In case you missed my review of Stephen Weiner's "Microarchaeology: beyond the visible archaeological record" in Geoarchaeology, one mild criticism I voiced was that he devoted a whole chapter (the concluding chapter) to a single technique, FTIR, which is only one of a bunch of techniques that can be useful in characterizing the micro-record. In planning my own research in Pacific coastal Soconusco, however, and partly as a result of some correspondence with Weiner, I later came to appreciate the importance of FTIR for characterizing components of archaeological sediments. The following miniproposal provides part of the rationale for FTIR in archaeology from my perspective.
We did secure funds and a Bruker Alphas portable FTIR was delivered in early August.
Mini-proposal: Rationale for Acquiring a Portable FTIR Spectrometer for IIRMES Archaeometry Research
Hector Neff
Applications of FTIR (Fourier-Transform Infra-Red) spectroscopy in archaeology range from dating (determination of obsidian-hydration rates) to provenance determination (mineralogical characterization of ceramics and lithics and comparison to source raw materials). Although acquisition of a FTIR instrument for the IIRMES archaeometry program could be justified on the basis of potential contributions in any of these areas, the most immediate and compelling need is for a portable instrument that can be used in the field during investigation of prehistoric industrial sites in Pacific coastal southern Chiapas, Mexico. A NSF proposal for this research, which preliminary indications suggest will be awarded, is appended to this mini-proposal. The project budget does not include funds for a FTIR instrument.
Prehistoric industrial activities on Mesoamerica’s coasts
The tropical coasts of Mesoamerica are lined with mangrove forests and estuaries that, while rich in many food resources, are of little use for agricultural production. As a result, once Mesoamerican people became fully committed to agricultural subsistence, probably during the Middle Formative period (~800 -- 400 BCE), human habitation shifted away from the coasts. The coastal margins continued to be utilized for hunting, fishing, and shellfish collecting, but increasingly over time as well for industrial production. Salt extraction is one well-documented activity (e.g., Andrews 1983; McKillop 2007), and several lines of evidence are now indicating that large-scale ceramic production was another such activity. Moreover, excavation data from the Caribbean coast of Belize (Murata 2011) and survey data from southern Chiapas, Mexico indicate that the two activities were often carried out side-by-side, perhaps by the same workers. The intimate connection between the two industries appears to have stimulated technological innovation, perhaps including the invention of an alkaline glaze by Plumbate potters of southern Chiapas (Neff 2010).
Identification of large-scale ceramic production facilities within the coastal wetlands may partially answer a question that has long puzzled Mesoamericanists, namely, why, given the super-abundance of archaeological pottery in post-Archaic Mesoamerican deposits, do surveys and excavations almost never encounter convincing evidence of ceramic production? At least for settlements proximate to coastal wetlands, the answer may be that ceramics were not produced near habitations but instead at special-purpose locations dedicated to industrial production that have rarely been the focus of archaeological investigation (Murata 2011).
The project described in the accompanying NSF proposal seeks to develop a historical record of wetland industrial activity and inland population trends over the past 2000 years in Pacific coastal Chiapas, Mexico. As discussed in the proposal, available evidence suggests several possible "collapses," one at the end of the Formative period (AD 1 - 200), one at the end of the Terminal Classic period (AD 1000 - 1200), and one at the beginning of the Colonial period (AD 1521). There was also, apparently, a dramatic population explosion associated with the Late Classic Plumbate ceramic industry, which eventually exported fancy pots to the farthest corners of Mesoamerica.
In large part, the southern Chiapas project is about building a better chronology of how humans adapted to and exploited the coastal zone over the past 2000 years. The archaeological chronology for Soconusco, like for many regions, is inherently discontinuous, so it is possible that "collapses," "abandonments," and "population explosions" are wholly or partly artifacts of the coarseness with which archaeologists measure time. Much of the planned fieldwork, therefore, focuses on collection of samples for chronometric analysis (radiocarbon and luminescence dating) and measurement of relative population levels (land clearance, intensity of littoral-zone industrial activity).
Additionally, if demographic change was rapid in some cases, can rapid growth or contraction be linked to environmental deterioration or amelioration, population movements, local political events, or organizational or technological innovation, i.e., to circumstances that are potentially observable in the archaeological and paleoenvironmental records? Sensible answers to these latter questions require that field and laboratory research efforts also be designed to infer what specific past human activities were increasing or decreasing in frequency. This is where the need for a portable FTIR instrument arises.
