CHAPTER

4

X-RAY FLUORESCENCE

LUC MOE S

University Ghem, Laboratory of Analytical Chemistry, Ghent, Belgium

ALEX VON BOHLEN

Illstillit fiir Speklrochemie und Angew{lm/te Speklroskopie, ISA S, Dortmund, Germany

PETER VANDENABEELE

Universi ty Ghent, ulboratory of Analy tical Chemislry, Ghem, Belgimll

4.1. INTRODUCfIO

X-ray fluorescence (XRF) analysis allows the concentrations of most ele­ments of the periodic table to be determined. The method has many advan­tages that make it very suited for the analysis of artifacts. It allows solid materials to be analyzed directly without necessitating dissolution or any other time-consuming or destructive sample preparation. Small objects can be brought to the XRF instrument and in many cases analyzed as such. The method is consumption free and, apart from occasional radiation damage, will leave the artifact unaltered and suited for analysis with other methods. Mobile XRF instruments allow larger objects to be studied on site. Ob­viously, it is also possible to sample the artifacts if such is ethically accept­able. A further advantage of the method is its capability to determine major, minor, and trace element concentrations. Finally, XRF is a well-established method, the possibilities and limitations of which are known and well under­stood. The operation costs are low and the analysis times are short.

Accurate quantitative analysis of unprepared artifacts may be difficult be­cause XRF (except total reflection XRF, TXRF) suffers from matrix effects for which it is not easy to correct. Also, the concentrations measured with

~odern Analytical Methods in Art and Archaeology, Edited by Enrico Cilibcno and Giuseppe I poto. Chemical Analysis Series, Vol. 155 SBN 0-47 1-2936 I -X c 2000 John Wiley & Sons, Inc.

55

56 x -RA Y FLUORESCENCE

P ..... 2%

Varnishes 2%

Ceramics 20%

Glass ,,%

Figure 4.1. Relative importance ( 1995- 1998) o f different XRF application fields in archaeology and art history.

XRF mainly reflect the composition of the outer layers of the material. For weathered objects this composition may considerably deviate from that of the unaltered material below the patina. These limitations are obviously alleviated when samples are taken from the artifact and sample preparation methods aiming at accurate quantification are applied. TXRF, under optimal circumstances, is free from matrix effects and quantification can be simple.

A literature search for the period 1995 to 1998 revealed that XRF analysis is still widely used to identify and characterize a variety of archaeological and art historical materials. Figure 4.1 depicts the different application fields and their respective importance (period 1996 to 1998). Metallic objects (in­cluding coins) and potsherds make more than 50% of the applications, but stone material and bones and ivory have been studied as well. Also painted surfaces and glass were found to have been the subject of XRF analysis.

In this chapter the XRF techniques used will be discussed, and the appli­cation field will be reviewed via a limited number of typical applications. Rather than emphasizing the analytical details, the archaeometric potential of the method will be illustrated.

4.2. NATURE OF X-RAYS AND X-RAY FLUORESCENCE

X-rays are part of the electromagnetic spectrum (cr. Fig. 4.2) and were dis­covered and described by W. C. Rontgen in 1895 (Bertin 1975; Broil 1996). Only a part of their spectral range between 0.1 and 100 keY or according to

4.2. NATURE OF X-RAYS AND X-RAY FLUORESCENCE

Frequency 1-I. ..... IIiI,jjW .. [s'l Energy leV]

Figure 4.2. Electromagnetic spectrum.

57

the relationship E = he/}., between 10 and 0.01 nm is used in X-ray spectral analysis for material sciences. The X-rays are generated by bombardment of a target by means of charged particles, electrons, or ions leading to X-ray emission or by high-energy radiation leading to X-ray fluorescence. The ob­served X-ray spectra are composed of a continuum with wide spectral range superimposed by well-defined X-ray lines.

When applying sufficient kinetic energy, both, radiation and panicles are able to penetrate into the atoms and to remove electrons from their inner shells. After the ejection of electrons, the atoms are in an energetically un­stable configuration, and electrons from outer shells fill up the vacancies. The energy difference between the shells involved in the transition process of electrons is emitted- in the case of inner electrons shells- as X-ray photons. The selection rules of quantum theory dictate which electron transitions are allowed. The nomenclature of emitted X-rays is based on conventional rules, as illustrated in Figure 4.3. For instance, in KPI, the capital K refers to the K shell (Bohr's atom model) from which initially an electron was expelled. Greek letters and digits are used to further denote the X-rays in a conven­tional and nonsystematic way; thereby reference is made to the orbitals from which an electron leaves to fill up the originally created vacancy and to the relative intensities of the X-rays. The wavelength of the emitted X-rays depends on the atomic number Z of the excited atoms. For a given type of X-ray (e.g., K, or Kpl) the dependence of its wavelength on Z is described by Moseley's law:

where }. is the wavelength and k, and k2 are constants. Each element emits X-rays with specific wavelengths that characterize the element. This char-

58 X-RAY FLUORESCENCE

..............................................

K--~~--L----------------J

Figure 4.3. Electron transitions for different shells involved in the emission of X-rays.

acteristic radiation can be detected and is graphically represented by a char­acteristic X-ray spectrum. in contrast to the atomic emission lines in the ultraviolet (UV) and visible part of the spectrum, the characteristic X-ray spectra are simple: whereas Fe has alone 6000 UV - vis emission lines, not more than 600 X-ray lines are relevant in X-ray spectrometry and are suffi­cient to characterize all the elements. In general, no distinction can be made between characteristic X-rays from isotopes of the same element or from atoms of the same clement present in different chemical compounds. Except for the two first elemeots of the perindic table of the elements, hydrogen and helium, that do not produce X-ray spectra, all elements are accessible to a qualitative and quantitative X-ray analysis. Thereby the wavelengths or energies of the emitted X-rays identify the elements contained in a sample, while the intensities of individual signals are proportional to the concen­trations of these elements. Main constituents up to 100% as well as minor constituents or even traces can thus be detemlined in a great variety of samples.

For detailed information concerning the physical nature, the instrumen­tation, the quantification, and classical applications of X-rays and X-ray spectrometry several monographs are available: Bertin (1975), Williams (1987), Jenkins (1988), and KIockenkiimper (1996).

4.3. X-RAY FLUORESCENCE SPECfROMETRY

Being a form of electromagnetic radiation, X-rays can be described in terms of wave phenomena or as corpuscles. In particular, X-rays show interference effects that can be explained effectively by wave theory (Bertin 1975). One of these effects is coherent scatter (Williams 1987) usefully interpreted as a

4.3. X-RAY FLUORESCENCE SPECTROMETRY 59

Secondary X-rays from the sample To the detector

Figure 4.4. Diffraction of X-rays by a crystal according to Br'dgg: ). = wavelength , 0 = Bragg angle of diffraction, d = interplanar spacing of diffracting planes.

wave phenomenon by Bragg, who considers it as a reflection on a stack or crystal planes (cr. Fig. 4.4). When a collimated X-ray beam or wavelength ). is directed at an angle 0 onto a crystal with crystal planes or spacing d, a reinrorced scatter orthe radiation can be observed ( Williams 1987) under the angle or reflection on condition that Bragg's law

11,1 = 2d . sin (J

is rulfilled ror an integer number 11.

4.3.1. Wavelength-Dispersive X-Ray Fluorescence Spectrometry

This so-called Bragg reflection is rully used in wavelength-dispersive X-ray flu orescence spectrometry (WDXRF ) to select the characteristic X-rays or interest originating rrom a sample, by selecting the corresponding angle O. A schematic representation or a WDXRF spectrometer is shown in Figure 4.5.

In WDXRF samples are irradiated by the primary X-rays or an X-ray Sourcc. The induced secondary X-ray radiation consists or different wave­lengths originating rrom the different elements or the sample and reflecting its elemental composition. The secondary (or fluorescence) radiation origi­nating rrom the sample is guided through a collimator that allows only rays with parallel propagation to pass. The X-rays reach a Bragg reflector, that is, a crystal, and arc reflected according to Bragg's law. Scanning the angle 0 by rotating the crystal and simultaneous displacement or the detector with a

60

Primary X-rays

X-ray tube

Sample

X-RAY FLUORESCENCE

Bragg reflector

Figure 4.5. Schematic representation of a wa\-'elength dispersive X-my ftuorcscence spectrometer (WDXRF).

high mechanical accuracy (better than 0.001 °) make it possible to sequen­tially detect X-rays of different wavelength. The radiation selected by the Bragg reflector will pass a second collimator before reaching tbe detector. By the aid of a computer, the signals are recorded and composed to a spectrum or when only some elements are of interest, the intensity of the X-rays of the corresponding wavelengths are reported.

Several excitation sources are used in X-ray spectrometry. The most common are X-ray tubes producing a broad beam spanning a large solid angle. These are built as end- or side-window tubes that are operated at high voltages up to 100 kV; the current (milliampere) is adjusted to a maximum load of 2 or 3 kW. Also fine-focus tubes (power 0.6 to 3 kW) or tubes with rotating anodes (power up to 18 kW) are used in WDXRF spectrometers.

