ORIGINAL PAPER

Multiple assessments of local properties, production,and performance in metal objects: an experimentalcase study from Petra, Jordan

Krysta Ryzewski & Brian W. Sheldon & Susan E. Alcock &

Max Mankin & Sugeetha Vasudevan &

Nicholas Sinnott-Armstrong

Received: 29 July 2010 /Accepted: 11 January 2011 /Published online: 22 March 2011# Springer-Verlag 2011

Abstract This object-specific case study focuses on cupreousartifacts excavated from the Great Temple complex of Petra,Jordan to demonstrate how the use of compositional X-rayanalyses alongside two experimental applications (ImageJsoftware and nanoindentation) have the potential to generatedifferent and otherwise unobtainable information aboutarchaeological metals. The study highlights the value of usingmultiple techniques as a means of resolving the ambiguitiesthat tend to arise from interpretations of single-sited measure-ments on objects and from single-instrumental analysesduring studies of production processes and consequentmaterial performance. Employing different techniques onmultiple localities within a sample permits the gathering ofprecise information about the behavior of and interrelation-ships between variables that affect the objects’ fabrication anduse, particularly composition, structure, and hardness proper-ties. The resulting data are interpreted in association withcontextual archaeological information from Petra to considerthe use-life and potential significance of these objects.

Keywords Material properties . Copper alloys . Digitalimage analysis nanoindentation . Petra

Introduction

Attempts to examine themanufacture of archaeological metalsbased primarily on elemental composition, constituent impu-rities, and microstructure are often complicated by thecomplex chemical and structural transformations that werecaused by the alloying, annealing, and working processes ofproduction, as well as by post-depositional conditions (Galeand Stos-Gale 1992; Henderson 2000; Rohl and Needham1998). As a result, studies of materials fabrication demandmultiple analytical techniques; this demand is especiallycritical in the absence of data linking excavated metal objectsto particular sites of forging or ore extraction (La Niece et al.2007; Ryzewski 2008).

Even when multiple characterization techniques areemployed, it can be extremely difficult to relate the dataobtained to specific aspects of the production process. Forexample, the size of the metal grains in a given specimencertainly reflects the annealing and working conditions thatshaped the object. There is a considerable body of materialsscience knowledge that has established quantitative relation-ships between forming conditions and grain size (Reed-Hill andAbbaschian 1992). However, thestudyofarchaeologicalobjectsrequires researchers to address the inverse problem—that is, byknowing the grain size and other measurable characteristics ofthe object, is it possible to “go backwards” to obtain insight intothe forming conditions or production processes?

A second related challenge is that researchers are oftenfaced with trying to understand a given manufacturing processwith only a limited number of archaeological specimens. Our

K. Ryzewski (*) : S. E. Alcock :M. Mankin :N. Sinnott-ArmstrongJoukowsky Institute for Archaeology and the Ancient World,Brown University,60 George St, Box 1837, Providence 02912 RI, USAe-mail: Krysta_Ryzewski@brown.edu

K. Ryzewski :B. W. Sheldon : S. VasudevanSchool of Engineering, Brown University,Providence, RI, USA

M. MankinDepartment of Chemistry, Brown University,Providence, RI, USA

Archaeol Anthropol Sci (2011) 3:173–184DOI 10.1007/s12520-011-0051-6

premise is that significant information about the fabricationprocess and use-life can be obtained by studying variations incomposition, microstructure, and properties locally within asingle object. This, we argue, requires enlisting multipletechniques in conjunction in order to gather data that articulateinterrelationships between these variables. In exploring thepotential for usingmultiple, spatially resolved characterizationtechniques on individual objects, this paper presents anintensive examination of two cupreous nails excavated fromRoman Period deposits in the Propylaeum of the GreatTemple in Petra, Jordan.

Several methods were used to document the spatialcorrelation of mechanical, optical, and structural propertiesof these metals, including nanoindentation, optical imagingin-tandem with ImageJ software, X-ray diffraction (XRD),X-ray fluorescence (XRF), and energy dispersive spectros-copy (EDS). These methods were selected because therelevant performance of architectural hardware, in general,and nails, specifically, is largely determined by composition,microstructure, and mechanical properties. The resulting dataon composition, structure, grain size, and hardness present adetailed account of the reasons for and implications of themetals’ material properties and performance during produc-tion and use. A series of subsequent questions are raised fromthe data about the objects’ function, techniques of production,and reasons for the metalworkers’ and architects’ choice of thematerial for these nails in relation to both their utilitarian andnon-utilitarian significance. In addition to introducing nano-indentation as a valid approach in archaeometallurgicalresearch, this case study offers information that will be usefulfor further analyses and comparisons of cupreous materialsassociated with the administratively, ceremonially, and rituallysignificant Great Temple complex located in the city center ofPetra. Not least it will ask: why make architectural hardwareout of a “soft” material?

