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The Study of Archaeological Floors: Methodological

Proposal for the Analysis of Anthropogenic Residuesby Spot Tests, ICP-OES, and GC-MS

William D. Middleton &Luis Barba &Alessandra Pecci &James H. Burton &

Agustin Ortiz &Laura Salvini &Roberto Rodriguez Surez

Published online: 19 June 2010# Springer Science+Business Media, LLC 2010

Abstract The identification of chemical activity residues on archaeological surfacesrequires the analysis of large numbers of samples, which can be costly and timeconsuming. Researchers wishing to apply sediment chemistry often are confronted witha dilemma of which technique to use and how to accommodate sediment chemistry intotheir budget. We propose an approach to the identification of chemical activity residuesin which semiquantitative spot tests, which are cheap, quick, and easy to apply, are

employed as an initial phase of analysis in order to leverage the results of more time-consuming and costly instrumental techniques. Three examples that pair spot tests withgas chromatography-mass spectroscopy and inductively coupled plasma-opticalemission spectrometry analysis show that spot tests successfully identify areas ofinterest. This approach can save both time and research funds.

Keywords Activity area analysis . Phosphorus . Chemical activity residues.

Spot tests . GC-MS . ICP-OES

J Archaeol Method Theory (2010) 17:183208DOI 10.1007/s10816-010-9088-6

W. D. Middleton (*)Department of Material Culture Sciences, Rochester Institute of Technology, Rochester,

NY 14623-5604, USAe-mail: [email protected]

L. Barba :A. OrtizInstituto de Investigaciones Antropolgicas, UNAM, Mexico, Mexico

A. PecciArchaeometric Laboratory, Department of Archaeology, University of Siena, Siena, Italy

J. H. BurtonLaboratory for Archaeological Chemistry, University of WisconsinMadison, Madison, WI, USA

L. SalviniCentro di Analisi e Determinazioni Strutturali, University of Siena, Siena, Italy

R. R. SurezUniversidad de la Habana, La Habana, Cuba


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Introduction

The archaeological analysis and interpretation of architectural space (the interpre-tation of the function and the patterning of activities within and around buildings and

structures) has traditionally relied on formal architectural analysis, spatial analysis,artifact distribution, paleobotanical and faunal remains, the byproducts of productionactivities, and other physical residues of past human behavior. As early as the 1920s,the potential of chemical activity residues on occupied surfaces was recognized(Arrhenius1929), but chemical analysis was neither widely nor intensively applieduntil the 1970s, when the analysis of chemical residues in floors was introduced toMesoamerican archaeology (Barba and Bello 1978). Since its renaissance inMesoamerica, the analysis of chemical residues in occupied surfaces has become apowerful technique to define the functionality of architectural spaces and has been

applied extensively and increasingly throughout Mesoamerica and other parts of theworld (Barba and Bello 1978; Barba and Denis 1985; Barba and Ortiz 1992;Manzanilla1993; Middleton and Price 1996; Middleton et al. 2005; Snchez andCaabate,1998; Terryet al. 2000; Wellset al.2000).

One result of this renaissance of chemical residue studies is a proliferation ofmethodologies for their detection and interpretation. We see variation in the class ofresidues identified, in the techniques for their extraction and quantification, and insampling strategy. Different techniques entail radically different cost and timerequirements, as well as providing different types and quantities of data. In the face

of such methodological heterogeneity, one might be tempted to ask

what is theright technique?, but this would miss the point: each of the many techniques offersa trade-off between three principal variables: time, cost, and data, so the righttechnique, in fact, often depends on the research question, its data requirements,and the funding available for chemical studies. We think that the question that weshould be asking and the question that we concern ourselves with here, is how canthe multiplicity of techniques for the analysis of chemical residues on occupiedsurfaces be employed to most expediently and economically provide the datanecessary to address the research question at hand.

During the first half-century, in which archaeologists were using chemical activityresidues on floors and other occupied or utilized surfaces (roughly the 1920s to the1970s), the techniques used were largely drawn from soil science (e.g., Arrhenius1929;Cook and Heizer1962, 1965; Heidenreich et al. 1971). These techniques producedaccurate, precise, and reproducible quantitative results, but determinations had to bemade for single elements, compounds, or other properties, one at a time. As a result,the analysis of large numbers of samples for multiple properties was prohibitively time-consuming and costly. This fact, more than anything else, can account for recognizedpotential of chemical activity residues being so sparsely utilized prior to the 1970s.

In the late 1970s, Barba and Bello (1978) introduced the use of chemical spottests to the study of archaeological house floors in Mesoamerica. While these spottests provide only qualitative or semiquantitative results, they can be used toelucidate multiple properties rapidly and inexpensively. As a result, the use ofchemical activity residues saw increasingly widespread use in archaeology.

By the end of the 20th century, advances in instrumentation had made fine-grained, quantitative analysis increasingly feasible. In organic chemistry, advances

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such as gas chromatography-mass spectroscopy (GC-MS) greatly facilitated theprecise identification of organic molecules. In inorganic chemistry, advances such asinductively coupled plasma-optical emission spectroscopy (ICP-OES) made itpossible to make rapid determinations of multiple elements for large numbers of

samples. These advances in instrumentation have opened the field of chemicalactivity residues to scholars from such diverse disciplines as archaeology, chemistry,and soil science. Not surprisingly, they bring with them a range of differenttechniques for the identification and analysis of these residues.