Since it is impossible to select all wavelengths using a single Bragg reflec­tor crystal, most X-ray spectrometers are equipped with several crystals that can be used according to the needs of the experiment. Tbe cboice will depend, for instance, on the elements to be detected, the required spectral resolution, or the stability of the crystal. A list of crystals regularly used in WDXRF and tbeir specificalions is given in Table 4.1. Detailed information can be found in the literature (Bertin 1975).

In WDXRF two detectors with different characteristics are normally used. These are arranged in a tandem geometry and are switched on de­pending on the wavelength of the radiation to be recorded. The first detector of this arrangement is a gas flow proportional counter for tbe registration of long-wave (low-energy) radiation. The second is a scintillation counter for the detection of the hard X-rays baving higher energies. Specifications and working methods are described in detail elsewhere (Bertin 1975; Heinrich

4.3. X-RAY FLUORESCENCE SPECTROMETRY 61

Table 4.1. Bragg Reflectors Regularly Used in WDXRF'

Crystal

Lithium fluoride, LiF (220) Lithium fluoride, LiF (200) Germanium, Ge (III ) Pentaerytrol PET (002) Thailium acid phtalate, TlAP (1010) Lead stearate decanoate, PbSD M uhilayer, W IC

2d (A)

2,848 4,208 6,532 8,742 25,75 100 120

- The crystallographic orienta tion is given in brackets.

Lightest element detectable (Z) at max 0= 70°

Using K Lines

V (23) K (19) S (16) Si (14) 0 (8) B (5) B (5)

Using L Lines

Pr (57) In (49) Zr (40) Rb (37) V (23) Ca (20)

1981; Klockenkamper 1980). The energy resolution of these types of de­tectors is poor and therefore a preselection by means of diffraction (Bragg reflector) is necessary.

4.3.2. E nergy-Dispersive X-Ray Fluorescence Spectrometry

The development of semiconductor materials had consequences in X-ray spectrometry. Indeed the energy resolution of lithium-drifted silicon ISi(Li)] detectors is sufficiently high to aUow X-ray spectrometry without previous dispers ion with a Bragg reflector. X-ray photons entering the Sit Li) crystal interact primarily by photoelectric absorption, producing electron- hole pairs. The number of pairs produced is proportional to the energy of the photon, thus making X-ray spectrometry possible. The produced charge is swept from the semiconductor diode by the applied voltage and produces an electric pulse. The signals produced by the X-ray photons are very weak and have to be amplified by a low-noise amplifier transistor (field effect transis­tor, FET ) ( Heinrich 1981). For optimal operation of the system and to freeze the lithium drifting process, effective cooling with liquid nitrogen is indispensable. By processing the signals with a fast multichannel analyzer (MCA), energy-dispersive X-ray fluorescence spectrometry (EDXRF) will produce X-ray spectra with a spectral resolution sufficient for most applica­tions. Tbough its spectral resolution is poor in comparison to WDXRF, EDXRF has the advantage of being simpler and cheaper and to allow truly Simultaneous observation of all X-rays.

As in WDXRF, in EDXRF different X-ray tubes are applied as primary X-ray Source. Additionally X-ray emitting radioisotope sources such as cadmium-I09 ( ' 09 Cd), americium-241 [241 Am), and others are used instead

62 X-RA Y FLUORESCENCE

of X-ray tubes. Radioisotope sources, among others, are used in compact portable spectrometers that are applied in field work for fast screening of the elemental composition of a material.

Recently, synchrotrons (Haller and Knochel 1996) have been used as extremely intense X-ray sources that make XRF analysis with a superior sensitivity possible (synchrotron radiation XRF, SRXRF). The high initial intensity available at the source moreover allows microbeams to be used, offering excellent spatial resolution (p-SRXRF) and good sensitivity.

4.3.3. Samples and Sample Preparation

In general , no restrictions exist concerning the samples or their preparation for either WD- or EDXRF. Conducting or isolating solids, powdered mate­rials, and liquids can be subjected to X-ray fluorescence analysis. One of the most prominent properties of XRF analysis is that the samples can be re­covered completely after the analysis. Unfortunately, some samples show alterations caused by the irradiation with X-rays and a change in color or a change in mechanical properties caused by radio lysis is occasionally ob­served. Some minerals, glasses, plastics, and organic materials are affected more seriously than others. The sample chambers of the spectrometers in general contain the end of the X-ray tube, parts of the spectrometer and of the crystal changer (WDXRF ), detectors or parts of these, collimators, and the sample stage. Usually the sample chamber is evacuated down to a pres­sure of the order of 10 Pa. Nevertheless, XRF instruments can be operated at normal atmospheric conditions or filled with helium depending on the sample and/ or the analytical tasks. Especially in the latter cases, the size of the sample chamber can be sufficiently large to accommodate small archae­ological objects (jewels, statuettes, etc.). The sample chamber among others is designed to prevent X-rays from escaping the instrument and thus is im­portant for safe operation. evertheless, open-beam instruments have been used, especially for in silll analysis of large artifacts (e.g. , paintings). Safety in this case will depend on keeping an appropriate distance during operation and on the use of protection shields.

4.3.4. Quantification

Unfortunately, WD- and EDXRF operated in conventional modes are affected by matrix effects that cannot be neglected. These are caused by absorption of X-rays in the sample and by fluorescence enhancements when the radiation is crossing the sample. Also effects related to the surface tex­ture, to lhe inhomogeneity of the sample, and to the particle size have been observed. All these effects lead to curved instead of straight calibration lines

z

4.3. X-RAY FLUORESCENCE SPECfROME"fRY 63

and have to be taken into account if quantitative evaluation of data and accurate determination of the elemental composition of a sample are the goals. Many calibration and correction procedures are being applied in XRF analysis (Bertin 1975):

I. Srandard Addition and Dilution Methods. The purc sample itself is measured as well as at least one sample to which a known amount of the clements to be determined (analytes) is added. Sample preparation is necessary to make the two specimens as similar in density, surface condition, and the like as possible. In this method, tbe sample actually provides its own standard(s) in its own matrix.

2. Thin-Film Methods. The specimens (samples and standards) are made so thin that absorption/enhancement effects become irrelevant.

3. Matrix Dilution Methods. The matrix of all specimens is diluted with a suited material so that the effect of the matrix is determined by the material used for the dilution.

4. Comparison Standard Methods. The X-ray intensities fwm the sam­ples are compared with those of standards, having the same form as the sample(s) and as nearly as possible the same matrix and analyte concentration.

5. IlIIemal Srandardi=tlfion. The comparison standard method is im­proved by quantitative addition to all specimens of an internal stan­dard element. The calibration function involves the intensity ratios of the analyle and internal standard lines.

6. Standardization lVith Scollered X-rays. The intensity of the primary X-rays scattered by the specimen is used to correct for absorption/ enhancement effects.

7. Experimental Correction. Various other experimental techniques have been deseribed to minimize or compensate matrix effects.

8. Mathematical Correction. Absorption/enhancement effects are cor­rected for mathematically by use of experimentally derived parameters.

The most prominent methods are the fundamental parameter approaches based on standard or on programs for the standardless analysis (Lachance and Claisse 1995). Monte Carlo calculations (Vince et al. 1995a, 1995b) and chemometric techniques (Adams and Allen 1998; Luo et al. 1998) are appli­cable as well. It is beyond the seope of this book to go into further detail on the different calibration methods. Suffice it to say that the development of POwerful hardware and software in combination witb new mathematical and computational techniques improved XRF in the last decades from a semi­quantitative to a fully quantitative method.

64

X-ray tube

X-RA Y FLUORESCENCE

First reflector Primary X-rays

Si(Li) -Detector

Fluorescence radiation

Sample carrier with sample

Totally reflected X-rays

Figure 4.6. Schematic representation of a total reflection X-roy fluorescence spectrometer (TXRF).

4.3.5. Total ReHection X-Ray Fluorescence Spectrometry

Total reHection X-ray Huorescence spectrometry (TXRF) is a newly devel­oped method that differs from conventional energy dispersive X-ray fluo­rescence (EDXRF) by the excitation geometry (Klockenkamper and von Bohlen 1992; Prange and Schwenke 1992; Klockenkamper et al. 1992; Klockenkiimper 1996). In TXRF a Hat, sheetlike primary X-ray beam is filtered and collimated when passing a first reflector (see Fig. 4.6). Then it is directed at a small glancing angle < 0.1 0 to a very nat and smooth surface of a sample carrier. The incident beam is totally reHected at the sample carrier's surface. It therefore hardly interacts with the carrier but effectively excites the elements of the sample. The Huorescence radiation emitted by the sample is recorded by a Si(Li) detector positioned directly above it. Usually TXRF instruments operate at normal pressure in the sample chamber. Absorption of X-rays by the gas in the chamber is minimized since the sample detector distance is very small.

The special excitation arrangement leads to a substantial decrease of the spectral background originating from the sample carrier. If applying only small amounts of sample (microgram or microliter size), a very effective excitation to Huorescence of the sample and an effective collection of the secondary radiation is observed. The detectable amount of a great number of elements is in the order of a picogram (10- 12 g). Moreover, if a critical sample mass (upper limit), depending on the matrix of the sample, is not exceeded, no matrix effects occur and multielement analysis with a simple

4.3. X-RAY FLUORESCENCE SPECfROMETRY 65

and reliable quantification becomes possible (Klockenkamper and von Bohlen 1988).