History and industrial archaeology of Petra

Petra is located in southern Jordan, 80 km south of theDead Sea and 400 km directly east of Cairo. In antiquity,Petra was, for a period, a powerful and prosperousmetropolis owing to the city’s position at the nexus oftwo major trade routes: an east–west route that connectedthe Mediterranean in the west to partners in the east,including India and ultimately China, and a slightly shorterbut no less important north–south route, which connected‘Aqaba, to Petra’s south on the Red Sea, with Damascus(Kennedy 1924; Joukowsky 1999; Young 2001). As aWorld Heritage Site (and as a recently elected ‘NewWonder of the World’), Petra is best recognized for theelaborate façade of El-Khazneh (the “Treasury”) andnumerous other tomb facades carved into the rose-red cliffs

encircling the site. Petra underwent a remarkable urbanefflorescence when it became capital of the Nabataeankingdom, a significant political entity in the power politicsof the ancient world during the last centuries BCE/firstcenturies CE. The Nabataeans were annexed by Rome in106 CE, and Petra—if less important in political andeconomic terms—remained a major center of activity untilat least the sixth century CE (Chancy and Porter 2001;Hammond 1960; Joukowsky 1998; Joukowsky 1999).Archaeological excavations at Petra, conducted by BrownUniversity since 1993, have primarily focused on the GreatTemple and structures in its immediate vicinity, which wereconstructed, renovated, and used during the Nabataean andRoman Period (roughly from the first century BCE throughthe second century CE).

Major industrial operations were required to supportPetra’s infrastructure and to sustain the production offoodstuffs and other materials necessary for the city’ssurvival and prosperity. Evidence of extensive hydrologicand irrigation systems, as well as large-scale metalproduction operations have, for example, been exploredin considerable archaeological detail in the proximateWadi Faynan and Wadi Dadi regions (Grattan et al.2007; Hauptmann 2000; Hunt et al. 2007). Extensivearchaeological research has determined that mass-scale copperore extraction and metalworking were thriving industrialoperations, especially during the Roman occupation ofsouthern Jordan (Gale and Stos-Gale 1982; Healy 1978;Hong et al. 1996; Muhly 1993; Patterson 1971; Zwicker etal. 1985). In the vicinity of Petra, for example, during theRoman Period, over 250 copper mines were worked in theore district near Khirbet Faynan alone (Hauptmann 2000,2003), including the Umm al-‘Amad, one of the largestknown and best-preserved mines in the Roman empire(Craddock 2007; Grattan et al. 2004, p. 99; Hauptmann andWeisgerber 1992; Weisgerber 2003). Once mined, the copper(ore) was prepared for working via extraction or smelting; itwas heated and mixed into alloys such as arsenic bronze, tinbronze, and brass and hammered and/or cast by Romansmiths into a finished object.

Craft production and water management strategieswithin the city of Petra itself were undoubtedly visibleand important parts of daily life within the city (Ortloff2005), but far too many aspects of these systems, remainarchaeologically underexplored. To date, only a singleexample of metallurgical production activities has beendocumented within the confines of Petra itself; recentexcavations of the Upper Temenos zone of the GreatTemple located an unusually high concentration of slag,metal waste, and copper ore, suggesting that metalprocessing activities occurred within the Temple complexitself, not far from where our sample nails were excavated(Hussein 2010:33).

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Samples

Two large nails, both approximately 10 cm in length and0.5 cm in width, were selected from the Petra metals collectionfrom the 2003BrownUniversity excavations. Of the 481metalobjects in the 2003 assemblage, 52were copper or copper alloyand the remainder were iron. The majority of iron artifactswere architectural hardware, including nails and hinges, whileall of the copper alloy objects were indeterminate fragments—with the exception of the two sampled nails. The two nailswere selected because they were among the only metals in theentire assemblage with interiors that were not partially orcompletely oxidized by corrosion processes. These samplesare highly intriguing, considering that copper and copperalloys are highly ductile and can be easily bent, making them aless-than-pragmatic choice to use for seemingly utilitarianobjects, like nails, designed to provide structural support.