Plan for fieldwork and in-field analysis
As mentioned above, there is mounting evidence that salt production and ceramic production were carried out intensively within the estuarine zone of Mesoamerica’s coasts from the end of the Formative period on. In Soconusco, the focus of this project, more than 30 mounds that appear to fit this “pyro-industrial” characterization have been located within about 40 sq km of mangrove forest. All have abundant clay cylinders that are thought to have been used as supports for brine boiling in the
sal cocida method of salt production (Andrew 1983; McKillop 2002; Murata 2011). Evidence of ceramic production is as-yet more circumstantial, but Plumbate, a technologically unique and widely traded ceramic type of the Late Classic and Early Postclassic periods has been unambiguously matched to raw clays from near the mouth of the Rio Cahuacan, which drains through the estuarine zone (Neff 2002, 2003), and Plumbate sherds constitute the dominant ceramic type at a number of the sites within this zone. Finally, as Figure 1 shows, the sediments making up the mounds is bright red in color, an unmistakable, qualitative, sign of exposure to high heat. These observations together with the relative dearth of domestic debris make a strong
prima facie case that these mounds accumulated as the result of intensive pyro-industrial activity.
Fieldwork on this project is intended both to collect samples from pyrotechnological and other features for chronological analysis and to document functional variability between mounds and between features. An exhaustive inventory of mounds within the region will be compiled from Airborne LiDAR data that were collected on April 30 and May 1, 2011. All mounds will be visited, and those with post-Formative remains will be investigated further with near-surface geophysical survey and subsurface testing.
A magnetometer will be used in gradiometer mode to identify areas on the mounds that might have been exposed to especially intense heat. Ground penetrating radar will be used to gain some preliminary understanding of the vertical extent of these features. Finally, subsurface samples will be collected by a combination of augering and split-core sampling on an evenly spaced grid placed over magnetic anomalies.
Samples obtained by split-core probes will provide charcoal for radiocarbon dating and sediment for luminescence dosimetry. Because ceramics are such a dominant component of these deposits, it is also anticipated that ceramic artifacts will be brought up by the subsurface sampling, and these will be retained for luminescence dating. Although large-scale excavations are not planned, small, 1 x 1 meter units may be excavated in order to obtained charcoal and other materials in secure association with pyro-industrial features.
Another major purpose of the split-core sampling is to obtain materials for archaeological-sediment characterization. A portable x-ray fluorescence instrument will be used for horizontal mapping of elemental concentrations across features detected with the magnetometer and for detecting down-core elemental variation. Fifteen to 20 elements will be measured, but the instrument will be optimized especially for detection of phosphorus, an indicator of organic accumulation (e.g., in middens) and calcium and potassium (e.g., in wood ash). Very high aluminum may indicate deposits of stored potting clay.
The FTIR for which funds are currently sought would be used primarily as a second kind of materials characterization for sediments recovered by the split-core sampling of features detected by geophysical survey. Portable-FTIR complements portable-XRF by providing information about the molecular structure of the deposits, both crystalline and non-crystalline. Since exposure to heat induces little or no change in elemental concentrations measured by XRF but may dramatically alter the chemical bonding of sedimentary materials, FTIR, a molecular-characterization technique, is ideally suited for investigation of pyro-industrial features such as the Soconusco estuary sites.
FTIR is widely used for investigating the firing history of archaeological sediments (Weiner 2010). Perhaps the best-known applications involve complementary use of micromorphology and FTIR to understand the depositional history of caves that were occupied by Paleolithic hominins (e.g., Berna and Goldberg 2008, Goldberg and Berna 2010). More closely analogous to the present study is a study of Late Bronze and Iron Age pyrotechnology at Tel Dor, Israel (Berna et al. 2007). A copy of the report on the Tel Dor study is appended to this mini-proposal.