In TXRF spectra the intensities [of the X-rays are recorded. If the above­mentioned conditions concerning the sample are fulfilled, no interelement effects are observed and the recorded intensities are proportional to the mass fractions of the corresponding elements. The intensities have only to be divided by their respective relative sensitivities S. The sensitivities are con­stants of proportionality that have to be determined only once by analyzing the residue after drying of aqueous standard solutions. By the absence of matrix effects these sensitivity factors can be applied to all kinds of samples.

If the sample mass is too low to be determined accurately, only relative mass fractions (In,) of the detected elements can be calculated by the fol­lowing equation (von Bohlen et al. 1994; Klockenkiimper 1996):

Thus, relative mass fractions with respect to the sum of the masses of all detected elements (k) can be calculated. However, a full quantification be­comes possible if the sample mass is known and the method of internal standardization is applied. For this purpose, a known quantity (l1Ii) of only one standard element, which is not contained in the sample, is added to the sample. After recording the spectrum, the absolute masses of the detected elements (III, ) are calculated from the following equation (von Bohlen et al. 1994; Klockenkamper 1996):

[,IS, nJ;c = . I S. x mint

lint tnt

and when relating tbe masses 111, to the total sample mass, mass fractions (e.g., in micrograms per gram) can be obtained. As mentioned above, the TXRF sample chamber is normaUy filled with air, so that the characteristic X-ray radiation of elements with atomic number" 13 is absorbed and these elements are not detectable.

TXRF instrumentation is, compared to other modem techniques, rela­tively simple. An excitation unit containing a high-voltage generator and an X-ray tube with housing, a total reflection module, and a detection unit composed of a Si( Li) detector and a multichannel analyzer are needed. The Operating and maintenance costs arc low.

TXRF proved to be a versatile tool for chemical analyses, especially for extreme trace and microanalyses and for direct analyses of solid samples as

r 66 X-RA Y FLUORESCENCE

was reviewed by several authors ( Reus and Prange 1993; Klockenkiimpcr et al. 1992; Prange and Schwenke 1992; Klockenkamper 1996; Moens et al. 1994).

4.4. APPLICATIO 'S

The examples cited here are far from a complete overview and should only show the versatility and the variety of applications of X-ray fluorescence analysis.

4.4.1. Analysis of Ancient Coins

Metal analysis is one of the most promioent applications of XRF in Hr­chaeometry. In particular, the analysis of ancient coins proved to be very useful. In general, coins were produced from well-controlled alloys by a known mint and often were provided with a date of issue. Many references concerning the technology, the typology, and the chronology can be found in historical documents and in modern studies. Coins are, except for the rough surface, ideal objects for XRF analysis. They can be investigated without any sample preparation and nearly all elements of interest present as major or minor constituents, as well as some traces can be detected in coins of all forms and sizes. Further advantages are their favorable physical properties allowing nondestructive multielement analysis in a reasonable time. Only layers of patina, bleaching, or plating have to be considered when discussing the results of elemental analyses. Indeed , the observed fluorescence radiation originates from the top layer of the sample, resulting in a "depth of infor­matioo" of I to some 100 J.1m, depending on the applied technique, the alloy, and the elements in question . Therefore XRF results do not necessarily rep­resent the composition of the bulk of the coin material.

Until now, the elemental composition of hundreds or even thousands of ancieot coins, predominantly of Roman origin, has been determined by XRF analysis. Very different approaches have been used for this purpose, and it is notable that XR F techniques often have been used io combination with other methods.

Condamin and Picon (1964) were the first to examine corrosion effects on dinarii and concluded that the surface silver content will be markedly higher than the silver content of the alloy within the coin. The notable discrepancy between the silver contents in Roman coins induced several approaches going hand in haod with the development of techniques to study problems of bleaching, plating, corrosion, and wear off by circulation. Walker (1976, 1977, 1978) presented an extended work related to the metrology of Roma n

4.4. APPLICATIONS 67

silver coinage. Several other works dealing with the examination of Roman coins (e.g., Carter 1978; Butcher et al. 1997) and related analytical problems (e.g., Lutz and Pemicka 1996) have been published. Some of these are pre­sented below.

In his approach, Klockenkiimper (1978) uses WDXRF and a scanning electron microscope with energy dispersive X-ray detection (SEM-EDX) for the characterization of more than 200 coins. The combination of the two techniques with different penetration depth of the primary X-rays and of electrons, respectively, into the material provided insight into the silver enrichment at the surface of dinarij relative to the silver content in deeper layers. The results showed the decay of values in the Roman Empire: Less silver was used for coinage alloys, but the surface of the coins was enriched starting from about the year A.D. 200. Similar results are reported for Roman coins (dinarii , quintarii , etc.) found in Switzerland, Augusta Raurica, and analyzed by Zwicky-Sobczyk and Stern (1997). They combined the results of EDXRF analysis using silver-K and silver-L lines and of density measure­ments to characterize more than 600 coins. The intensity ratio of silver X-ray lines in modern silver alloys is, theoretically, a clue to the presence of plating, but with most ancient silver coins this ratio is influenced by the surface morphology and by corrosion. However, the silver-K intensities of massive and of uncorroded plated silver coins are clearly different.

Though most work has been done on Roman coins, other coins of very different regions and ages were analyzed using XRF as well. Ancient dir­hams (silver coins) from the Abbasid period 158- 218 Hirji (A.D. 775 to 833) of the Great Islamic Empire were analyzed by AI-Kofahi et al. (1997). The dirhams were found to have silver (about 41 to 84%) as a major constituent and minor quantities of 10/0 or more of silicon, copper, mercury, gold, and lead, of O. I to 1% of aluminium, phosphorus, iron, and tungsten and less than 0.1% of titanium, nickel , and zinc. The results of the analyses show that the dirhams coined in the capital of the state, Madillat Essalam, are of good quality, having a silver concentration of at least 80%, whereas those fab­ricated in locations far from the capital have significantly lower silver con­centrations (41 to 54%).

Genuine and counterfeit German Reichsgoldlllllll:en dated 1872 to 1914 made of gold were studied by Klockenkiimper et al. (1990). A gold content of about 90% and a silver content of about 0.4% were found to be reliable criteria of genuineness. Counterfeits frequently show more gold and often more or less silver. Additionally, several counterfeits have a gold enrichment at the surface.

Early Russian platinum roubles, of the beginning of the eighteenth cen­tury, Showing gold inclusions and lower fineness than reported by the mint were examined by Auer et al. (1998). In earlier times platinum ores contain-

68 X-RA Y FLUORESCENCE

128.B

97.28 PI PI

L~' La'

76.88

u 58.88 .. PI <II

43.28 J!l L~213 c: 311 ••

" PI Pt 0 19.28 PI u La2 .>< Ljl4 18.88

Au Pt Au PI I, Pt I, 4._ I, Pt Au L~ L~ La La'

Cu K~

1.21!11 .-B.1!'lII B.l. 8.118 B.128 8.138 8.148

Wavelength, nm

Figure 4.7. WDXRF spectrum ora platinum rouble of the year 1838 showing gold and iridium lines next 10 those of platinum; analyzing crystal L1F (220); rhodium X-ray lube 100 kV, 30 rnA.

ing ca. 64% platinwn found at the Ural could be refined only by a complex chemical procedure. The refined platinum was minted in a powder metal­lurgical process to coins of 2, 3, or 6 roubles. These should have 96 to 99% of platinum and up to 0.5% of ruthenium, 0.25% of palladium, 1.2% of iridium, and traces of chromium, iron, and copper. The analysis of 2 rouble coins revealed the presence of additional elements like manganese, zinc, silver, and gold. An XRF spectrum of a platinum rouble with gold inclusions is shown in Figure 4.7. The detection of manganese, iron, nickel, copper, zinc, palla­dium, silver, iridium, and gold suggests a mixing of refined platinum and original platinum ores for the coinage.

The transfer of gold and silver originating from the ew World starting about the year 1500 and the propagation in the Old World and later mixing with European, Russian, and Ottoman gold used in coinage was studied by Guerra (1995, 1998). Typical element traces, for example, arsenic in copper, indiwn in silver, rhodium, palladium, and platinum in gold, present in coin­age metals of different provenance were analyzed by a combined procedure including methods of neutron activation, laser ablation mass spectromerty, proton-induced X-ray fluorescence, and XRF.

4.4.2. Pottery and Porcelain

Among the most important archaeological finds are ceramic materials, and it should not come as a surprise that numerous XRF studies deal with pottery or porcelain. Small samples are usually taken and either analyzed as such

b

4.4. APPLICATIONS 69

(LaBreque et al. 1998) or prepared for analysis (Punyadeera et al. 1997) following standard XRF procedures. Pottery samples c.1n often be linked to a common provenance based on petrographic and chemical characteristics originating from the clay matrix or the temper. The interpretation of the analytical information in most cases will be based on multivariate statistical methods. XRF has been used for the determination of major, minor, and trace elements characterizing pottery of known or unknown provenance. For instance, LaBreque et al. (1998) have used radioisotope (cadmium-109) XRF to study the origin of 12 Majolica ceramics, found in the Americas, by com­paring the concentrations of lead, rubidium, strontium, and zirconium in these with the concentrations of the same elements in 29 samples of known provenance. The lead tin enamel of the shards was removed with a diamond saw, and the remaining clay fraction was prepared for analysis by grinding and sieving of the obtained powder. A fraction of the powder was trans­ferred to a sample holder and measured as such. Principal component anal­ysis was used to establish the provenance of the unknown samples.