Although there is considerable variation in the techniques used by variousscholars to extract and identify various chemical activity residues, we do not see thisas a major concern. Empirically, many, if not most techniques yield useful,comprehensible results. For the most part, scholars are working with the resourcesthey have at their disposal and addressing specific research questions of interest to

them. We do not see this as an issue of any particular technique being intrinsicallybetter than any other. Instead, we are interested in exploring how differenttechniques can be employed in tandem to maximize data recovery and minimize costand time requirements.

We present three examples in which spot tests are paired(rather than compared)with instrumental analyses. The spot tests, which are quick, cheap, and easy butsemiquantitative, provide a preliminary assay of the sampled area. These results canthen be used to select sub-areas or sub-sets of samples that are likely to provideuseful results with more precise, quantitative instrumental analyses (GC-MS and

ICP-OES). With this staged sampling and analysis strategy, it is possible to samplethe largest possible area with spot tests and to focus the more costly and time-consuming instrumental analyses on the areas where they are most likely to provideuseful data.

Chemical Activity Residues

The utility of chemical activity residues stems from the fact that many humanactivities generate residues that are deposited on the surface upon which the activitytook place. These residues can vary in size from the macro-scale, such as lithicflakes, bone fragments, pieces of plant or animal tissue, wood chips, etc., to themicro-scale, such as ions and molecules in solution and fine particulates produced bysawing, grinding, abrasion, crushing, mechanical dissolution, and so on. Unlike themacro-scale residues which can be physically picked or swept up and discarded,micro-scale residues can be absorbed by and incorporated into the surface on whichthey are deposited (Barba1986; Barba and Ortiz1992; Middleton1998; Middletonand Price1996).

Both organic and inorganic residues gradually accumulate in the pores of thematerial and/or become chemically bound to or complexed with various constituentsof the material (for example, organic colloids adsorbed by clay particles), while areasthat were not in contact with the activity residues remain chemically clean.Although some residues are more labile than others, once the residues have beenabsorbed by the surface, there tends to be relatively little movement outside of thephysical disturbance of the surface itself (Middleton1998). Because surfaces such as

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plastered or beaten earth floors tend to be fairly homogenous in nature, the residueshave the effect of altering the chemical composition of the surface. This is the sameprinciple employed in the analysis of organic residues in ceramics to reconstructpatterns of ceramic use. Pottery vessels are, as plastered floors, originally clean

when they are manufactured. During their use, they absorb the substances that comein contact with them. The chemical analysis of residues in pottery allows theidentification of the substances stored or cooked in the vessels (Condamin et al.1976; Evershed1993; Evershed et al.2002; Evershed et al.2001).

While the detection of chemical activity areas is fairly straightforward, theidentification of the behaviors that generated the residues is not, particularly in thecase of inorganic chemistry. Equifinality, the existence of multiple pathways to the stateresult, say a high concentration of phosphorus, is a problem. Phosphorus, for example, isan extremely good indicator of human activity because it is ubiquitous in the organic

materials that humans use: plant and animal tissue, bone, wood ash,etc. all contributehigh levels of phosphorus to a floor surface. Therefore, multiple properties must beassessed to differentiate between different residue sources (see Middleton 2004). Inorder to make these distinctions, ethnoarchaeological studies are an absolute necessity(Barba and Ortiz1992; Middleton1998,2004; Middleton and Price1996).

Ethnoarchaeological studies in present-day beaten earth and plaster-flooredhouses allow us to match chemical activity residues to the behaviors that generatethem. Food preparation and consumption, for instance, are characterized by the useof substances containing organic compounds such as proteins, fats, oils, and resins,

as well as inorganic compounds such as phosphates, that are absorbed by thesurfaces on which they are deposited. Wood ash tends to elevate concentrations ofpotassium, manganese, sodium, and phosphorus, whereas in situ burning can beidentified through the thermal alteration of clays which enhances the extractability ofiron and aluminum. Ethnoarchaeological studies also provide a caveat to the overlysimplistic interpretation of chemical activity residues. A single space often is usedfor more than one single activityit may be used simultaneously, sequentially, orcyclically for a series of different activities. Therefore, it is a mistake to assume thatthere is always a one-to-one correspondence between a single chemical activityresidue and a single activity (see Dore and Lpez Varela, this volume).

Thus, anthropogenic chemical activity residues can be a powerful tool for theinterpretation of architectural space and the identification of patterns of activity.Numerous studies over the past three decades have validated the approach and havedemonstrated that a variety of analytical techniques can be applied to different typesof samples from interior and exterior surfaces with a variety of treatments, such asstuccoed, plastered, or earthen floors, as well as exterior spaces (Barba1986; Barbaand Denis 1985; Barba and Lazos 2000; Barba and Ortiz 1992; Linderholm andLundberg1994; Lpez Varela et al. 2005; Middleton1998, 2004; Middleton andPrice1996; Middletonet al.2005; Ortiz and Barba1993; Terryet al.2000; Wellsetal. 2000). Nevertheless, chemical activity residues constitute only one set of data,and the interpretation of archaeological spaces must also be based on theinterpretation of other traditional archaeological indicators such as the distributionof ceramics, lithics, animal bones, and botanic remains, the presence of features suchas fireplaces, altars, storage bins, or other structures, and to architectonic character-istics such as rooms size, the presence/absence of windows, doors, and other spaces.

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Sampling and Analysis Problems

The study of anthropogenic chemical activity residues on archaeological surfacesrequires the analysis of a large number of samples. In order to capture the footprint

of all activities, extensive surfaces must be intensively sampled: complete rooms, thewhole archaeological structures, and even whole sectors of a site should be sampled.Depending on the scale of the landscapes, sites, and/or intra-site structures that haveto be studied, as well as the goals of the study, different sampling strategies may becalled for (see Wells, this volume).