Both of the nails were excavated from trench 95, locus 10of the Propylaeum, the monumental entrance and staircaseapproach to the Great Temple complex from the main urbanthoroughfare of Petra. The Propylaeum underwent multiplerenovations throughout its history, but the 2003 excavationsdetermined that major revisions to the Propylaeum architec-ture occurred during the Roman Period (Joukowsky 1999).The context of the nails places them in a period of majorarchitectural modifications to the structure following theRoman annexation of 106 CE (Joukowsky 2007, pp. 73–74).

The first sample, nail 1, was bent, while the second, nail2, was slightly bent. In order to illustrate the potential ofusing traditional and new techniques in tandem to examinecomposition, structure, and mechanical properties, intensivesite-specific data were gathered from these two samples bycutting four longitudinal cross-sections across each nail,resulting in eight individual samples named according totheir position within each sample (i.e., sample 1.1 is the

cross-section closest to the head of nail 1, sample 2.4 is thecross-section at the tip of nail 2; Figs. 1 and 2).

The eight cross-sections were mounted, ground, andpolished in cylindrical resin blocks. Small metal disks wereattached to their underside to secure the samples to amagnetic plate in the nanoindenter. Each sample was etchedfor 3–10-s, as necessary in a FeCl3/HCl solution (adaptedfrom Petzow 1978) in order to achieve high contrastbetween grain boundaries.1 Initially, the composition ofeach sample was presumed to be bronze based on visualinspection of the exterior surfaces, but analyses determinedthat the two nails were different in composition from eachother (Hussein 2010, p. 37). Due to their compositionaldifference, the samples could not be directly compared withone another. Instead the analyses focused on each nailindividually to examine how microstructural trends existedand varied between the samples.

Methods, research focus, and data

Following sample preparation, five complementary techni-ques were used to examine the nails’ properties and how theirproperties affected performance in production and use: XRF,XRD, SEM-EDS, optical imaging, and nanoindentation. Inconjunction, these techniques facilitate the examination ofinternal variations along the length of specific nails. Further-more, the same sample preparation procedure—consisting ofmounting, polishing, and etching—yields specimens whichcan be readily subjected to each of the above methods ofanalysis. These methods were selected to investigate the

Fig. 2 Nail 2. Cross-section locations are circled in red

1 Etching was conducted before XRF data were collected. Whileetching can affect the composition of parts of the specimen’s etchedsurface, X-rays penetrate deeper (∼5–10 μm) than the 1 μm of likelyetchant penetration.

Fig. 1 Nail 1. Cross-section locations are circled in red

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primary factors that influence a metal’s material properties:composition (XRF, SEM-EDS), structure (XRD), grain size(optical imaging), and work hardening (nanoindentation andoptical imaging).

Composition and structure

XRF and EDS were used to evaluate bulk composition andto examine the consistency of composition along the lengthof the nail samples. The XRF measurements were takenusing 90-s scan time2 on an Innov-X Systems Alpha 4000handheld XRF standardized against NIST standard 2702.3

Two specific drawbacks were recognized in using the XRFto examine metals. First, due to the low energy of X-rays, onlysurface compositional information can be obtained. In thiscase, the samples were cut so that the surface was an interiorportion of the nail free of surface corrosion. Additionally,XRF may not be quantitatively accurate in measuring traceelements due to matrix effects and other complications.However, the comparison of the XRF data with EDSmeasurements and XRD data demonstrate that, in this study,XRF is useful as a semi-quantitative tool.4

The comparisons of the XRF spectra of samples 1.1 and2.1 in Fig. 3 clearly illustrate the presence of Sn in nail 2but not in nail 1; nail 2 was a low-Sn bronze, while nail 1was pure copper.

X-ray fluorescence data indicated that copper is the onlysignificant element found in all cross-sections of nail 1(Tables 1 and 2). Co, Ni, Ag, Cd, Hg, and Se were underthe detection limit of the XRF, while K, Ca, Ba, P, S, Cl,and I were ignored as corrosion or resin block elements.Rb, Sr, Zr, Mo, and Sb were found in trace amounts.

As Table 2 illustrates, XRF data from nail 2 indicated thepresence of a small amount of Sn present in each of the fourcross-section samples. Converting the Sn from parts permillion values to weight percents yields 1.7 wt.% Sn forsample 2.1 and 1.3 wt.% Sn for samples 2.2, 2.3, and 2.4.5

However, this discrepancy in Sn is probably due to the factthat sample 2.1 was encased within several layers ofcorrosion measuring up to 3 mm in thickness on all sides.It is known that bronze corrosion is Sn-enriched (Scott2002), so given the handheld XRF’s low resolution (e.g.,large ∼1-cm spot size), the presence of more Sn in relationto the thick corrosion ring is unsurprising.

