion with respect to the source of the ash used in their pottery. Similar to the difficulty encountered in definitively determining the source of pumice in Guangala ceramics, there is a question of source with respect to the volcanic ash found in the ceramics of the Lowland Maya. The material in the Maya ceramics, however, refers more to glass shards than to pumaceous volcaniclastic material (2). Anna O. Shepard, using petrographic thin section analysis, first identified the volcanic ash in the Maya ceramics (12). Ford also has used petrographic studies of Maya ceramics, but she concludes that the source of the ash is in the Guatemalan Highland volcanic chain. The ubiquity of ash in the Maya ceramic vessels, combined with the impediments and impracticality
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associated with its long-distance procurement, have led Ford to investigate ashfall as the possible explanation for the means by which the Maya acquired ash for habitual use in ceramic vessels. This hypothesis, however, is conditional on the assumption that consistent volcanic activity in the Guatemala Highlands would have to have occurred in order to account for the ash present in ceramics spanning the entirety of the Late Classic Period (600-900 AD). Furthermore, the employment of volcanic ash as a tempering agent in both elite and utilitarian/domestic vessels strengthens the assumption that volcanic ash was widely available and obtainable across many social echelons. It is, however, improbable that the nonlocal volcanic ash constituting the ceramics from the Late Classic Period derived from a single ashfall, as this circumstance would be inconsistent with the short-term planning paradigm common to most societies. Several weaknesses in Ford’s model are identified when applied to the Guangala study of ceramics. In the Guangala study, in contrast to Ford’s hypothesis, trade between the Highlands and Lowlands is already known for another volcanic material, obsidian, and therefore it may have transpired for ash and pumice as well. It is important to note that while striking parallels exist between this provenance study and Ford’s research, the latter is only intended as a means of comparison rather than as an identical situation. Nonetheless, Ford’s data points on which her ashfall hypothesis is built are useful for testing against the hypothesis of the present study; that raw material arrived via trade, not from the sky. For this reason, the characteristics used by Ford were selected and evaluated in the Guangala archaeological ceramics and experimental samples. These characteristics are shape, homogeneity, the presence of phenocrysts, and the size of inclusions, though it must be noted that these characteristics may not necessarily be revealing of temper source, despite their use as a main support for Ford’s ashfall hypothesis (2). Although Ford viewed these characteristics as indicative of ashfall, some may also be characteristic of mined or traded material. This point will be discussed further below in Methods and Results.
MATERIALS AND METHODS
The methodology employed in this study was designed to determine the source of the pumice found in the pottery of the Guangala people. There are three hypotheses for the source of the pumice: (i) the ash utilized “fell from the sky” (carried by wind from volcanic eruptions in the Ecuadorian highlands); (ii) the potters traded for it with the dwellers of the highlands; or (iii) the vessels were produced in the highlands and traded to El Azúcar. To test these three hypotheses, the methodology focused first on defining the test implications for each model. For the first hypothesis, ashfall characteristics defined by Ford and stated by her to be indicative of ashfall were used and compared with the appearance of inclusions in the Guangala archaeological ceramic samples. For the second, trade from the highlands, the methodology focused on examining potential raw materials as well as creating experimental samples with different pumice and pumice related materials, including materials from the Highlands to compare their appearance with the appearance of the inclusions in the archaeological samples. For the third, that the vessels were produced in the Highlands, the conditions for this model (i.e. pottery color, style and cultural characteristics of the Highlands were present) were evaluated. In this way, the source of the pumaceous ash could be determined based on which hypothesis had the support of the data.