When investigating the pattering of activities at a domestic scale, a fairly small-sample interval is called for; studies typically use between 50 cm and 1 m (Barbaand Lazos 2000); however, the number of samples collected for any given spaceincreases exponentially as the sampling interval decreases in size. As a consequence,

with a sample area of any size, it is necessary to analyze hundreds or even thousandsof samples. For instance, in the study of the Oztoyahualco apartment compound inTeotihuacan (Ortiz and Barba1993), close to 500 samples were analyzed. For theTeopancazco compound, in the same archaeological site, more than 800 sampleshave already been analyzed, and the project is not finished yet (Pecci 2000; Peccietal.2010). At atahyk, Turkey, over 650 samples were analyzed (Middletonet al.2005).

Whatever the sample interval is, the number of samples to be analyzed increaseswith the sample density and dimensions of the area to be covered. This also

increases both the money and the time spent in collecting and processing the samplesas well as shipping them if they are not to be analyzed in the field. These economicand logistic factors become important considerations in planning and executing astudy.

A second and even more important issue in the use of anthropogenic chemicalactivity residues for the functional analysis of archaeological structures is theselection of which technique or techniques will be used in the study. Such questionsas: What kind of analysis is the right one?orAre some techniques more useful todetect specific compounds than others?, and Which technique will tell me what Imost need to know? are all extremely relevant, but so too is What do I have thetime and funding to do?.

As mentioned above, different research groups have developed different techniquesin the last decades. Since the late 1970s, the Archaeometric Laboratory of the Institutode Investigaciones Antropologicas at the UNAM, Mexico has focused on the use ofspot test analyses for a variety of sediment attributes that allow the rapid processingof large numbers of samples, in the field if necessary, to study the distribution ofchemical compounds in order to elucidate the different uses archaeological spaces.These spot tests can detect the presence of phosphates, carbonates, fatty acids, proteinresidues, carbohydrates and to measure the pH level (Barbaet al.1991). These studieshave been supplemented, both in Mexico and in Italy, with GC-MS analysis of organicresidues to better understand the origin of the organic acids found with the spot tests(Barba et al.1998; Pecci2004; Pecciet al.2010).

On the other side, Middleton, Burton, Wells, Terry, Parnell (Middleton and Price1996; Parnell et al. 2002; Middleton 2004; Wells et al. 2000; Wells 2004), andothers have applied ICP-OES focused to the analyses of approximately a dozen



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major elements, some of which are particularly useful in the archaeologicalinterpretation. It is a very precise quantitative analytical technique that providescopious data that are amenable to powerful inferential statistical analysis. RichardTerry and his group, in the Brigham Young University have concentrated their

attention on the development of quantitative analysis for phosphates in soil samples,focusing upon the study of large open areas such as plazas (Terry et al.2000).

A Comparison of Techniques

Archaeologists have therefore to make some choices on the number of samples to beanalyzed and on the analytical techniques to be used depending on time, resources,and goals of the project. Below, we compare the various capabilities, requirements,

and drawbacks to three major techniques for the detection of chemical activityresidues. We reiterate, however, that this is decidedly not from the point of view ofany of the techniques being superior to any of the others. Furthermore, this is notintended to be to be a comprehensive comparison of techniques, but rather acomparison of the techniques that we have employed in our research.

Spot Tests

As stated above, spot tests established in Mexico are aimed at detecting the presence

of phosphates, carbonates, fatty acids, protein residues, and carbohydrates at asemiquantitative level, and measuring the pH value (Barba et al. 1991). They aresimple techniques of analysis that can be performed by trained archaeologists andstudents in the field if necessary (in a laboratory exercise, one of the authors has aclass of 20 students that analyze 100 sediment samples and plot their data in a singleclass period using the Eidt Ring Test (Eidt1973)). Spot tests are quick and cheapcompared to other techniques. As a consequence of using spot tests, it is possible toanalyze a large number of samples quickly at a very low cost.

Once the results of each spot test are obtained, they are plotted on the archaeologicalmap of the site or on the architectonic plan of the structure in order to obtain distributionmaps for each compound. These maps show the concentrations or absence of thecompounds. Comparing the distribution maps of all the chemical indicators to thepresence of archaeological materials and the architectural characteristics of the space, itis possible to identify activity areas. Both the presence and absence of specificcompounds can provide useful information on the utilization of space.

The advantages of spot test analysis are evident. On the other hand, they also havedisadvantages such as providing only a relative idea of the abundance of the chemicalresidues. Furthermore, with spot tests, it is feasible to detect organic acids' presence, butit is not possible to identify their origin, and therefore know if they are related to resins,waxes, fats, or oils. Protein residues likewise cannot be related to a particular origin.

In the interpretation of the activities performed in specific spaces, it is importantto correlate the results of all the chemical compounds analyzed. For instance, foodpreparation activity areas are usually characterized by concentrations of fatty acids,protein residues, phosphates, and carbohydrates present in the food that will beassociated with high values of pH due to the presence of ash as a combustion

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byproduct. The animal stables are mainly characterized by the homogenous highvalues of phosphates but low levels of other chemical indicators.

Gas Chromatography-Mass Spectrometry

As discussed above, spot tests cannot give information on the origin of the organicresidues often constituted by complex mixtures. Frequently, it is necessary to identifyeach component of these mixtures, and for this purpose, gas chromatography coupledwith mass spectrometry (GC-MS) is one of the most suitable techniques. This kind ofanalysis allows the separation and identification of many substances, lipids in particular.This permits the interpretations of the residues absorbed in the archaeological floors andceramics, through the identification of specific markers that are characteristics of thedifferent substances (Evershedet al.2001).