To confirm XRF data, sample 2.2 was evaluated withEDS, which reported 4 wt.% Sn. This discrepancy betweenthe XRF and EDS measurements arises from the fact thatthe EDS had extremely high resolution compared to theXRF. Nevertheless, this low percentage of Sn does littleexcept to slightly disrupt the FCC structure of the copper,resulting in decreased hardness of the metal, whichrendered the nail easier to bend during production and use.

2 A 90-s scan for heavy elements and 90-s scan for light elements3 The XRF measured each element in the standard to within ∼5% ofthe accepted values. Also, we acknowledge that there may be adiscrepancy between the NIST soil standard and the metal samplesdue to matrix effects but, at present, there is no easy way to accountfor the difference given the nature of the instrumentation and theabsence of discussions about these discrepancies in the literature.4 The EDS measurements were obtained from an INCA EDSaccessory from Oxford Instruments attached to a Zeiss LEO 1530VPFESEM, operating at an accelerating voltage of 20 kV and workingdistance of 8 mm.

Fig. 3 Overlaid XRF spectra of 1.1 (blue) and 2.1 (pink). The arrowsmark Sn kα and kβ (left and right, respectively) peaks visible in 2.1but not in 1.1

Table 1 Relevant XRF data in weight percent from cross-sections ofnail 1

1.1 1.2 1.3 1.4

Ti 0.121 0.355 0.251 <LOD

Cr 0.021 <LOD <LOD 0.025

Mn 0.060 0.066 0.099 0.065

Fe 0.386 0.366 0.396 0.438

Cu 99.010 98.689 98.673 99.072

Zn 0.311 0.431 0.508 0.318

As 0.075 0.066 0.073 0.067

Sn <LOD <LOD <LOD <LOD

Pb 0.017 0.028 <LOD 0.015

5 The XRF data was normalized to 100 wt%. This process involvedadding all parts per million measurements of major elements, taking apercent of each element by dividing the parts per million of an elementby the total, and then normalizing by weight of the element. Oneimportant reason for normalizing the XRF data included the potentialfor making these data directly comparable to other data normalized bythe same process.

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XRD was employed to evaluate the consistency ofstructure (and thus qualitative composition) along thelength of the nail. The XRD measurements were collectedusing a Siemens Diffractometer 5000. Source entrance,source exit, scatter, and detector slit sizes were 1.0, 0.6, 0.2,and 0.1 mm, respectively. XRD was incorporated into thisstudy because, in addition to structural information, itprovided qualitative clues about the chemical compositionof the samples. This is particularly the case whenexamining samples containing impurities because theseimpurities can disrupt a pure lattice away from its typicalstructure, shifting XRD peaks up to several degrees awayfrom the peaks of a pure crystal.

X-ray diffraction data are seen in Figs. 4 and 5 for nail 1and 2, respectively. The spectrum for nail 1 matches that ofFCC copper perfectly. The peaks in the spectrum of nail 2are shifted slightly, indicative of a measurable Sn impurity.The peaks around 2Theta = 37°, 57°, and 78° in the 1.3,1.4, and 2.1 spectra correspond precisely to cuprite (Cu2O),the first corrosion product which forms on copper surfaces.Cuprite’s presence is understandable as some of thesamples had been polished and etched, allowing for easycorrosion, before scanning in the XRD. Finally, XRD alsoconfirmed that the bronze along the length of nail 2 was ofthe same composition throughout the sample, despite theXRF results, as there were no shifts in the peaks betweensample 2.1 and the other nail 2 cross-section samples.

Grain size

The microstructures within each sample were examined forevidence of constituents, working processes, and hardness,all of which are visible in the sizes, dislocations, phasecontrasts, and orientations of the nails’ grain structures.Multiple techniques were involved in determining grainsizes and spatial properties. First, optical microscopy wasused to image the distribution of grain sizes along thelength of the nail and within individual cross-sections. Then,

images were analyzed using ImageJ, an open-access softwarethat has been applied widely in the natural and physicalsciences and has found recent applications in archaeologicalscience to examine patterning within materials other thanmetal artifacts (Araujo et al. 2008; Delaney-Rivera et al.2009; Greenlee and Dunnell 2010; Hufthammer et al. 2010).Of note is that ImageJ allows for a quantitative and statisticalanalysis of grain areas rather than a visual comparison ofscale bars with images.