Optical petrography of thin sections was used as the primary method to analyze and examine the characteristics of the rock materials and the inclusions in the experimental samples
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and the archaeological ceramics. This method was introduced to American Archaeology by Anna O. Shepard and provides insight into the mineralogical and material makeup of ceramics as they are a type of fired geological material. Petrographic analysis allows pottery to be examined as a geological material, focusing on mineral and rock fragment content and microstructure (14). Petrographic microscopes employ polarized light microscopy through the use of polarizing filters and analyzers and a rotating stage. Minerals also polarize light, and when viewed through this type of microscope the light rays combine and interfere in unique patterns, revealing the crystalline structure unique to each particular mineral and rock fragment being viewed. Both plain polarized light and crossed polarized light were used, as each identifies different components of the minerals and rock fragments under examination. The use of plain and crossed polarized light in juxtaposition allows for identification of isotropic vs. anisotropic minerals. For example, iron is known to be anisotropic, so when a large black or red Figure under the microscope looks the same under both plain and crossed polarized light, one can confidently identify the Figure as iron. Furthermore, isotropic minerals, such as pumice, do depend on the direction of light, and will look different under both light filters.
The first step in our procedures to test the above hypothesis was to establish what pumice and related pumice-like materials look like in thin section. Previously prepared thin sections of rock and mineral types were examined, and their general characteristics under polarized and plain polarized light were noted (Table I). The inclusions in the Guangala ceramics had been fired and therefore would likely have been altered by the firing process. Therefore, the second step in our procedures was to create experimental ceramics with known inclusions of pumice and pumice-like materials.
Table I: A log documenting the mineralogical contents of the type samples examined.
Eight different test material samples were ground individually with a mortar and pestle. Great care was taken to avoid cross contamination between samples, as this could have caused a misidentification of materials. The test samples were made using a commercial clay, which did not naturally contain pumice or ash. A control without any test materials was made to ensure that neither pumice nor ash was present. Two briquettes were formed from each sample in case one
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was destroyed in the firing, and each set was rolled in one of eight tempers (the human-added inclusions). Each pair of briquettes was placed in a separate petri dish to prevent contamination. Furthermore, to permit easy identification of the samples after they were fired, the clay briquettes were etched with a label. The inclusions were listed in the sample log spreadsheet, along with the name of the sample, the label used, and notes about the sample.
One clay briquette of each temper and one control briquette were placed in the kiln, a Barnstead Thermolyne 1300 Furnace. Rapid firing of clay can cause a build-up of steam and concurrent explosion of clay objects. To avoid this result, a gradual firing schedule was devised; the kiln was originally set to 100 degrees Celsius, and the briquettes were fired over a two hour and fifteen minute period. Every fifteen minutes, the temperature was raised 100 degrees Celsius, culminating with a final temperature of 650 degrees Celsius. The highest temperature was determined by information on the temperatures for low-fired ancient pottery of the Guangala which estimates temperatures between 600-800 degrees Celsius (8). The lower temperature range was selected because industrial clays often contain fluxes that lower the temperature at which chemical changes occur in firing. Small sections of the briquettes were needed for thin section (.03mm in size) preparation; therefore, a Ray Tech Jem Saw 45 was used to cut the briquettes into flat, rectangular sections. The samples were sent to the National Petrographic Service to be prepared as thin sections, as thin section analysis allows for the identification of minerals and rock fragments. Preparation standards were requested of impregnation with clear epoxy due to the friable nature of low fired ceramics and a coverslip to reduce cost and the difficulties of polishing friable ceramics (Table II).
Table II: A log documenting observations of the experimental samples. The ground pumice samples from Cotopaxi National Park are labelled C97-1, C97-2, and C97-3; the ground burned clay sample is labelled “100”; the El Azúcar 2010 ash fall collection from the Tungurahua volcanic eruption is labelled “Ash Fall”; the ground pumice rock from the Rock Collection is labelled “Rock Box 40”; and ground pumice found in an excavation is labelled B5-9.