For example, it is possible to distinguish between animal and vegetal fats. Inparticular, it allows identifying vegetable oils. Furthermore, GC-MS permits theidentification of markers characteristic of other vegetal substances common inancient European diet such as wine (Guash-Jan et al. 2004) and Brassicaceae(Charters and Evershed1995; Charterset al.1997). The presence of animal stablescan also be detected monitoring the presence of coprostanol (Bull et al. 2003;Evershedet al.2001). On the other hand, Mesoamerican ritual areas were enrichedwith blood and copal. Resins burnt during ceremonies were detected by GC-MSanalysis (Barba et al. 1996). The advantages of GC-MS are therefore evident, and

they are particularly related to the possibilities of knowing the specific substancesinvolved in ancient activities. On the other hand, the main disadvantage of this kindof chemical analysis consists mainly in the complex and expensive instrumentationrequired. Furthermore, the sample preparation is time consuming, as preparing agroup of 10 samples can take 1 day for each extraction, and the analysis of eachprepared sample lasts more or less 1 h (not taking into account the time needed forthe interpretation). Considering that different procedures are often carried out toextract different compounds (Mottramet al.1999; Guash-Janet al.2004), it can bean extremely time-consuming technique. The study of the distribution of organicresidues on archaeological floors requires the analyses of many samples, andprobably for these reasons, no case studies exist as far as we know of this kind ofstudy performed only by GC-MS analysis.

Inductively Coupled Plasma-Optical Emission Spectrometry Analysis

Another approach to studying archaeological floors and the function of archaeolog-ical structures is the use of ICP-OES analysis of an acid extract of floor sedimentsamples. This technique is particularly useful in the identification of anthropogenicchemical residues present in archaeological surfaces (Middleton 1998, 2004;Middleton and Price1996; Middletonet al.2005). It entails the characterization ofa suite of elements that includes both anthropogenic and geochemical indicators,thus making it possible to distinguish between anthropogenic and natural processesin the composition of the archaeological surfaces. A variety of extraction techniquescan be used, ranging from a buffered neutral extraction to a total digestion (Middleton2004), the particular technique employed in a given case depends upon the purpose



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and goals of the study. In the present study, samples were extracted at roomtemperature in a weak acid for the period of 2 weeks (see Middleton and Price1996).

The advantages of this technique is that it is highly sensitive to subtle differencesin the chemical composition of floor sediments, making it possible to distinguish

between a wide range of human activities and to distinguish between humanactivities and geochemical processes. The copious quantitative data produced by thistechnique readily lend themselves to powerful inferential statistical analysis. FactorAnalysis of Principal Components and Numerical Classification recognize patternsof covariance in the enrichment and depletion of elemental concentrations in the datathat help to identify the patterns of activity that took place upon the surface. Both theraw data and derivative statistics, such as factor scores, can be surface plotted toprovide a visual aid to the interpretation of patterns of activity. Both the distributionof the factor scores and raw elemental data often mirror the areas of most intense

activities and reflect activities such as food preparation and washing or thecontribution of human metabolic byproducts in the soil. Because data can rangeover several orders of magnitude, data are usually converted to a logarithmic scalefor analysis and surface plotting (Middleton2004; Middleton and Price1996).

Although a powerful technique, there are limitations to this approach, particularlyin comparison to spot tests in the field. Foremost, the length of time involved insample preparation and extraction would bar this approach for use in the field inmost cases, whereas spot tests can provide immediate results in the field, which canbe used to guide excavation and interpretation. A second and significant drawback is

the cost of the instrument. Beyond instrument cost, however, ICP-OES analysis isfairly economical. Finally, although sensitive to some constituents of organic mattersuch as phosphorus, ICP-OES analysis cannot identify organic compounds. SeeTable1for a schematic comparison of the various techniques.

A Step Further: Integrated Approaches, Examples

Above, we have outlined some of the issues in the identification of chemical activityresidues and their analysis. With these points in mind, we show some examples inthe following paragraphs of how different techniques can be integrated in such a waythat the strengths of one technique (spot tests) can be used to leverage the strengthsof another (GC-MS and ICP-OES) to maximize the data obtained while keeping thetime and expense of analysis to a minimum. These examples also show that whilespot tests are less precise and only semiquantitative, they successfully identify thesame areas of interest that the instrumental techniques do, even if they cannot aseffectively elucidate the activities that created them. The first two examplespresented are related to the integration of spot tests and GC-MS, while the lastone is concerned with the spot tests, ICP-OES integration, and comparison.

Example 1Ritual Activities in Templo Mayor (Mexico)

Among the most impressive discoveries made in Templo Mayor (Mexico City)during the fieldwork in 1978, was the House of the Eagles structure thatcorresponds to Phase VI of the Templo Mayor (ca. 14861502 A.D.) (Barba et al.

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1997; Barbaet al.1998). The House of the Eagle Warriors comprises five main areas(rooms 14, and the patio; Fig.1). The floors of the entire house are made of limeplaster, a mixture of hydrated lime and ground tezontle (fragments of volcanicscoriaceous rock).