After uploading the optical microscope images into theImageJ program, threshold values were set to distinguishbackground color from the rest of the image and tohighlight the lighter grains within each sample. The“Analyze Particles” function was set to calculate the areaof the grains in the pixels squared (Figs. 6 and 7). Given themagnification and the use of a simple pixel-to-centimeterconversion, the area of the grains was calculated in standardunits.

Grain sizes were also determined by examining transectsof optical microscopy images collected along each sample(Fig. 8c). From these digital images, we analyzed the totalamount of color change within each image with a fixed

Fig. 4 XRD spectra of nail 1 cross-sections. Dashed red linescorrespond to pure FCC copper

Table 2 Relevant XRF data in weight percent from cross-sections ofnail 2

2.1 2.2 2.3 2.4

Ti 0.194 0.186 0.201 0.204

Cr 0.035 <LOD <LOD <LOD

Mn 0.057 0.074 0.060 0.089

Fe 0.544 0.297 0.237 0.333

Cu 96.886 97.464 97.767 97.408

Zn 0.319 0.495 0.258 0.514

As <LOD <LOD <LOD <LOD

Sn 1.719 1.325 1.319 1.312

Pb 0.246 0.159 0.156 0.140

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field of view and measured the approximate size of thegrains present in the image. These measurements involvedcalculations of the approximate area of the grains andanalysis of the grain boundaries. In order to obtain thesedata, the images were loaded into Photoshop CS4, and theirlocal contrast was increased using a high-pass filter. Theresults were then blurred to reduce microcontrast (anddeemphasize minor variation, such as scratches and coring).

Using an edge detector, we then found the grain boundariesand summarized the edges using ImageJ. In these cases,color change is proportional to the number of grains.

ImageJ was a useful technique for examining the spatialrelationships of microstructural constituents within thesesamples. This was the first time, to our knowledge, that thesoftware has been used in an archaeometallurgical exami-nation. As such, it is important to recognize the short-comings with ImageJ that we encountered during thisresearch. The most notable challenge is that ImageJ cannotdistinguish between neighboring darker-colored grainsbecause of their low-contrast boundaries. Also, high-contrast boundaries can be falsely introduced by twinning,scratches, and cold working lines. While ImageJ is apotentially useful analytical tool, it is recommended thatfuture analysis with ImageJ be undertaken in carefulconsideration and cross-referencing of optical micrographsand data from other techniques such as nanoindentation.

Mechanical properties

Nanoindentation is a relatively new application in archaeo-metric studies, and the few previous applications that haveused it have examined the properties of bone (Riede andWheeler 2009), phytoliths (Sanson et al. 2007), and lithics(Lerner et al. 2007). This is the first known application ofthe technique to metal archaeological samples. However,nanoindentation is widely used by materials scientists tocharacterize the mechanical response of a small volumeof material (Liu et al. 2008; Maier et al. 2002; Ziegenhainet al. 2009). By accurately controlling a small tip, theresponse of the material to a controlled load can beprecisely measured. For this study, a standard Berkovichtip was used. During loading (i.e., as the tip pushed intothe material), the sample’s resistance to the probedetermined the depth of the indentation that is produced.This is reported as a hardness value. The response of thematerial during the unloading (i.e., as the tip is retracted)

Fig. 7 Average grain sizes in nail 2 cross-sections as computed usingImageJ. Error bars correspond to standard deviation in grain sizes

Fig. 6 Average grain sizes in nail 1 cross-sections as computed usingImageJ. Error bars correspond to standard deviation in grain sizes

Fig. 5 XRD spectra of nail 2 cross-sections. Dashed red linescorrespond to pure FCC copper

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is then used to determine the elastic modulus of thespecimen (Fig. 9) (Oliver and Pharr 1992).

Nanoindentation offers significant potential for under-standing the behavior of archaeological metals in relation totheir properties, manufacture, and use. For example, duringthe production process, cold working and certain forge

conditions can cause the structure of cupreous metals toreach a threshold at which the material becomes saturatedwith dislocations. Evidence of this activity can be seen withnanoindentation because such saturation prevents disloca-tions from nucleating and moving to new areas; this is aprocess that affects the resistance experienced by the indentertip as it pushes into the material. As such, dislocationsaturation results in higher hardness of the material.