Guangala archeological samples were selected from collections of ceramics from a site in El Azúcar Valley. Nine of these samples were selected for analysis. Three were sombreware (black pottery), the Early Guangala fineware, three were bichrome, and three were polychrome, constituting a representative sampling of Guangala fineware pottery. An iPhone with a Proscope
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Micro Mobile was used to photograph the flat side of each piece of pottery. The samples were identified in the sample log under the subsection "archaeological samples" (Table III). Thin sections of each archaeological sample were available and were analyzed under the petrographic microscope, magnification 40x, with crossed polarized light (xpl) as well as plain polarized light (ppl) to identify inclusions. Pumice was identified in all of the archaeological samples. Pumice, as seen in the type samples and also observed in the ceramics, is isotropic black-gray in xpl, but has a bubbled and vitreous as well as transparent appearance in ppl. Other minerals as well as voids (pores) could be confused with pumice due to their black-gray color in xpl. Feldspar exhibits twinning (pieces of feldspar alternately turning black and white as the slide was rotated), and quartz displays extinction (quartz disappearing from view), when the stage is rotated. Voids show no change when the stage is rotated.
Eight photographs of the field of view with pumice for each sample were taken using an Apple iPad (with a styrofoam eyepiece adaptor): two under ppl with 40x, two under xpl with 40x, two under ppl with 100x, and two under xpl with 100x. In addition, the pumice fragments once identified were analyzed for the presence of phenocrysts or volcanic crystals, which according to Ford were present in the volcanic ashfall she identified in Maya ceramics. Attributes of appearance, shape, size, density, and presence or absence of phenocrysts for the pumice inclusions were documented in the sample log spreadsheet, for purposes of relative compositional analysis (Table III). On this basis, the pumice inclusions in the experimental samples (where the source of the temper was known) could be compared to those in the archeological samples to establish or eliminate possible sources of the pumice found in Guangala ceramics.
A similar procedure was followed for examining and recording the test tempers in the experimental fired clay samples in order to allow comparison and help verify the identity of the materials in the archaeological samples. Attributes recorded were limited, however, to appearance, shape, and presence or absence of phenocrysts because we did not control for the size or density of the test materials when they were added to the industrial clay.
The final step in the procedure was to compare the attributes of the pumice inclusions identified and analyzed in the archaeological ceramics with the attributes of the raw materials in thin section and the attributes of the test tempers in the experimental clay briquettes. The attributes in the archaeological samples were also compared with characteristics defined by Ford in order to allow us to test the ashfall hypothesis. The data recorded on inclusions is summarized in Table III and discussed below.
RESULTS
The archaeological and experimental samples provided a window into the composition of Guangala ceramics. All nine archaeological samples contained inclusions of pumice (Table III, Figure 6). All of these inclusions were identified as pumice based on their isotropy and their vacuous appearance with small black “bubbles” which is a part of their larger volcanic “flow structure”; this flow structure is exemplified in Figure 9. Pumice stone itself is characteristic of “flow structure”, yet flow structure is not necessarily indicative of pumice stone. Figure 10 juxtaposes both pumice stone and the burnt clay, both of which demonstrate macroscopic flow structure; notwithstanding, burnt clay proved to be incompatible with the Guangalan temper
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because even though microscopically the burnt clay did demonstrate a flow structure, it was not parallel with the pumice’s and it also was anisotropic.
Table III: A log documenting the characteristics of the archaeological samples as a means of testing against the ashfall hypothesis
All pumice inclusions were evenly spaced, which is also known as “well sorted”, throughout the ceramic paste. Nine of the samples had a pumice density between 0-10%, while
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one had a density of 10-15%. In addition, while the pumice inclusions in the archaeological samples were of variable shapes, mainly subrounded and subangular, the sizes remained fairly consistent, averaging approximately 0.02 mm, (Figure 6). Phenocrysts were present in a minority of the observed inclusions but this was not a common characteristic of the pumice inclusions in the archaeological samples. The material identified in the archaeological samples as pumice or pumaceous ash is a very distinctive material. Pumice is formed from the rapid cooling of gas filled lava. Pumice can be exploded from the volcano or form in beds near a volcano. The materials which travel in the air and are exploded from a volcano can include pumice but also can include a range of “pyroclastic” or exploded molten materials (e.g., crystals of different compositions or phenocrysts, glass shards or ash; 15).