Yu and Miao (1996) used EDXRF forthe determination of trace elements in Chinese blue and white porcelains of the Ming dynasty, in imitations, and in a modem porcelain object. The porcelain objects were analyzed as such. For calibration, samples of broken objects from the Ming and Qing dynasties were taken and analyzed with EDXRF and inductively coupled plasma atomic emission spectroscopy (ICP-AES). For the latter analysis the outer I-mm layer of the shards was taken as a sample. The material was ground and sieved prior to dissolution and ICP-AES analysis. The quantitative re­sults were used for principal component analysis. Using the concentrations of 13 trace elements, this allowed porcelain from different periods during the Ming dynasty to be distinguished and imitations to be separated from au­tbentic artifacts.

Punyadeera et al. (1997) used EDXRF for a provenance study of Lron Age pottery from South Africa. They compared the amounts of 10 transition metals in 107 potsherds originating from four archaeological sites. The shards were ground and the powder was homogenized, mixed with a binder, and compressed to a pellet of standardized dimensions and with a well­polished flat surface. Pellets prepared from blank samples and from four geological samples of known composition were used for calibration pur­poses. The data obtained were subjected to correspondence analysis to dis­tInguish between groups. The groupings were interpreted in terms of social and cultural interactions between sites.

4.4-3. Obsidian

ObSidian was widely used and traded in prehistoric times as raw material for tool making. Provenance determination of obsidian found at archaeological

70 X-RAY FLUORESCENCE

sites therefore is an interesting tool for studying trade patterns and intcr­cultural contacts. Among others, Vazquez and Escola (1995) have used WDXRF to detennine the concentration of five trace elements (rubidium, zirconium, strontium, titanium, and manganese) and one major element (iron) in obsidian samples from archaeological and geological sites. Obsidian flakes were ground and the powder was sieved and compressed to a pellet using cellulose as a backing. Synthetic standards, mimicking obsidian, were made by mixing known amounts of the elements of interest with basic com­ponents (mainly silicon dioxide) that reflect the major element composition of obsidian. It was found that rubidium, zirconium, and strontium are very reliable indicators for the provenance determination of the obsidian found at the site of Casa Chavez Monticulos (450 B.C. to A.D. 650) in Argentina. The obsidian deposit of Ona, at a distance of 80 to 90 km from the site, turned out to have been the source of the material used at the site.

4.4.4. Ivory

The analysis of mammoth ivory was the subject of a study (though not a purely archaeological study) by Shimoyama et al. (1998). The Convention on [nternational Trade in Endangered Species (C ITES), while protecting elephants has also created a market for smugglers. Archaeological mam­moth ivory is obviously not meant in the CITES, but it has been discovered that elephant ivory is transported together with and as mammoth ivory. Visual distinction of these materials is extremely difficult and therefore a method was designed whereby nondestructive XRF analysis of the dentine is used to distinguish mammoth ivory from modern elephant ivory. Use was made of glancing angle XRF for the determination of the intensity ratios of the K. X-rays of strontitun and calcium and of bromine and phosphorus. These ratios are sufficiently different for both types of ivory to allow mam­moth ivory clearly to be distinguished from smuggled African elephant ivory.

4.4.5. In Situ Element Mapping with EDXRF

Element mapping of small objects can be performed with a small diameter X-ray beam and by stepwise moving the object, at each position performing XRF analysis. EDXRF equipment can be made to be portable or at least displaceable. Thus it becomes feasible to transfer the equipment to a museum or any site where artifacts need to be studied. Moreover the equipment can be attached to a mechanical system that allows it to be positioned at will with Te pect to large artifacts. The primary X-ray beam is aimed at a selected spot on the surface of the object and the characteristic fluorescence X-rays

?

4.4. APPLICATIONS 71

are measured. Schreiner et a!. (1992) have used such a system for in silll mapping of elements in large objects such as paintings. In one such appli­cation, they performed element mapping of Indian miniature paintings (seventeenth century) in the Millionenzimmer at the Schonbrunn palace in Vienna (Austria). The spatial resolution of the mapping was I mm and the measuring time per spot was 30 sec. This led to a long total analysis time (up to hours), but it was possible to identify most of the pigments used: red lead, vermilion, white lead, a green copper pigment, azurite, and ochre. In addi­tion silver and gold were found to be present in metallic form. Some pig­ments could not be identified and were assumed to exclusively consist of light elements that escape XR F analysis under the conditions used (e.g., ultra­marine or organic pigments). The mapping allowed origina l sections to be distinguished from later additions.

III silll analysis of paintings is hampered by the fact that the observed X­rays originate not only from the pigments under study but also from super­ficial contamination, varnish, underlying paint layers, or even the carrier. Critical interpretation of this mixed information is therefore necessary and in most cases sufficient to come to relevant conclusions.

4.4.6. Glass

Glass Analysis .. il" SRXRF. Janssens et a!. (1996) and Adams et a!. (1997) have used synchrotron radiation for the analysis of glass. These authors studied 90 Roman glass samples (about A.D. 4 to 68) originating from Qumran (Jordan) and detemlined tbe concentrations of trace elements with II·SRXRF. Unlike the major element composition, the trace element pattern showed considerable variation and allowed the objects to be divided in two different groups (Fig. 4.8). It was concluded that objects from each of both groups originated from the same batch of glass, suggesting a local production Or import from the same source. The same authors also used the method for trace element mapping, making line scans over glass sections in a direction perpendicular to the surface. Thus the alteration of the glass could be studied.

Glass Analysis II'il" TXRF. The characterization of valuable glass objects exhibited in museums becomes possible when no samples need to be taken or When only a few grains of material are required. Samples of ca. 100 ~g can be taken witb a diamond drill from the bottom of the artifacts (Wegstein 1997). The technique was applied to sample artifacts in the Hessisches Landesmuseum Kassel , Germany. The small samples were stored in plastic containers to avoid loss or contamination during transport to the laboratory.

Subsamples of only one or two grains were used to perforol semiquanti­tatIve analyses of a Bohemian Reic!Jsadlerhllmpell dated back to the year

72 X-RAY FLUORESCENCE

~r------------------------------------------------'

,.,

.. : .. E

'" " ", E

! 300 1I C , 0 200 E • ,.,

Cr2Ql NiO CuO ZIIO Rb20 SrO YlOl Zr02 M0203 Sn02 Sb205 a.o TII205 PbO

.,.,L-__________________________ --'

Figure 4.8. Concentrations detcmlined by SRXRF in two differenl types of Roman glass from Qurnnin (Jordan).

1572. Relative amounts referring to the amount of iron (-100 arbitrary mass units) were determined with TXRF: potassium = 2160, calcium = 2260, manganese = 147. Minor constituents and trace elements were titanium, zinc, rubidium, strontium, barium, and lead. [n the glass of this tumbler all elements found in the ashes of native plants (potassium, calcium, manga­nese, iron, zinc, rubidium, strontium, barium), sands (titanium, iron), and lime are present. The presence of other traces (e.g., lead) can be explained by the usual recycling of old glass. Furthermore, potassium- and manganese-rich components must have been added to the glass because the amounts present cannot be explained by the inclusion of native plant ashes alone. For com­parison, glass fragments from old Prague (sixteenth century) and from the glassworks in the Vogler (a forest in Lower Saxony) were analyzed. Nearly identical element ratios of potassium: calcium of I : I and of manganese: iron of 2: I were found as in the Bohemian Reichsadle,humpell. In contrast to these results, the analysis of recent glass from glass-making regions in Lower Saxony and Thuringa shows a potassium:calcium ratio of I: 5 and less manganese than iron.

4.4.7. Analysis of Artist's Pigments

The analysis of valuable works of art such as paintings, miniatures, or poly­chrome sculptures is ethically acceptable only if no damage is inflicted to the

4.4. APPLICATIONS 73

artefact (as in direct XRF without sampling) or if the damage is negligible. For TXR F only microsamples (less than I Ilg) are required, and a gentle microsampling method was developed, consisting of rubbing a dry cotton wool bud (Q-tip) over the painted surface. This will remove a minute amount of material, without causing any visible damage. A fraction of the material can next be transferred to the TXRF sample carrier.

TXRF was extensively used for identification and fingerprinting purposes. Apart from being practically nondestructive, the method proved to be ad­vantageous because samples can easily be taken by restorers or art histor­ians, the response time is short (analysis time of a few minutes), and the costs are very low. However, sampling is possible only when the pigment layer is not covered by varnish. For most old paintings, therefore, samples can only be taken during restoration. Also, the sampling only concerns the surface of the object, and the analysis consequently does not reveal any information on deeper paint layers.