Table 1 Schematic Comparison of Spot Tests and Instrumental Analytical Techniques

Spot tests Instrumental analysis

Organic Fatty acids, proteins, carbohydrates GC-MS

Advantages: rapid, inexpensive,high volume, can be done on-site

Advantages: high precision, quantitative,detection of multiple compounds

Disadvantages: single tests,semiquantitative

Disadvantages: low volume, high samplepreparation time, moderate analytical time,moderately expensive, requires laboratory

Inorganic Phosphates, carbonates, pH ICP-OES

Advantages: rapid, inexpensive,high volume, can be done on-site

Advantages: high precision, rapid, detectionof multiple elements, high volume

Disadvantages: single tests,semiquantitative (except pH)

Disadvantages: moderate sample preparationtime, moderately expensive, requires laboratory

Fig. 1 Schematic diagram of the Templo Mayor



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A large number of ancient documents testify the kind of activities and thematerials that might be involved in the activities carried out in this area. For instance,in the Tudela Codex (1980), several individuals are practicing self-mortification infront of a deity, inserting the blooded spines in a zacatapayolli and burning copal

(Bursera jorullensis) in the braziers (Figs. 2and3). This historical information inconjunction with an accurately documented excavation and well-preserved floors,provided a unique opportunity to test the hypotheses on the rituals carried out in thearea, to identify the precise areas where they took place and to identify thesubstances used by the priests.

Samples were collected from the plastered floors of each of the four rooms. Allthe samples were analyzed with spot tests carried out in the ArchaeometryLaboratories of IIA-UNAM following the procedures established by Barba et al.(1991). The results of the spot tests (Fig. 4) show that one of the intensively used

areas was located in front of the main altar in room 2. Here, high contents of lipidswere probably related with the burning of resins in front of the main altar and in thebraziers. Other areas in which spot tests have high values are the entrances. In rooms3 and 4 and in the patio, the highest values are directly associated with the locationof the braziers and altars. In some of these areas, the presence of fatty acids isstrongly related with the distribution of other organic residues, such as carbohydratesand proteins. This is probably due to the blood offerings. In general, the highestlevels of fatty acids and other chemical indicators were found in the areas around thebraziers and in front of the ceramic sculptures and the zacatapayolli representations,

where rituals should be carried out.

Fig. 2 Aztec ritual, Tudela Codex

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Based on the results of the spot tests and the distribution maps, 20 samples wereanalyzed with GC-MS at the Laboratorio de Cromatografa de Gases of theDepartment of Analytical Chemistry of the UNAM following the methodologydeveloped in Evershed's laboratory in Bristol (Evershed1993). The GC-MS analysispermitted the identification of differences between the organic acids found in themain altar and those found at secondary altars, suggesting that different materialswere involved in specific rituals. The residues present in the samples taken from theclose to the main altar were characterized by high molecular weight and wereidentified as copal residues (Barba et al. 1996). The samples recovered fromsecondary altars showed, in general, fatty acids with lower molecular weight.

The chemical analysis of the floors of the House of the Eagles showed thepresence of three main areas for ritual. They were located in front of the altars,around the braziers, and in the entryways, where clay figures representing EagleWarriors and Mictlantecuhtli were found. In those places, ritual activities involvedthe spilling of fluids on the floor, whose chemical compounds were absorbed andfixed and whose analysis permitted the reconstruction of past human activities.

Furthermore, the analyses carried out at the Templo Mayor of Tenochtitlanshowed that where spot tests indicated the presence of high concentration of lipids infront of the main altar, GC-MS analyses confirmed the data and showed that the fatswere mainly due to the burning of copal and other resins, confirming the ritualfunction of the area investigated. Due to the order used for applying combined spottest and GC-MS, it was possible to determine the relative content of organic acids inthe 500 samples, to represent their distribution, to select eight of the samples for

Fig. 3 Self mortification, Codex Maglabechiano



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further analysis depending on the chemical results, and to conduct a very specificstudy to verify the presence of copal.

This is an example of the use of spot tests for the study of many samples, which isnecessary for the study of activity areas, but also for the screening based on chemicaldata of samples that need more specific analyses.

Example 2Food Production Activities in Donoratico (Italy)

In Italy, an integrated approach that used spot tests and GC-MS analyses has beenapplied in the study of several sites, in order to understand the function of differentareas. At first, ethnoarchaeological cases were studied (Pecci 20032004), in orderto understand how different activities could be related to chemical traces, following

Fig. 4 aDistribution of fatty acids. bDistribution of phosphorus

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the methodology established in Mexico at the end of the seventies (Barba and Bello1978; Barba and Denis1985).

There was a need to go back to this kind of studies because the majority of thesubstances used and consumed in Precolumbian Mesoamerica were quite different

from the European ones, so that some of the enrichment patterns established forMexico might have different interpretations in Italy. For example, wine and olive oilthat did not exist in Mesoamerica were widely produced, consumed, and stored inEurope. For this reason, modern production areas of these products were sampled.Stables were also sampled, as animal breeding was an important activity in Europe.Furthermore, some experiments were carried out cooking different meat broths andvegetable soups (traditionally Italian) in unglazed coarse ware ceramics (Pecci20032004). All the samples were analyzed with spot tests at the ArchaeometryLaboratory of the Department of Archaeology of the University of Siena. Some of

the samples were recovered from the wine and olive oil production areas, and all theexperimental samples were analyzed also with GC-MS at the Centro di Analisi eDeterminazioni Strutturali (CIADS) of the University of Siena. Comparing theresults of the two approaches, it is interesting to note that when the presence oforganic acids was detected with spot tests, the analyses performed with GC-MSconfirmed it; and when no fatty materials were detected with spot tests, the analyseswith GC-MS again confirmed their absence.

One of the projects in which the systematic combination of the two approacheswas applied was the study of some structures of a Medieval Castle in Central

Tuscany, Donoratico (Bianchi 2004). The site was excavated under the scientificdirection of Prof. R. Francovich and the field direction of Prof. G. Bianchi of theDepartment of Archaeology of the University of Siena, as part of the MedievalLandscape Project.