Figure 6. Archaeological samples imaged using optical petrography. Images represent thin sections of Guangala ceramics with inclusions of pumice. (ppl, 1.6mm field of view at 100x)
Looking at the experimental samples (Table II, Figure 7), the first observation was that the control contained inclusions of mica, which can make it difficult to distinguish inclusions due to the bright colors of the mica under the petrographic microscope. A non-mica industrial clay would have been easier to analyze. After careful examination it was determined that the control sample, and therefore, the industrial clay, did not contain pumice. The scoria sample contained iron-rich glass, which indicates that the scoria vitrified during the firing process and it did not have any characteristics which would make it appear to be pumice or pumice-like even though in hand-specimen it has a bubbly, vesicular appearance. The three pumice samples from Cotopaxi National Park underwent the vitrification process during the firing process, but only fused with the clay and were still visible as pumice-like, bubbly with a vesicular appearance. All three samples under the microscope contained identifiable pumice inclusions that were angular and subangular, and were all relatively large when compared to the inclusions in the archaeological samples (>0.02 mm, Figure 7).
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Figure 7. Experimental samples imaged using optical petrography. Left image represents a thin section of ashfall-tempered industrial clay. Right image represents a thin section of industrial clay tempered with pumice from Cotopaxi National Park. (ppl, 1.2mm field of view at 100x)
Figure 8, however, images how pumice can completely vitrify within the firing process. This experimental sample illustrates the flat, glass appearance that some phenocrysts can take, and even though we know it was originally pumice, the image is not indicative of pumice. The burnt piece of mud-brick, although it looked like pumice in hand specimen due to its bubbly, vesicular appearance, did not contain any pumice-like inclusions in the fired experimental clay thin section and instead appeared brown and vitrified in the industrial clay. The sample with ashfall which was collected after “falling from the sky” in El Azúcar after travelling from Tungurahua, a volcano in Ecuador, appeared to be the most promising. However, close observation of this sample in the fired experimental clay thin section revealed that there was no evidence of pumice in the sample and instead there were only pieces of gray clay (Figure 7). It appears that the material as noted in the thin section which was made of this material is primarily glassy ash fragments and phenocrysts but no bubbly, vesicular material is present. The rock which had been identified as pumice in a reference collection from GeoScience Industries called “Rocks, Minerals, and Gemstones” fused with the clay and appeared angular in the sample and did not have the vesicular, bubbly appearance typical of pumice. Finally, a fragment of vesicular like material which could have been pumice found in the Guangala excavation also fused with the clay and did not appear like the material in the archaeological samples.
Figure 8. Experimental sample imaged using optical petrography. Image represents vitrification of pumice. (ppl, 1.6mm field of view at 100x)
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Figure 9: Experimental sample made by Dr. Masucci of Ecuadorian clay and added pumice. (ppl, 3.0 mm field of view at 40x)
Figure 10: Macroscopic images of pumice stone (on the top) and burnt clay (on the bottom), by hand lens.
DISCUSSION
Comparison and analysis of the type, archaeological, and experimental samples reveals a number of insights into the characteristics of Guangala pottery and furthers understanding of the source of the pumice inclusions. The experimental sample with “ashfall” temper (the material which fell from the sky in El Azúcar in 2010 from the Tungurahua volcanic eruption) contained no pumice or material with a vesicular, bubbly appearance, while the archaeological samples all contained pumice (Figure 7). The ashfall consisted primarily of glass and glass shards with occasional phenocrysts such as biotite (15). This decreases the possibility of ashfall as a viable option to explain the pumice inclusions in Guangala pottery. Had ashfall been a source of the archaeological temper material, the sample collected should have had at least some fragments of pumice. Also, the material in the ceramics is only pumice, not pumice mixed with other pyroclastic materials, and therefore the ashfall would have had to be “pure pumice.” This decreases the likelihood of Ford’s hypothesis
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as a viable explanation for the pumice temper in Guangala ceramics and opens the door for alternative hypotheses such as trade.