In this study, nanoindentation was performed on twosamples, one from each nail. A magnetic metal wafer wasglued to the bottom of each resin block to keep the samplestable in the indenter. Each sample was placed in thevacuum chamber of a Hysitron TI 900 TriboIndenter. Usingthe Hysitron software, the machine was programmed toindent ten times along two perpendicular transects, whichintersected at the middle of the sample. The nanoindenta-tion produced high-resolution behavioral information oncertain localities within the metal samples. These micro-scale data were gathered from 20 locations along thesurfaces of each sample; at each location, indentationdepths measured 1 μm or less and were penetrated by theindenter’s tip, which was 5 μm in width.

The resulting nanoindentation measurements providedata about mechanical properties which, in turn, relate tothe production techniques used in the nails’ manufacture. Inparticular, the mechanical properties of the nail cross-sections were measured to gather information about workhardening and performance. Work hardening is thestrengthening of metal by plastic deformation that occurswhen materials are subjected to forces at temperaturesbelow their recrystallization temperature (i.e., cold work-ing), which results in dislocations. Copper and copperalloys are known to be highly strengthened by workhardening (Smith and Hashemi 2005).

Fig. 8 b ImageJ and c photoshop analysis of an a optical image ofcross-section 2.2. ×160

Fig. 9 Nanoindentation loading and unloading curves (load versusdisplacement) for one location on nail 2. The upward sloping loadingcurve corresponds to the nanoindenter entering the sample. Thedownward sloping unloading curve corresponds to the copper expand-ing elastically and pushing the indenter tip up. The unloading curve isused to determine hardness and Young’s modulus using the Oliver andPharr method (1992)

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Measured hardness and elastic modulus values are alsoparticularly important for understanding the nails’ perfor-mance. The modulus value is a measure of the fundamentalelastic response of the material, which is essentiallydetermined by the stiffness of the interatomic bonds. Forthe copper alloy samples from Petra, this is primarilydetermined by the local composition of the material underthe indentor. When the indentor samples a large number ofgrains, hardness is generally inversely proportional to grainsize (primarily because grain boundaries impede dislocationmotion). However, in the materials studied here, the grainsizes are much larger than the size of the indentations; thus,for the most part, we expect each indent to primarily sampleone grain. With this in mind, variations due to grainorientations are probably responsible for much of theindent-to-indent differences that were measured in eachsample. Differences in the average hardness and modulusvalues for a given sample are thus likely to reflectdifferences in dislocation behavior.

Here, a higher hardness value could be caused byhardening to high dislocation densities or by compositionaleffects that decrease dislocation mobility (e.g., impurities inthe Cu lattice). Figure 10 shows the hardness across eachnanoindentation transect for the cross-sections of nail 1.The x and y axes roughly correspond to the edges of thesample. Similarly, Fig. 11 shows the elastic modulus

across each nanoindentation transect for the cross-sectionsof nail 1.

The mechanical properties along the length of nail 1 aresummarized in Fig. 12. The same tests were performed onnail 2. A summary of the mechanical properties of nail 2 ispresented in Fig. 13.

An additional appeal of nanoindentation as a potentialarchaeometric technique is that the data it generatescomplement data from other archaeometric methods; theuse of these multiple techniques and results in tandemprovides access to variables of archaeological objects’properties and performance that cannot be garnered fromXRF and grain size analyses alone. This is detailed in theinterpretation of the nanoindentation data about the nails’mechanical properties and work hardening in light of datacollected about grain size and composition.

Interpretations

Combining data from these individual analyses, it is possibleto comment in detail on the relationships between the nails’material properties, structure, and composition. Data fromeach of the four samples along each nail were synthesized inorder to demonstrate variability along samples and also tocompare material properties between the two nails.

Fig. 10 Spatial distribution ofnanoindentation hardness valuesof nail 1 cross-sections. a 1.1, b1.2, c 1.3, d 1.4

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First, it is valuable to consider variations within a givensample. Using data from ImageJ and nanoindentation toexamine grain sizes along transects, we determined subtledifferences between sections of the same nail (nail 1) thatrelated to conditions during manufacture. For example,section 1.1 has what appears to be a random grain sizedistribution along its two transects. Section 1.3, however,displays a clear trend of size increases to one side of the

sample, which is indicative of differential cooling on one sideof the nail. It is possible that the nail was placed on an anvil orsome other surface out of the forge to cool, resulting in adifference in temperature between the side resting on a surfaceand the side exposed to air. It is also possible that the metal inthis portion of the nail was folded as it was worked. Theseobservations are consistent with the nanoindentation datawhich, as Fig. 10a shows, recorded a relatively random

Fig. 11 Spatial distribution ofnanoindentation elastic modulusvalues of nail 1 cross-sections. a1.1, b 1.2, c 1.3, d 1.4

Fig. 12 Summary of the mechanical properties along the length ofnail 1. H is hardness and Er is elastic modulus

Fig. 13 Summary of the mechanical properties along the length ofnail 2. H is hardness and Er is elastic modulus

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distribution of hardnesses in sample 1.1 (this part of thesample would have been surrounded on all sides by air as thenail was positioned on its head), while Fig. 10c shows aclear increase in hardness to one side of cross-section 1.3(which would have conducted heat differentially due to itscontact with the work surface).