Here, several rooms couldn't be interpreted solely on the basis of the excavationdata and the study of recovered materials such as ceramics, metals, and botanic andanimal remains. Chemical analyses of floors were therefore carried out (Pecci2004).As in the Templo Mayor case study, spot tests were carried out on all the samplesrecovered from the floors of the rooms. Some of the samples that were analyzed withthe spot tests, which showed to be enriched in lipids, were selected and subjected toGC-MS analyses.

The results obtained for the study of a cellar room of the twelfth century,characterized by a small channel that lead directly outside the site defensive wall,were particularly interesting (Fig.5). The room was at first interpreted, based on thearchitectural characteristics, as a possible stable or a cellar to store food, in particularolive oil. The presence of the channel suggested that it was used to get rid of thewaste and that the activity carried out in the room was probably a dirtyone. Theresults of the spot tests were consistent with the hypothesis of a stable, because highvalues of phosphates were present (Fig. 6). However, organic acids and proteinresidues were present as well, which is not typical of this kind of space (Fig.7).

Five samples were therefore selected for the GC-MS analysis in order to identifyexcrement markers, such as coprostanol (Bull et al.2003; Di Pasqualeet al.2010)or/and olive oil markers, such as oleic, azelaic, and 9,10-dihydroxyoctadecanoicacids (Condamin,et al.1976; Dudd,et al.1998). The analysis with GC-MS on thesamples didn't show the typical compounds that are usually found in stables (such as



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coprostanol), nor olive oil traces, as C18:1 was very small, C16:0 and C18:0 werethe main components of the chromatograms and no azelaic nor 9,10-dihyroxyocta-decanoic acids, usually considered the markers of olive oil, were identified (Fig.8).These results, together with the presence of cholesterol, indicated on the contrarythat the fatty acids had an animal origin and suggested that the room was a foodpreparation or production area. Here, animals were probably killed and/or someanimal products, such as meet or cheese, could have been prepared and the wastethrown out of the room through the channel.

In the same site of Donoratico, a Late Medieval round structure was sampled andanalyzed to support the archaeological hypothesis that it was a device for oilproduction (Fig.9). In order to test this hypothesis, not only the structure, but alsothe floor around it was sampled, as the sampling of modern oil mills showed that oilproduction also leaves traces on the floors (Pecci20032004). The spot tests didn'tshow any presence of fatty acid in the plaster of the structure and of the floor aroundit (Fig. 9). The GC-MS analysis of four samples confirmed the absence of fattyacids, and in particular, of olive oil (no relatively high values of C18:1, no azelaic,nor 9,10-dihydroxyoctadecanoic acids were identified). Therefore, the structureprobably was not used to produce olive oil (as originally thought) but for somethingelse. The absence of lipids in the samples shown by the spot tests and the GC-MSanalysis suggests the grinding of some substances that didn't leave fatty residues.

Fig. 7 Distribution map of the results obtained with spot test for the determination of organic acids in thesamples of the cellar



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Fig. 9 Possible mill and distribution map of the results obtained with spot test for the determination oforganic acids in the samples of the mill and the floor around it

Fig. 8 Chromatogram obtained with the GC-MS analysis of one sample of the channel of the cellar

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These data, together with the form of the structure, suggest that it might be a mill forflour production.

The entire site of Donoratico is under investigation, but these examples alreadyshow how the information coming from different approaches can help in

understanding the use of some rooms, and more in general, some aspects of thelife of the ancient inhabitants of Donoratico and their economy. The results obtainedsuggest that the two different approaches to the study of the organic acids present inarchaeological materials (spot tests and GC-MS analyses) are complementary andcan be successfully combined in archaeological research for the study of both floorsand ceramics. Again, as in the Templo Mayor case study, the fact that there was anorder in the application of the techniques (at first, all samples were analyzed withspot tests, and afterwards, only some of them were tested with GC-MS) gave goodresults. Additionally, it is important to stress that the results show that absence of

residues is an important archaeological indicator, as is their presence.

Example 3Muxucuxcab (Yucatan, Mexico)

The third case study that we present concerns the comparison of the data obtainedwith the analysis of samples carried out with spot tests and ICP-OES. In this case, allthe samples were analyzed with both techniques. As stated above, in order to verifythe reliability of spot tests for the identification of phosphate and carbonate analysisand to eventually establish the relationship between the two techniques, the same

samples were analyzed with semiquantitative techniques (spot tests) to determinepH, carbonate, and phosphate content at the UNAM and with ICP-OES at theLaboratory for Archaeological Chemistry at the University of WisconsinMadison.Here, the samples were analyzed following the multi-elemental protocol establishedthere: room-temperature, weak acid extraction process followed by determination ofthe elemental concentrations using ICP-OES. Elemental concentrations for elementsthat have proven to be strong anthropogenic indicators (e.g., Na, Mg, Ca, P) as wellas elements that are more reflective of geochemical processes (e.g., Al, Fe, Mn) weredetermined.

The Archaeological Prospection Laboratory of the UNAM in the Ninetiesundertook an ethnoarchaeological study of a recently abandoned household inMuxucuxcab, Yucatn, Mexico (Barba et al. 1995). The study site is a largedomestic compound. It has been occupied on-and-off since the colonial period andwas finally abandoned about 30 years ago. During its occupation, a number ofstructures were constructed, none of which were simultaneously occupied (Fig.10).This study focuses on the southwest quadrant of the compound, which had beenmost recently occupied (Fig.11).