The basic design of the Tel Dor study (Berna et al. 2007) can be borrowed and implemented in other studies of archaeological sediments associated with pyro-industrial activity. The Tel Dor investigators first characterized transformations of natural sediments from the region as they were subjected to different temperatures in controlled (furnace) and open firings. Changes in the FTIR spectra with increasing temperature could be attributed to breakdown of clay minerals and formation of high-temperature silicate minerals. Applied to sediments from the excavations fire-affected sediments could be easily identified (despite the absence of reddening), firing temperature could be estimated for the fire-affected sediments, and, in combination with micromorphological observations, secondary deposits (e.g., kiln clean out remains) could be distinguished from sediments fired in-situ and left undisturbed. FTIR together with XRF identified sediments associated with bronze smelting in some levels.
In the coastal Soconusco project, baseline data on firing changes in sediments will be generated on samples obtained from the mudflats of the estuary zone, which are the most likely sources of sediments accumulated on the mounds. Samples will be fired to a variety of temperatures in a muffle furnace installed in the project field laboratory, and FTIR and XRF characterization will then be undertaken on the fired and unfired samples. Model FTIR spectra will also be obtained from other materials that might be encountered in subsurface sampling, such as wood ash from burning of mangroves and other available fuels, mixtures of fired sediments and wood ash, sediments that are mixed first with wood ash and then fired, mixtures of fired sediments and midden materials, etc. The library of FTIR spectra of known derivation will serve as a starting point for interpretation of spectra obtained from the archaeological sediments.
Beyond materials identification, the FTIR spectra can be quantified in various ways and the measurements can then be mapped in a mode analogous to mapping of elemental concentrations. This would provide a means to map variables that correlate with firing duration and temperature, proportion of wood ash in the sediments, etc., across the magnetic anomalies selected for subsurface sampling. Positions of IR peaks, ratios of peak heights, and other quantified descriptions of features in the spectra can be chosen so as to optimize their value as indicators of pyro-technological variation. Spectra representing maximally high or low points on the generated surface can then be subjected to more detailed analysis and interpretation. Similarly, down-core variation in quantified spectral features can be used to examine intensity of pyro-industrial activity over time and whether the pyro-industrial activities were episodic or continuous.
A key feature of the overall research design for this project is that collected materials will be analyzed either immediately in the field or within a short time in the field lab. This will allow adjustments to be made as appropriate. For instance, materials that do not match any of the model sediments can be evaluated more carefully both elementally and by comparison of FTIR spectra to available libraries. Preparation of additional model pyro-industrial materials may provide secure identification.
Some technical considerations
A research program stressing rapid in-field measurements requires instruments that are portable and, ideally, battery powered. Our Bruker Tracer portable XRF spectrometer is an example of such an instrument. Field FTIR analysis entails similar demands for durability, portability, battery power, and rapid sample throughput but without sacrificing repeatability of spectral measurements. Both Bruker and Thermo-Nicolet market instruments as “portable” FTIR instruments, and both would be evaluated.
An important consideration in implementing FTIR in a field or field-laboratory situation is whether analyses will be conducted in transmission or ATR mode. The traditional approach, which permits very good stability and reproducibility, is transmission mode. Stephen Weiner (personal communication April 2011) favors this approach and states that sample preparation, which involves pressing a KBr wafer mixed with a small amount of the sample powder, takes only a few minutes.
Andrews, Anthony P. (1983) Maya Salt Production and Trade. Tucson, University of Arizona Press.
McKillop, Heather (2002) White Gold of the Ancient Maya. Gainesville, University Press of Florida.
Murata, Satoru (2011) Maya Salters, Maya Potters: The Archaeology of Multi-crafting on Non-Residential Mounds at Wits Cah Ak’al, Belize. Unpublished Ph.D. dissertation, Department of Archaeology, Boston University.
Neff, Hector (2002) Sources of raw material used in Plumbate pottery. In Incidents of Archaeology in Central America and Yucatan, edited by M. Love, M. P. Hatch, and H. Escobedo, pp 217-231. University Press of America, Lanham, MD.
-- (2003) Analysis of Plumbate pottery surfaces by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Journal of Archaeological Science 30(1):21-35.
-- (2010) Plumbate technology revisited. Physical and Chemical Methods in Archaeology. Proceedings from the 2nd Latin-American Symposium on Physical and Chemical Methods in Archaeology, Art and Cultural heritage Conservation and Archaeological and Arts Issues in Materials Science, Cancun, August, 2009.
Weiner, S. (2010) Microarchaeology: Beyond the Visible Archaeological Record. Cambridge University Press, Cambridge.