Information about consistency in the production processescan be inferred by comparing the results obtained at the fourpositions within a given nail. In general, the measureddifferences in composition along the length of each nailwere relatively modest. This suggests that the materialsused in the production of each nail were relatively wellmixed. The grain size differences along each nail are alsorelatively small; however, they do suggest some modestvariation in microstructure development along the lengthof each nail. Temperature differences alone could certainlyproduce the observed variations in grain size. However,differences in working could also be a factor here—forexample, different amounts of cold work could alter the finalgrain size during firing, even if the temperatures wereuniform. Because processes like grain growth and recrystal-lization are highly sensitive to impurity effects, the relativelysmall differences in composition that were observed couldalso play a role here.

The nanoindentation results provide the most significantevidence of differences along the length of each nail. Therelationship between nanoindentation hardness and modulusvalues in a polycrystalline metal has a complex dependenceon the structure of the material, with variations due tocrystallographic orientation of the grains, dislocation struc-tures, and possibly grain boundary effects. However, ingeneral, modulus and hardness values typically follow similartrends. In nail 1, this type of similar trend is observed incomparing samples 1.1, 1.2, and 1.3. The modulus andhardness show similar relative changes between these threepositions. The sample-to-sample differences in these valuesare slightly larger than the error bars, which suggests that thenanoindentation detected a real difference in the material thatprobably reflects some non-uniformity in the forming con-ditions. This suggests that nanoindentation may be sensitive torelatively small changes in the microstructure that were notdetected by the other methods that we employed. Forexample, the distribution of impurities at grain boundariesand other defects is not detectable with XRF, and this can havea significant effect on the measured properties.

In sample 1.4, the hardness and modulus values deviatefrom the trend seen in samples 1.1, 1.2, and 1.3. Here, theaverage hardness value is comparable to sample 1.3, but themodulus is notably lower. Although the cause for the dropin modulus is not completely clear, this suggests that theend of this nail was cold worked differently after it cooled.One possibility is that small differences in compositionproduced the decrease in modulus, while more extensive

cold working produced hardening. This type of explanationneeds to be evaluated more carefully. The type of workingthat may have been done here is also open to additionalquestions. While it is probable that cold working occurredat a site of metal working, it is difficult to assessindependently the extent to which subsequent use of thenail in construction activities may have also impacted thematerial properties.

The relative variations in nanoindentation modulus andhardness also provide an interesting basis for comparisons innail 2. Overall, the modulus values are somewhat lower here,which may be due to the presence of Sn. The hardness andmodulus values follow similar trends in samples 2.2, 2.3, and2.4. However, in sample 2.1 the relative modulus is notablyhigher, while the largest impurity levels (Sn and Fe) aresomewhat higher. The relationship between these impuritiesand the higher modulus is not immediately obvious, but itseems likely that the head of the nail was subjected tosomewhat different forming conditions, either during heatingor subsequent cold working. In looking at the hardness values,the tip was probably hammered more than the head to make itinto the thin squared-off shape, resulting in higher hardness inthat micro-region, and this is consistent with the increase inhardness along the length of the nail.

Why copper?

Returning to the compositional data in light of theinformation generated about the nails’ material properties,it is interesting to consider why copper and bronze wereselected to make these large nails. By the Roman Period,iron had superseded copper as the primary material forcrafting utilitarian objects, such as nails, for two reasons.First, iron was more widespread and easier to locate,resulting in greater availability and cheaper costs ofutilitarian objects in comparison to copper counterparts(Zaccagnini 1990). In addition to lower economic cost, ironwas also more cost-effective in terms of its performance asa metal in architectural contexts; in particular, iron is moreresistant to bending than copper, offering greater durabilityand longer use-life—qualities essential for nails. Neverthe-less, Roman smiths were known to have an excellentunderstanding of and control over the raw materials theyused in copper manufacture as well as how to alter theirproduction processes to deliberately manipulate the finalcopper product’s material properties (Dungworth 1997).