Pierrebourg excavated the site and collected sediment samples for chemicalanalysis. Samples were collected at a 2-m interval for exterior spaces and a 1-minterval for interior spaces. In addition, the former residents of the household wereable to provide a descriptive narrative of life and activities in the household. Withthese data in hand, it was possible to assess the impact of formation processes onboth the material and chemical archaeological records and the fidelity with whichthese records could be interpreted. Years later, the same samples were sent to theLaboratory for Archaeological Chemistry at the University of WisconsinMadison,



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where James Burton analyzed the samples following the multi-elemental protocol.Together, the results provide us with a good example of how specific activitiesgenerate identifiable residues.

Fig. 10 The Muxucuxcab study site

Fig. 11 The sampled area at Muxucuxcab

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The distribution of artifacts recovered generally follows the pattern of chemicalenrichment of pH, carbonates, and phosphates. pH values are elevated, that is, morealkaline, around the structures, particularly the kitchen area, and in the washing area(Fig.12). Sources for pH enrichment include wood ash, lime, or calcium hydroxide

used in processing maize (nixtamalization). The pattern for phosphates' enrichmentshows that the space around the structures, the washing areas, and the latrine haverelatively high levels, as does a peripheral area at the eastern edge of the sampledarea. Sources for phosphate enrichment include organic matter and human metabolicbyproducts. Organic matter often accumulates in peripheral areas, and this area alsois adjacent to the latrine, so the distribution isn't surprising. Carbonates are enrichedin the areas around the structure, the washing area, and the latrine. Much of thecarbonate enrichment is probably due to nixtamalization and residue from washing.

In general, patterns of chemical enrichment are clearer in the areas of more

intense human activity. The patterns of chemical enrichment also show that exteriorspaces around the structures are more intensively enriched than the interior spaces.This pattern, although it might be surprising, is quite typical of many of theethnoarchaeological study sites that have been investigated by the UNAMProspection Laboratory (Barba and Ortiz 1992), particularly when house size isrelatively small. Also in Neolithic China, the same pattern can be observed(Middleton et al. 2010). This is a clear indication that exterior spaces must be asintensively investigated as interior spaces in domestic archaeology.

While not identical, UW results fairly closely follow those patterns found in the

UNAM study. Typically, an ICP-OES analysis is carried out for concentrations of 12elements, but problems with the instrument forced to use only nine at that time:aluminum, barium, calcium, iron, magnesium, manganese, phosphorus, strontium,and zinc. This is unfortunate because several of the excluded elements areparticularly useful anthropogenic or geochemical indicators. The pattern of calciumenrichment (Fig. 13) rather closely mirrors that of carbonates and is reflectingessentially the same factors. It shows that the main residue is calcium carbonate as abyproduct of the use of calcium hydroxide as lime during the use of this space. Limewas used to soften the corn kernels, and the calcium hydroxide solution was spilledover the floor. Lime was also used as a powder over the fecal materials in the latrine,and finally, lime was a byproduct of the clothes washing process.

Fig. 12 Spot test results, pH, phosphate, and carbonate



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Phosphorus enrichment does not follow that of spot test phosphate that well(Fig.13): phosphorus enrichment is more tightly restricted to the kitchen, washingareas, and the latrine. Sources for phosphorus enrichment are the same as forphosphate enrichment, but a difference in the extraction process is likely selectingphosphorous that is bound differently in the soil matrix.

A detailed examination of each element and its patterns of enrichment would betoo long, but when we look at their distributions, we can see some interestingpatterns. Some elements, such as strontium and magnesium, have a fairly strongpattern of covariation with calcium (Fig.14). They are also related with the chemicalsimilarities of these elements that reflect the intense levels of activity in the areasaround the kitchen, washing area, and latrine: in this case, most likely associatedwith food residues. Other elements, such as iron and zinc have entirely independentpatterns of enrichment (Fig. 15). Iron is relatively depleted in and around thestructures and washing area, probably reflecting sediment deflation due to heavytraffic, while zinc is clearly reflecting activities in the latrine. Finally, aluminum andbarium appear to reflect primarily geochemical processes. Their patterns ofenrichment appear to be following sediment characteristics rather than any humanactivity. These patterns can help us to understand natural variation in the sedimentthat might have an influence on the anthropogenic residues that we detect.

As stated above, although multi-elemental characterization is more expensive andtime-consuming than semiquantitative techniques, one important asset is that it

Fig. 13 ICP-OES results, calcium, and phosphorus

Fig. 14 ICP-OES results, strontium, and magnesium

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provides copious data that are amenable to powerful inferential statistical analysis,such as cluster analysis and principal component analysis, which explore relation-ships between multiple variables that are difficult or impossible to identify throughdescriptive statistics alone. In this case, Principal Components Analysis allows us toidentify three principal components that account cumulatively for over 90% of theobserved variation. Principal component one is determined by the covariance ofcalcium, magnesium, phosphorus, and strontium (Fig. 16). The distribution of thefactor scores, clearly mirrors the areas of most intense domestic activity and reflectsuch activities as food preparation, cooking, and washing. Principal component twois determined by the covariance of aluminum and barium (Fig. 16). The distributionof factor scores here is most likely reflecting the composition of the parent sediment.Principal component three is determined by zinc alone, and the distribution of factorscores appears to reflect the contribution of human metabolic byproducts to thesediment (Fig.16).