TXR F proved to be an excellent method to identify artists' pigments via the detection of key elements, a principle also applied in the in silll XRF pigment analysis outlined before. Especially ancient pigments are predomi­nantly of mineral origin and can indeed be characterized by the presence of one to three typical elements (Klockenkiimper et al. 1993; Moens et al. 1995). In addition, TXRF allows the mixing ralios of different pigments, used to produce a paint of a particular hue, to be determined. Finally, it is also possible to determine minor and trace element concentrations that reflect the characteristics of the minerals used, of the refinement process, and of the actual paint-making recipe used in tbe artist's workshop. This con­centration pattern is often a unique result of various parameters and there­fore can be used, with the necessary precautions, to tell different workshops apart.

Apart from this, pigment identification and characterization can provide data for various other investigations: general art historical or archaeological studies on the materials used, conservation and restoration, deteclion of forgeries, and relative dating. The latter two aims are within reach because some pigments are known to have appeared (dating POSI quem) on artists' pallets and/or to have disappeared (dating anle quem) at a certain time.

Apart from the detection of anachronistic pigments, the detection of pig­ments that were not found in any of an artist's authentic works can also reveal falsification. For instance, in 1994 a privately owned painling claimed to be painted by Modigliani was offered to the organizers of a comprehen­Sive exhibition on the work of this master. The pallet of Modigliani has been studied by Delburgo et al. (1981 ), who analyzed 15 authentic oil paintings With in Silll XRF. It was reported that in those paintings Modigliani only u.sed cadmium and chrome yellow, ochre, vermilion, chromium green , Prus· sian blue, wbite lead, zinc white, and organic pigments. In the painting dis-

74 X-RA Y FLUORESCENCE

cussed here, cerulean blue and emerald green where detected via TXRF analysis of microsamples, which could indicate falsification (Devos 1996). Further radiographic research revealed anomalies in the painting technique as well , and the painting was rejected by the exhibition.

4.4.8. Analysis of Ink

Medieval manuscripts are often illuminated with miniatures and decorative motives (Van Hooydonk et al. 1998, Vandenabeele et aI. , 1999). The pig­ments used have been studied among others with TXRF. Also writing ink, in most cases gall ink, was found to yield interesting information on the writer. Gall ink was made by adding iron sulfate to gall extract. A complex between iron and gallic acid is formed that , only after the writing, turns into a black ink by oxidation of the iron (Wunderlich 1994). In the scriptorium new ink had to be made quite regularly, and it is likely that the impurity pattern was slightly different for each new batch and quite different for different scripto­ria. This allowed constructing a chronology of undated lellers and notes by Galileo based on impurity patterns determined with proton-induced X-ray emission ( PIXE) and comparison of these patterns with those of inks on dated bills and diaries (Giuntini et al. 1995). Recently, ink analysis was used to study a series of manuscripts from the collection of Raphael de Merca­tellis (1437 to IS08), native son of Philip the Good, Duke of Burgundy (Derolez 1979). One of the manuscripts ( Decretum Gratiani) consists of three volumes from which microsamples where taken from the ink on several pages. The relative concentrations of a number of elements, as deternlined by TXRF are shown in Figure 4.9. All samples show a similar pattern, except one, a sample from a colophon, mentioning the acquisition date (ISOS ). The

100 90

Q) 80

> 70 ~ 60 ~ 50 C 40 1l ~ 30 Q) a. 20

10 0

K Ca Mn Fe Ni Cu Zn Sr Hg Pb [---""* '""!"" ....--CoOophon I

Figure 4.9. Impurity pattern of gnU inks on different pages of n Medieval manuscript.

b

4.4. APPLICATIONS 75

analysis suggests tbat the colophon was not written at the same time or the same place as the manuscript.

4.4.9. Violin Varnishes

Historical violin varnishes are mixtures of natural products in which the main components are organic substances such as drying oils, essential oils, resins, waxes, organic colorants, spirit, and others. Beside these major com­ponents some inorganic substances were added to control the properties of the varnish. It is indeed possible to influence thc color, the hue, the trans­parency, the hardness, and also to reduce drastically the drying time of oil­based varnishes. A great variety of old fOrIllulas describing how to produce and how to process varnishes for violinmakers are known, but unfortunately those of early Italian masters were lost. Especially these varnishes, applied by prominent violinmakers and families of violinmakers remain unattain­able. The particular beauty of these varnishes and their capability to preserve the wood and thus the sound quality of the instruments can thet:efore not be reproduced.

Small flakes of varnishes were collected from stringed instruments that were under restoration. The flakes were removed carefully from the pre­cleaned surface of the varnishes by the aid of a scalpel and deposited in small paper bags for transport and storage. Fractions of such samples with masses below 20 fig were analyzed directly by TXRF without any additional prep­aration except the deposition on Plexiglas carriers by means of wooden toothpicks (von Bohlen 1999; von Bohlen and Meyer 1996). The multiele­ment analyses were perforIlled in a counting time of 100 to 300 sec. More than 20 elements (silicon, phosphorus, sulfur, chlorine, potassium, calcium, titanium, chromium, manganese, iron, cobalt, nickel , copper, zinc, arsenic, bromine, rubidium, strontium, barium, lead, potassium, calcium) were de­tected simultaneously and relative detection limits of the order of 10 pgfflg for elements with atomic number Z ~ 24 were estimated. In Figure 4.10 a TXRF spectrum of an Andrea Guarneri varnish of a violin of ca. 1660 is Shown. Elements such as iron, arsenic, and lead could be related to pigments added to the varnishes, whereas manganese, cobalt, and lead are related to siccatives accelerating the drying process of the oil-based varnishes. Occa­sionally elements are detected (e.g., silver or tin) that seem to have been Introduced accidentally.

Unfortunately, the masses of the analyzed flakes were too low to be weighed accurately, and, consequently, it was impossible to calculate the mass fractions of the detected elements in relation to the total sample mass. However, a possibility for quantification is given when relative mass frac­lions of the detected clements are calculated with respect to the sum of all

76 X-RAY FLUORESCENCE

A Guarne ri, Violin ca . 1660 1500

~ K

'" Fe

8 1000 - Ar Pb J!l

" 500 " 8 0

0 2 4 6 8 10 12 14 16

Photon Energy, keV

Figure 4. 10. TXRF spectrum of an Andrea Guarneri varnish obtained from a violin of about 1660. A sample with a mass of less than 20 ~g was used for the analysis. Excitation: molybde­num X-ray tube, SO kV, 38 rnA and spectrum acquisition lime of 100 sec.

detected elements (see before). After nonnalization of the data, a generation and visualization of multielement patterns was performed by a graphical representation of TXRF results using star plots (cf. Fig. 4.11). This combi­nation makes an easy classification of element pattern in violin varnishes possible. It can be used to characterize unknown historical varnishes, to distinguish retouches made on historical stringed instruments using modern varnishes, and for general classification of the elemental distribution in varnishes.

4.5. CONCLUSION

XRF-based methods, versatile and various as they are, will continue to be of great use to archaeometric research. For qualitative analysis the non­destructive character and the speed of analysis allow the element composi­tion of an artifact to be known instantly and without damaging the object. This infonnation is orten sufficient to determine the nature of the material used for making the object. Quantification may require expert knowledge, but numerous examples show that accuracy and precision are within reach. The useful infonnation present in the vast amount of data, generated by the simultaneous multielement analysis, orten is extracted from the data set with the aid of statistical analysis.

In recent years the means have been created to perform microanalysis and spatially resolved analysis with good resolution. Microsamples, as required for TXRF, can be taken from most artifacts without inflicting any visible damage and can easily be analyzed quantitatively. Microbeams, as generated

REFERENCES 77

A. Guarneri, 1670 A. Guarneri, 1690

s s

Co Co

L. Mausslell, 1750 E. Fran~ais, 1950

s s

Zn H-f--'f-

Co Co

Figure 4.11. Radial plots for the characterization of elemental patlern of varnishes of twO dif· ferent celli made by A. Guarneri, of a varnisb of a cello made by L. Maussie ll , and of a modem violin varnish made by E. Franc;:ais. Relative concentrations were obtained by TXRF analysis of small varnish Oakes.

by special X-ray source setups and synchrotrons, allow element mapping at submilimeter scale. It is expected that new applications of these new possi­bilities will continue to be developed, making XRF an even more attractive analytical tool than it is today.

REFERENCES

Adams, M. J. , and J. R. Allen (1998), J. Anal. A 10111. Speclrolll. 13, 119.

Adams, P., A. Adriaens, A. Aerts, I. De Raedt, K. Janssens, and O. Schalm ( 1997), J. Allal. Atom. Spec/rom. 12,257.

78 X-RA Y FLUORESCENCE

AI-Kofahi , M. M., K. F. AI-Tarawneh, and J. M. Shobaki (1997), X-Ray Spectrom. 26, 10.

Auer, E., Th. Rehren, A. von Bohlen, D. Kirchner, and R. KJockenkamper (1998), Metal/a 5, 71.

Bertin, E. P. (1975), Principles and Practice of X-Ray Spectrometry. 2nd ed. , Plenum. New York.