So why are these nails made of copper and not iron? Thechoice of copper and low-Sn bronze over iron need notnecessarily convey that these were objects of social,esthetic, or ritual significance. Copper and especiallybronze nails were not uncommon in the Roman Empire(Ponting 2002). Bronze nails, for example, were usedregularly in Roman ship construction to fasten hull planks;

182 Archaeol Anthropol Sci (2011) 3:173–184

smaller bronze nails from Petra have been associated withthe roofing structures of the Great Temple precinct (Eisman1975; Hussein 2010; Manning 1985; Roebuck 1969).Additionally, Petra’s proximity to hundreds of copper minessuggests that copper and copper alloy objects were readilyavailable throughout the site’s long history.

And yet the choice of material here may well be moreconsequential given where the nails were found. Theirspecific archaeological context—within the Great Temple’sPropylaeum in the period of its Roman renovation—does raisethe strong possibility that these bronze and copper nails wereincorporated into the complex’s monumental architecture forreasons transcending their immediate utilitarian function andlikely relating to the nails’ conspicuous and decorativematerialproperties, such as color and size. Propylaea, which markmajor points of entrance and exit in ancient architecturalsystems, are often highly elaborated and ornamented struc-tures. Comparative examples in which bronze nails werechosen for color contrast and design are visible in otherimportant Roman architectural complexes throughout theRoman Empire, not least the famed Pantheon in Rome (Smith1901). As these and other cases demonstrate, in antiquity, thereddish copper luster and green patina of bronze and copperbuilding materials were highly valued for their visualproperties and the esthetic highlights that they added tosignificant buildings (Scott 2002). The changing functions ofthe Great Temple in the period following Roman annexationremain uncertain but are believed to have encompassed arange of political, ceremonial, theatrical, and ritual elements(Joukowsky 2007, pp. 350–355). The evidence of these nails,in their modest way, testifies to the building’s continuingimportance and prestige in the life of Roman Petra.

Conclusions

The analysis of the internal variation of Petra nails throughthe use of several techniques in conjunction clarifies themultiple properties of these objects as they relate to thenails’ performance in production and use. Compositionaldata confirm that the nails were two different metal types,copper and low-Sn bronze. The grain size and hardness ofthe nails provide further detail about the craft process,which involved the metals’ exposure to variable processingand cooling conditions and differential work hardening bythe smiths. The examination of these metals in relation totheir excavated archaeological and architectural contexts,the Propylaeum of the Great Temple at Petra, is anopportunity to consider these types of nails not only asutilitarian but also as aesthetic objects.

This case study of Petra nails recognizes that themultitude of factors involved in causing changes to thegrain sizes and composition in archaeological metals cannot

be reasonably understood through the use of one source ofmeasurement or through focus on one aspect of an objects’structure or properties. The use of multiple techniquesdemonstrates the importance of articulating the relation-ships between composition, structure, grain size, andhardness in examining archaeological metals, and thetechniques used suggest possible new directions in whichfurther research might well expand. In particular, nano-indentation measurements are highlighted as another sourceof data complementary to data on structure and compositiongathered from digital image and compositional analyses.The combination of these techniques has the potential toprovide more precise information on how materials andobjects were produced and how they performed over thecourse of their use-life.

In the future, more detailed studies on these and othermetal finds from Petra will involve the collection of anincreased quantity of optical transects which, whenaligned with nanoindentation data, will permit higher-resolution spatial correlations between optical and mechanicalproperties. Nevertheless, the applicability and potentialusefulness of handheld XRF and ImageJ software areapparent in this case study, although refinements to bothof these techniques are necessary to improve theiraccuracy and reliability in archaeometallurgical studies.Nanoindentation, in particular, offers promise for increasingthe level of detail available about metal properties, whichcould improve our understandings of metal behavior. The useof nanoindentation data, in conjunction with other techniques,can be particularly useful in examining questions about theperformance, use-life, and traditions of metalworking inarchaeological contexts.

Acknowledgements The authors wish to thank Anthony McCormickfor assistance with SEM/EDS, David Murray for access to the XRF, PaulWaltz for guidance with the XRD and the sample prep laboratory, andElizabeth Murphy for her support in various stages of this research.Additional thanks toMartha Sharp Joukowsky for providing feedback onPetra and the excavation contexts of the metals. Funding for instrumentalanalysis was provided by several sources at Brown University includingthe Dean of the College, the Division of Engineering, the JoukowskyInstitute for Archaeology and the Ancient World, and an Office of theVice President for Research (OVPR) Seed Grant.

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