Although the correspondence between spot test and ICP-OES analysis isn'tperfect, both techniques are sensitive to the same chemical residues and provideessentially similar results. The lack of perfect correspondence between thesemiquantitative and quantitative results deserves a more thorough study, particu-larly looking at mechanisms of extraction and how these might be influencing the

results. This study also demonstrates the importance of both ethnoarchaeological

Fig. 15 ICP-OES results, iron, and zinc

Fig. 16 ICP-OES results, principal components factors 1, 2, and 3



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studies in sediment chemistry and the utility of making determinations of multiplesediment properties.

Discussion

Experimental, ethnoarchaeological, and archaeological studies have shown that theresults obtained with the organic acid spot tests and results of the GC-MS analysesare consistent with one another (Barba et al.1996; Lazos1997; Pecci20032004).In particular, the analyses carried out at the Templo Mayor of Tenochtitlan, in theHouse of the Eagles showed that where spot tests indicated the presence ofconcentration of organic acids in front of the main altar, GC-MS analyses confirmedthe data and showed that these concentrations were mainly due to the burning of

copal and other resins. This confirmed the ritual function of the area investigated.In Italy, ethnoarchaeological studies of olive oil production areas, experiments inthe cooking of various meat broths and vegetables, the analysis of archaeologicalsamples taken from archaeological floors and ceramics, in particular at Donoratico,gave similar results. Here as well, GC-MS analyses confirmed the presence of fattyacids, as indicated by the spot tests and specified their origin. Furthermore, areas inwhich had been provisionally identified as having been used for olive oil productioncould be eliminated through their lack of organic residues. The comparison betweenspot tests and ICP-OES showed that a good correspondence exists between

carbonates and calcium concentrations. Furthermore, the same disposal areas wereidentified by both techniques using differentmarkers: phosphates for the spot testsand zinc and phosphorous for the ICP-OES. In all cases, the identification of theanalysis of residues was useful in the interpretation of the function of thearchaeological sites.

The stepwise application of spot test and more intensive instrumental analysisappears to work well. In both cases, the results of spot tests were confirmed andfurther elucidated through both GC-MS and ICP-OES analysis. The use of spot testsprovides useful data that can be obtained, if necessary, during field operations;instrumental analysis can provide further insights into the sources of the residuesidentified through spot tests. The lack of perfect correspondence betweensemiquantitative phosphate analysis and quantitative phosphorous results deservesa further study.

In general, these examples show that human activities leave detectable andidentifiable residues in the material upon which they take place and that theirchemical analysis can therefore be an extremely powerful tool in the interpretation ofthe archaeological record, particularly if no other artifactual remains are available. Asecond point is that both semiquantitative spot tests and GC-MS and quantitativeICP-OES provide powerful insights into the nature of anthropogenic chemicalresidues. While the precision of quantitative techniques is extremely useful, thesetechniques are more expensive, time-consuming, and often impossible to apply inthe field.

The ICP-OES analyses are very efficient to determine the chemical enrichmentwith inorganic residues and are especially good to determine metallic particlesproduced by pre-industrial production activities. In addition, it provides reliable data

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for statistical analysis for more specific information. GC-MS is very important foridentifying the specific origin of organic residues, particularly of food residues.

Spot tests are inexpensive, rapid, and can be easily performed in the lab or thefield. A number of scholars have published important results obtained through field

analysis (e.g., Terry et al. 2000). The results of pH, phosphate, and carbonatedeterminations together with organic residues tests actually support essentially thesame inferences as those obtained through multi-elemental characterization. In someother cases, spot tests (especially those that detect organic compounds) could giveinformation that is complementary to the ICP-OES data.

Conclusions

As a conclusion, we can suggest that the ideal investigation requires theintegration of different techniques for the chemical analysis of archaeological floorsthat will allow us to achieve as much information as possible, with the optimumcosts and time. This is related to the financial resources of the projects, to theavailable analytical instruments, the qualified personnel, the time allowed, and, ofcourse, the specific archaeological question.

Our proposal suggests an order with which the different techniques should beapplied. Simple techniques that allow the analysis of many samples with low costsand short time should be applied first and, if possible, in the field. All the samples

from the entire investigated area should be analyzed with these techniques. Oncepatterns have been recognized and hypotheses have been established, they can betested applying more sophisticated instrumental analyses.

ICP-OES analysis should be applied in order to obtain information about theinorganic elements and their distributions. If possible, all the samples should also beanalyzed with this technique. If this is not possible, it would be particularly useful tofocus on specific areas that have been identified through spot tests as having a highpotential for yielding useful results, for example, areas in which metallic ions couldbe present, such as in production areas or in ritual areas (cinnabar has been found inritual Mayan areas) and in the study of refuse patterns (as shown above, zinc isrelated to latrines, refuse areas are rich in phosphorous). Terry et al. (2000)suggested that market residues could be identified, providing information concerningthe use of these large open areas.

When archaeological problems deal with organic materials, GC-MS should becarried out on specific samples in order to answer questions arising from spot testanalyses. As stated above, this analysis is particularly useful in identifying thepresence or absence of specific organic substances such as resins, food residues,fecal material,etc. Although spot tests give reliable information on the function ofthe structures and on the spatial distribution of activities, the interpretation is moreprecise when the analysis is combined with the GC-MS data.

To conclude, another important point that must be stressed is that theinterpretation of anthropogenic sediments must be empirically based on ethno-archaeological and experimental studies in which the patterns of residue formationcan be documented. Initially, sediment residue studies featured post hoc interpre-tations of observed patterns of variation without empirical verification, what we



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might describe as a sort of laboratory-based armchair archaeology. You can't reliablyidentify an activity if you don't know how the activity generates residues and howthose residues are incorporated into the porous materials. This is why ethno-archaeological studies should have important part in archaeological investigation.

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