Broil, N. (1996), J. Phys. IV 6,583.

Butcher, K., M. Ponting, and G. Chandler (1997), AJN Second Series 9, 17.

Carter, G. F. (1978), Numismatic Chronicle J9, 67. Condamin, J., and M. Picon (1964), Archaeometry 7, 98. Delbourgo, S. , and L. Faillant-Dumas (198 1), L'Etude au laboratoire de Recherche

des musees de France, in Amadeo Modiglini, 1884- 1920, MusCe d'Art Moderne de la Ville de Paris, Paris.

Derolcz, A. (1979), The Library of Raphael de A1ercafellis, Story Scientia, Ghent. Devos, W. (1996), Ph.D . thesis, Laboratory of Analytical Chemistry, Universi ty of

Ghent.

Guerra, M. F. (1995), Appl. Radial. Isol. 46, 583.

Guerra, M. F. (1998), X-Ray Spectrom. 27, 73.

Giuntini L. , F. Lucarelli , P. A. Mando, W. Hooper, and P. H. Barker (1995), Nile/. Ills/rUIIl. Methods Phys. Res. B 95, 389.

Haller, M., and A. Knochel (1996), J. Trace Microprobe Tech. 14, 461 .

Heinrich, K. F. J. (1981), Electron Beam X-Ray Microanalysis, Van Nostrand Reinhold, New York .

Janssens, K., A. Aerts, L. Vincze, F. Adams, C. Yang, R. Utui , K. Malqvist, K. W. Jones, M. Radtke, S. Garbe, F. Lechtenberg, A. Knochel , and H. Wouten; (1996). Nile/. IlIsrrllln. Methods Phys. Res. 8 109/110, 690.

Jenkins, R. (1988), X-Ray Fluorescence Spectrometry, Wiley-Interscience, London.

Klockenkamper, R. (1978), Fresenills Z. Anal. Chem. 290, 212.

Klockenkamper, R. (1980), in VI/malls Ellcyklopadie der tecllllischell Chemie, Verlag Chemie GmbH, Weinheim, p. 501.

Klockenkamper, R. (1996), Total Reflection X-Ray Fluorescence Analysis, John Wiley & Sons, New York.

Klockenkiimper, R., and A. von Bohlen (1988), Spectrochim. Acta 844, 461.

Klockenkiimper, R., and A. von Bohlen (1992), J. Allal. AIOIll. Spectrom. 7, 273.

Klockenkamper, R., M. Becker, and H. Ouo (1990), Spectrochim. Acta 845, 1043.

Klockcnkiimper, R., J. Knoth, A. Prange, and H. Schwenke (1992), Allal. Chem. 64, 1115A.

Klockenkamper, R., A. von Bohlen, L. Moens, and W . Devos ( 1993), Spec/roc"im. Acta 8 48, 239.

LaBrecque, J. J., J. E. Vaz, J. M. Cruxent, and P. A. Rosales (1998), Specrrochim. Acta 853, 95.

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Lachance, G. R., and F. Claisse (1995), in Quantitative X·ray Fluorescence Analy sis: Theory and Applicalioll , G. R. Lachance and F. Claisse, Eds., Wiley, New York.

LulZ, J ., and E. Pemicka (1996), ArchaeomeirY 38, 313.

Luo. L. , A. Ji , G . Ma, and C Guo (1998), X-Ray Spectrom. 27, 17.

Moens, L. , W. Devos, R . Klockenkiimper, and A. von Bohlen (1994), TRAC Trend Anal. Chem. 13, 198.

Moens, L. , W. Devos, R . Klockenkiimper, and A. von Bohlen (1995), 1. Trace Microprobe Tech. 13(2), 119.

Prange, A., and H. Schwenke (1992), Adv. X-Ray Allal. 35, 899.

Puyandeera, C , A . E. Pillay, L. Jacobson, and G . Whitelaw {I 997), X-Ray SpeClrolll. 26,249.

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Schreiner, M., M. Mantler, F. Weber, R . Ebner, and F. Mairinger (1992), Adv. X­Ray Anal. 35, 11 57.

Shimoyama, M., T. Nakanishi, Y. l-iamagana, T. NinomiY3, and Y. Ozaki (1998), J. Trace Microprobe Tech. 16, 175.

Vandenabeele, p" B. Wehling, L. Moens, 8. Dckeyzer, B. Cardon, A. "Von Bohlen, and R. Klockenkiimper (1999), The Analyst 194, 169.

Van Hooydonk, G ., M. De Reu, L. Moens, J . Van Aelst, and L. Milis (1998), Eur. J. Illorg. Cite",. 5, 639.

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Chapter 12

X-Ray fluorescence of obsidian: approaches to calibration and the

analysis of small samples Jeffrey R. Ferguson

Introduction

For decades, archaeologists have used a number of techniques to dClcnnine the

compositions of geological sources of chemically homogenous materials and then

attempted to match artifacts to sources to understand trade and exchange of material

objects. Such provenance research is common with glassy rhyolites (obsidian) that

were lIsed wherever available as a source of raw material for flaked slone artifacts

such as projectile points and cutting tools. Handheld X-ray fluorescence. of all of

the analytical techniques available for obsidian compositional analysis, has the

potential to make the greatest impact by combining non-destructive analysis with

rapid results, relatively low equipment and analysis cost, and the option of in-field

analysis. While handheld XRF is a powerful research tool, its successful use in

obsidian provenance research requires an understanding of X-ray physics. igneous

petrology, the calibration process, and the ability 10 test a sufficient variety of

homogeneous and well-characterized reference materials suitable for developing

a valid calibration curve. The potential for "point and shoot" handheld XRF is

hampered in pan by the all of lhese factors.

This chapter addresses precision requirements of a matrix-specific calibration,

the choice and analysis of reference standards, spectral normalization to account

ror differences in sample geometry and size, and the use of secondary targets to

fine tune the spectra to maximize sensitivity to the elements of interest. While the

focus here is on obsidian provenance research, many of the same basic principles apply to almost all applications of XRF.

Compositional studies of anifacts have been an integral pan or archaeological

"cientific investigations for decades, providing insight into many aspects of past

human behavior, and obsidian sourcing studies are often presented as the "poster

child" of adherence to the provenance postulate (Weigand, et al. 1977; Neff 1998).

The requirements for materials to permit successful provenance studies as stated in the provenance postulate are that measurable intersource variability must exceed

intrasource variability. Obsidian is a nearly ideal material for provenance studies

402 Jeffrey R. Ferguson

because it occurs in a relatively limited number of geologic contexts and typically

exhibits uniform chemistry within particular outcrops. There are numerous

problematic cases of sources with complex and even overlapping chemistry

(Braswell and Glascock 1998; Glascock et al. 1999: Ambroz et al. 200 1), but in

general obsidian studies are extremely successful at linking artifacts with geologic

sources wherever it was used in the past.

Obsidian is a glassy rhyolitic rock, meaning that it has high silica content and

a disordered atomic structure (Shackley 2005). The disordered atomic structure

allows obsidian to fracture predictably and with extremely sharp edges. Obsidians

arc formed on ly in specific volcank events when magma of the right chemistry

is cooled under specific conditions making it a remarkably rare material. The

fracturing ability and limited natural outcrops made obsidian a valuable resource

that in many cases was carried or exchanged across great distances.

Provenance studies are particularly appealing to archaeologists because they

can document migrations. logistical movements, exchange relationships, and

tool production , use. and discard in a spatial context (Duff 1999; Shackley 2002:

Carter et al. 2007: Eerkens el al. 2007; Ferguson et al. 2010) . Obsidian has the

additional benefit of potentially providing direct temporal informalion through the

measurement of water absorption; a process known as obsidian hydration (Friedman

and Trernbour 1978, ] 983; Origer 1989). When coupled with temporal information

acquired either through obsidian hydration analysis or by other means . such

temporally-sensitive artifact types , stratigraphic sequences, or direct association

with dated materials, it is possible to examine changes in obsidian procurement

patterns over time spans ranging from decades to hundreds of thousands of years.

No doubt the reasonable cost of obsidian sourcing relative to the valuable cultuml

information obtained has entrenched obsidian provenance in the world of contract

archaeology, driving further research in the development of analytical techniques.

geologic characterization, and the cultural interpretation of sourcing results.

This chapter is aimed at providing a technical background for the analysiS

of obsidian using XRF, with a particular focus on proper calibration techniques

and the analysis of small artifacts. This chapter is included in a volume focusing

on handheld XRF because handheld XRF users tend to be most likely to

underestimate the complexity of proper analysis. The analytical issues presented

here are common to both handheld and lab-based XRF. This paper is not intended

to discuss the geologic context of obsidian in detail , proper methods of source

sampling and characterization, or the anthropological implications of obsidian

provenance studies. There are a number of excellent publications on these topics.

starting with volumes by Shackley (l998a, 2005) and edited volumes by Glascock

and others (Taylor 1976; Hughes 1984, 1989; Glascock 2002; Glascock el al.

X-ray fluorescence of obsidian 403

2(07) and these volumes contain references to many of the hundreds of relevant

publications.