Pumice fragments found in the Ecuadorian pots were of a relatively consistent small size and similar shape with only extremely small and rare phenocrysts. This clear pattern of size and shape within the different archaeological samples indicates a consistent and similar source of pumice with similar physical properties. This would argue for consistent trade with Highland pumice sources which would facilitate such consistent material size, shape and nature as observed in the archaeological samples. Of course the consistent size is most likely due to the potters’ preparation and selection of a size range but the consistency of the type of material could not be selected out of a mixed source such as an ashfall. The industrial clay used in the project and the clay used by the Ecuadorians were different, as was the firing process. These factors could have accounted for the vitrification observed in some of the experimental samples including the fusing of inclusions to the clay. Chemical interactions due to the chemistry of the industrial clay and the use of a modern kiln could have affected the inclusions differently than those of an open pit firing and the Ecuadorian clays used by the Guangala potters. The different tools used in the crushing of the pumice could have been a factor as well. Regardless, the closest match to the archaeological pumice tempers and our experimental test types both as a raw material in thin section and fired in an industrial clay and observed as thin section were the highland pumice samples from Cotopaxi National Park. Ground commercial pumice added to an Ecuadorian clay from previous research is also a close match. Future studies should include the origin of the use of pumice in the Guangala pottery which could help further the study of its source. It is possible that migrations of highland peoples fleeing volcanic eruptions might have brought use of highland materials to the lowland cultures. In addition, continued investigation into former trade routes of other goods which existed between Highland and Lowland communities could open the door for understanding the exchange of pumice.
CONCLUSION
The Guangala was a group of people that lived hundreds of years ago in southwest Ecuador. Although not much is known about the ancient civilization, most archaeologists believe their use of pottery technology was very advanced compared to other pottery technology used by similar civilizations during this time period. The use of pumice as a temper to make very thin, finely decorated pottery is one of their technological innovations. However, the lowlands in which the Guangala people lived showed no abundant source of pumice. The only possibilities that may explain as to how there was pumice is if the pumice was transported through ash fall from highland volcanoes or if the pumice was traded with the people from the highlands. The contemporaneous people living in the highlands did not produce pottery similar to that of the Guangala and therefore it is unlikely that they produced Guangala pottery to be traded. Because the origin of the pumice is unknown, the goal of the project was to try and determine the source of the pumice. The experiment included eight different substances that could potentially have been the pumice used by the ancient Guangala. Industrial clay was mixed with each substance, and fired to a specific temperature to mimic how the Guangala made their pottery. These
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materials were then compared with the pumice in the archaeological samples using thin section petrography.
The project’s results showed that out of the eight experimental tempers tested, only four were pumice or contained pumice. Three of the four experimental samples came from an extensive pumice deposit in Cotopaxi National Park; the pumice looked very similar to the pumice used in the Guangala pottery. This material is ancient and would have been available to ancient inhabitants of the highlands contemporaneous with the Guangala. However, there were some differences. Although there was pumice, all three samples included subangular-looking pumice, while the actual pumice from the ancient pottery was more subrounded. The pumice shape was very different. The pumice in the experimental fired clay also seemed to have melted into the clay during the firing, while the actual Guangala pumice was not. The clay used in the experiment is not, however, the same clay used by the Guangala people, which may have skewed the results. Further research in this area should mimic exact temperatures for the firing of the clay and the temper while also using attempting to use a clay local to the Guangala area.
Ford examined one explanation, claiming that the ash used in Maya ceramics “fell from the sky”; the hypothesis encompasses the possibility that wind currents carry ash clouds from volcanic eruptions to coastal areas. The 2010 Tungurahua volcano eruption in El Azúcar produced an ash fall that could have carried pumice particles to the Guangala area. The ash was tested during this experiment to see if any pumice was found; there were none, demonstrating that it is highly unlikely that the pumice came from the sky. The most plausible possibility as to where the pumice could have been obtained is trade.
ACKNOWLEDGMENTS
We would like to thank Dr. Maria Masucci, Dr. Adam Cassano, Dr. Steve Surace, Anna Mae Dinio-Bloch, Janet Quinn, New Jersey Governor’s School in the Sciences and Drew University.
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