Analytical techniques for obsidian

Unlike many other geologically-derived materials typically recovered from

archaeological sites, obsidian sources lend to follow the requirements of the

provenance postulate for major, minor, and trace elements, permitting the

successful application of a variety of analytical techniques. Specific details of

the analytical techniques are readily found in existing publications (i.e. Shackley

1998b) so the discussion here is limited to the differences between the techniques

and their advantages and limitations as they relate to obsidian analysis.

Visual sourcing is occasionally put forth as a more cost-effective method for

examining large assemblages in contexts where all geologic sources are well

known (Weisler and Clague 1998), however the reliability of visual sourcing is

highly questionable (Bettinger et al. 1984; Braswell et al. 2(00). On more than

one occasion 1 have seen archaeologists working in the American Southwest

get excited after finding an artifact with a green translucent color, a feature

characteristic of the Pachuca source in Central Mexico. In one case it turned out

to be from Pachuca, comprising one of the handful of Mexican obsidian artifacts

recovered from north of the Mexican border (except for a small cluster in the

southern tip of Texas [Hester et al. 1991: Ferguson and Skinner 2006]). In the

second , the artifact was from the Antelope Wells source in southwestern New

Mexico that is somewhat similar to the Pachuca source as it also very high in

zirconium and has a green translucent color. While there may be a few cases of

material so visually unique as to aUow some correct visual source assignments ,

there is an error rate most researchers are not comfortable accepting.

Neutron activation analysis (NAA) was one of the first techniques used for the

chemical characterization of archaeological obsidian (Gordus et al. 1968; Glascock

et al. 1994; 1998). NAA allows for the precise determination of a wide range

of clements that can vary depending on the nuclear properties of the elements ,

irradiation conditions, and timing of the gamma count(s), but it is an expensive

technique and typically requires the destructive analysis of at least some portion

of the artifacts (although, as described later, there are options for non-destructive

NAA of small samples). NAA also requires considerable sample preparation time

and cost in the fonn of irradiation and the use of high-purity sample containers.

Another common technique is inductively coupled plasma mass spectrometry

(ICP-MS), often incorporating a laser (LA-ICP-MS) to ablate, or vaporize, an

404 Jeffrey R. Ferguson

extremely small ponion of the anifact for analysis in the ICP-MS. LA-ICP-MS

can provide data on a wide range of elemental compositions, comparable to NAA,

but can suffer from short and long term difficulties with analytical accuracy due

to difficulties with consistent calibration (Speakman el al. 2007). LA-ICP-MS is

often louted as a non-destructive analytical method, but almost every instrument

in use today requires the removal of a portion of a sample in order to fit inside the

laser ablation chamber.

Techniques with limited application to the compositional analysis of obsidian

include proton induced X-ray emission (PIXE: Pollard el al. 2007; Bellot-Gurlct

el al. 2008) and electron microprobe analysis (EPMA; Merrick and Brown 1984;

Pollard el al. 2007). These techniques often involve the same basic method, .,

XRF, but they vary in the means of removing the electron from the inner shell of

the atom in order to create the emitted X-ray as a result of outer shell electron~

dropping down to fill the void. Many of the same issues discussed here for XRF

apply to these techniques as well. Additional analytical techniques include, but are

not limited to: natural radioactivity (Leach el al. 1978). obsidian density (Reeves

and Armitage 1973), fission track analysis (Bigazzi el al. 1998), and numerous

chemical methods (Tykot 2004).

XRF offers the potential for totally non-destructive analysis, ponable and

handheld operation , minimal or no sample preparation. rapid results. widely

available analytical instrumentation, low cost. and quantitative analysi, of some

of the most important elements used for discriminating obsidian sources and

providing provenance data. All of the other techniques described above requ ire

large stationary equipment - ranging from the size of a chest freezer to a nuclear

reactor - and require the sample to fil the space requirements of an instrument

sample chamber rather than introducing a smal l instrument to the object of interest

as is possible with handheld XRF. Most of the commercial obsidian provenance

laboratories employ non-ponable energy-dispersive (ED-XRF) as their main

instruments , yet portable instruments can provide high quality data in the lab while

also aJlowing in-field analysis including obsidian artifacts in countries or curation

facilities that will not allow their removal of artifacts. While the decreasing co ... 1.

increasing availability, and portability have all contributed to making XRF a more

attractive analytical technique to a broader spectrum of researchers across man)

disciplines. the fundamental physical principles behind the technique and the need

to understand them have not changed. The following section describe' some of

the most imponant issues in handheld XRF analysis of obsidian, with panicular

attention to some of the issues encountered in the research at the Archaeometr)

Laboratory at the University of Missouri Research Reactor (MURR).

X-ray fluorescence of obsidian 405

XRF methodology

One of the firM considerations in XRF analysis is whether or not quantitative

analysis is necessary, or even possible, with the sample set under investigation.

While qualitative comparative spectra] overlay can be used to assign individual

obsidian artifacts to particular sources, it is not the most efficient means of

processing large numbers of samples and can lead to difficulties in discriminating

geologic sources with similar elemental ratios (i.e. the shape of the spectra are

similar but the magnitudes differ). Without the ability to extract quantitative

concentrations for certain clements. it is not possible to compare new analyses with

previously published compositional data, limiting comparisons to those samples

previously analyzed using the same equipment. Thus, quantitative analysis is

necessary in obsidian studies. In contrast, qualitative analysis can be highly useful

in. for example. attempting to determine if some large pit features were used to

smelt iron or to fire limestone to produce lime. In this case qualitative spectra]

overlay comparing areas inside the features to areas of the surrounding natural soil

;howed significantly higher calcium and strontium levels in deposits at the bottom

orthe features compared to outside the features and no difference in the amount of

iron. Additional tests may be warranted. but it seems likely thatlhe features were

used as lime ki lns strict ly based on spectral comparisons (Ferguson 20 10).

The main reason for not perfonnjng quantitative analysis on the pit features

described above is I have not yet developed a comprehensive calibration for the

analysis of soils. and the difficulties in properly quantifying such a heterogeneous

material in the field make the process difficult at best. The "point and shoot" claims

by some instrument manufacturers have led to misleading data being published

without a proper matrix-specific calibration (i.e. Morgenstein and Redmount 2005;

Goren el 01. 20 I I). A single calibration is simply incapable of such a broad range

of different matrices, and matrix-specific calibrations are required for accurate

quantitative analysis due to a number of factors, including absorption propenies of

the matrix, secondary fluorescence, and different concentration ranges for specific elements.

Developing accurate quantitative approaches for obsidian analysis allows

the rapid (although not fool-proof) assignment of geologic sources to artifacts

Using a variety of multivariate statistical methods, something not possible with

qualitative spectral overlay. Proper calibration is the key to efficient and accurate

SOurce assignment. MURR has a long history of obsidian analysis by NAA and

consequently a massive source library including almost every known obsidian

SOurce in the world. The MURR handheld XRF generally uses a calibration based

...

II ~ III I" '

1'1'

II II

I

I

I, ".:

406 Jeffrey R. Fergu,on

on compatibi lity with NAA data in order to maximize the potential to find sources

not anticipated in a panicular study, and thus not yet fully characterized by XRF.

The most common deviations between M URR XRF data and that produced at other

labs using only XRF are in the iron and zirconium concentrations. Unfortunately

NAA is not as capable of producing rughly accurate zirconium concentration, due

to the production of both "Zr and "Zr during "'u fission. It is possible to correct

for the uranium interference, but the correction has a large uncertainty (Glascock

el al. 1986).

The differences between the iron concentrations reported by the Berkeley

XRF Laboratory and MURR are more complicated to explain, but Mike Glascock

(pers. com. 201 I) provides a possible explanation. The published values for iron

in many USGS standards used by some XRF labs for calibration arc generally

listed as total iron. USGS standards, as a historical convention resulting from the

use of gravometric techniques, report concentrations for multiple iron oxides. It is

possible to convert the weight percent into ppm concentrations, but there is often

the false assumption that all of the iron exists as the single oxide reported. Thi,

can result in inaccurate iron concentrations entered into the calibration standards.

NAA iron concentrations are based on a single 1ST standard (fly ash) and the

concentration in the standard is not reported as an oxide, but as total iron. AA

calibration is linear and only requires a single standard for iron. The MURR

XRF calibration is based on the data derived from AA measurements of the

40 calibration standards and thus is not susceptible to the oxide weight percent

conversion problem.

Obsidian XRF calibration

Proper calibration is not a simple process. It is important to have a broad range

of concentrations in one's standards that cover the full range of concentrations

anticipated in the unknown samples. This was made clear with the initial analysis

of obsidians from the Central Rift Valley of Kenya. The initial calibration included

samples from 40 well-known obsidian sources in the Americas]. For these

obsidians, iron rarely exceeds 6-7 percent. zirconium is rarely above 1,000 ppm,

and niobium is rarely over 2oo ppm. It was clear from the spectral overlay that

many of the Kenyan samples had abnormally high concentrations of these three

elements , yet the concentrations calculated by the calibration produced lower

concentrations as the peak areas increased. Bruce Kaiser (personal communication

2oo9) has termed this phenomenon the "wet noodle effect" in which peak areas for

panicular elements that exceed the range of the standards used in the calibration

X-ray fluorescence of obsidian 407

can trail off in extreme directions. sometimes leading to inverse relationships

between peak areas and concentrations.

The data from the 2009 Kenya source samples (n:14I , collected by Stanley

Ambrose) serve as an example of the need to include the complete range of

variability for each element. Figure 12.1 is a plot of the NAA-determined

concentrations for iron (X -axis) and the XRF-detcrmined concentrations for

iron (Y-axis) for both the original and extended calibrations. As noted above, the

original calibration produced decreasing iron concentration for larger peaks once

the concentrations exceeded the range in the calibration. The problem was corrected

by including a couple Kenya obsidian samples with high iron , zirconium, and

niobium concentrations. previously analyzed by NAA. The corrected calibration

produces XRF results comparable to NAA .

15000 +--~-__ -~-~-_--~ lSOOO 25000 lSOOO 4SOOO SSOOO 65000 75000

Iron (ppm) from NM

Figure 12.1. Comparison of the initial and extended MURR XRF calibrations for iron. The lower curve (triangles) are the inItial concentrations reported for a group of 141 source samples from Kenya. The upper linear spread ("x' symbols) are the same analyses recalculated using the extended calibration. The Y-axis values are the XRF concentratIons for Iron and the X-axis values are concentrations determined by NAA.

The current MURR calibration uses either 40 or 50 different obsidian secondary

standards previously characterized by NAA at MURR (the main calibration uses

only 40 standards, and the calibration for high iron , zirconium, and/or niobium

samples uses an additional 10 standards, this is explained in greater detail below).

The calibration produces concentrations for 19 elements (Na. AI, K, Ba, Ti, Mn.

I'

408 Jeffrey R. Ferguson

Fe, Co, Ni, Cu, Zn, Ga, Pb, Th, Rb, Sf. Y, Zr, and Nb). Although not a ll these

elements are relevant to the overall composition of obsidian, they are included to

improve the overa ll calibration . Only six elements are routinely used in obsidian

provenance studies. These six elements and their concentration ranges are listed in

Table 12.1. It is necessary to not only bracket the expected range of concentrations,

but to have uncorrelated variation between the elements in the suite of standards.

This multi-elemental variability helps the ca~bration algorithms account for peak

interferences and secondary fluorescence (especially for the elements Rb- 'b

where the spacing of the peaks allows for the K~ peaks from a particular clement

to interfere with the Ka peaks of the element two units greater in atomic number).

It is also important to test calibrations using samples of known composition that

are not included in the calibralion (Mauser 2006).

Mmimum (ppm) Maximum (ppm)

Fe 3554 69319

Rb II 48 1

Sf 0 488

Y 12 662

Zr 61 3455

Nb 4 372

Table 12.1: Minimum and maximum concentrations for standards used in extended MURR XRF calibration.

Unfortunately, concentrations for yttrium and niobium are not avai lable using

NAA. and AA has a relatively high detection limit (approximately 50ppm) for

strontium. Standard values for yttrium, niobium. and low strontium concentrations

were averaged from the ED-XRF analysis of the same standards at twO other

obsidian studies laboratories: Northwest Research Obsidian Studies LaboralOry

in Corvallis Oregon , and the Berkeley Archaeological XRF Lab althe Unive"ity

of California, Berkeley. Methods for analysis and calibration used by these

laboratories arc available through their respective web sites (Northwest Rc!)carch

Obsidian Studies Laboratory 20 II ; Shackley 20 II a).

Shackley (2010) recently commented on the need to use certified international

standards in calibration. This is certainly an ideal goal that would help lO increase

inter-laboratory compatibility, but there are a number of problems with this

approach. First of all, most certified standards are available only as powders.

While the powders can be pressed into pellets there is the potential for analytical

II X-ray Ouorescence of obsidian 409 I'!

~I differences when analyzing solid glass samples using a calibration based on !.

pressed powders. Empirical tests of this powder versus solid sample difference

have shown minimal , if any. effecl. Shackley (201Ib) has analyzed both solid

and pressed pellets from the single boulder used by USGS to create the RGM-

I standard and found very little difference. A recent test at MURR examined a

polished solid piece of obsidian from the Whitewater Ridge source in Oregon and

compared the spectra to loose powder and a pressed pellet from the same rock.

There was a reduction in the backscatter for the powder and pellet samples , but

there was no apparent difference in the peaks of the main elements of interest (Rb-

b). Others have observed morc significant differences as a function of grain size

of powdered samples. Liritzis and Zacharias (20 II) observed differences between

solid and powdered obsidians ranging from up to 5% for K. Ca, Sr, Zr, and Rb,

to 20 and 25% for Fe and Ti respectively. The different matrix effects of different

types of geologic materials (i.e. basalt versus rhyolite) may be more problematic

but are not yet fully studied.

Second, there are very few certifieq,&Iandards for obsidian or compositionally

similar materials . The inter-elemental interactions in XRF are incredibly complex,

and it is desirable to have a large number of calibration standards that not only

bracket the range of concentrations in the samples, but aJso include a large mix

of these elements. The goal of the MURR calibration is to use a large number of

secondary standards in the actual working range for obsidian rather than include a

number of certified standards with concentrations irrelevant for obsidian analysis.

It is critical to create a matrix-specific calibration. Finally. it is unlikely given

the compositional variations of obsidians that si mply using a series of common

standards will allow one to match source and artifact data collected at different

laboratories and using different instruments. Proper calibration using appropriate

standards can increase the probability of identifying possible sources for an

unassigned artifact in published data using other laboratories or techniques. but

assigning an artifact to a particular source unless the sample and source material

are analyzed on the same instruments using the same settings and calibration is

unwise. Some other analytical techniques. such as NAA. are much morc stable

due. in part , to linear standardizations and allow for calibration using a small

number of standards. These techniques are not susceptible to matrix effects,

and thus specific procedures to allow for inter-laboratory exchange of data are

warranted (Graham ef al. 1982; Glascock ef al. 1998). but XRF is based on

different excitation and absorption phenomena and requires a more conservative

approach. A recent inter-laboratory comparison of quantitative XRF of copper

alloys found that the accuracy of the data was a function of variables such as the

type of calibration used (fundamental parameters with and without standards and

II •

III

I"

Ii: I::

410 Jeffrey R. Ferguson

empirical calibrations). the number of standards used in the calibralion, bUl other

variables such as delector type and valid count rates, had Iiule apparent impact on

the accuracy (Heginbotham 20 I I) .

'"

12C

'0<

~

E Q. eo Q. ~

.D Z ou

Mule Moun"lns

,,>U 200 jW J~"

Rb (ppm'

Figure 12.2: Bivariate plot of rubidium and niobium for source samples from Mule Creek. The open symbols are source samples analyzed at MURR and the solid symbols are source samples analyzec at the Berkeley XRF Lab. The specific samples are not the same for each lab. Ellipses represent 90 percent confidence Intervals for membership in the group.

The goal of archaeological obsidian provenance stud ies is to match obsidian

artifacts to specific geologic sources of obsidian . While every effort should be

made to detcrmjne accurate elemental concentrations by using appropriate

standards in a robust calibration , consistent separation of sources is the ultimate

goal. Shackley (2005:7) correctly described the balance between precision and the

ultimate goals of obsidian provenance research: "[sjome physicists and chemists in

the past have made what 1 consider the mjstake of focusing on precision versus the

archaeological accuracy we seek in source provenance studies, and archaeologistS

have been guilty of the opposite - ignoring precision and accuracy in provenance

studies by wholly trusting the analyst because they don ' t understand the process."

In the case of XRF at MURR, the calibration is more valuable the closer it matches

the NAA data to facilitate the identification of artifacts from sources not yet

analyzed by XRF. The differences between the MURR calibration and other major

X·ray fluorescence of obsidian 411

labs are relatively minor. For example, Table 12.2 provides a comparison belween

the Be rke le y Lab and M U RR fo r som e o f the m ajor source s in N e w M ex ico. While

the re a re s uffi c ie nt differences to complic ate inte r- labo ratory s haring o f data , both

labs produce quantitalive elemental concentration data capable of discriminating

most sources, including two compositionally-similar sources from Mule Creek

( Figure 12.2).

Fe Rb Sr y Zr Nb n mean s.d mean s.d. mean s.d. mean s.d. mean !..d. mean

EI Rechuclos Bcrkelcy 15 6676 296 152 6 9 3 23 I 77 3 47 Rh)olile I MURR It 3803 114 138 7 I I I 22 2 7 1 4 39

rObSidian Ridge Berkeley 20' 9735 666 207 II 5 3 63 3 183 7 98

MURR 10 7289 185 186 5 6 I 54 4 168 7 89

f(-cITO del Medio Berkeley 2' 103 15 18 17 160 9 10 I 43 I 172 4 54

MURR 22 7053 396 143.6 7 10 I 4 1 3 167 9 50

Bcar Springs Berkeley 24 ' 6593 43 1 116 5 43 4 2 1 2 108 4 53 Peak

M URR 7 4877 206 110 4 51 5 20 I 108 5 47

~ran" Ridge Berkeley 10 8253 474 564 34 4 I 76 2 11 8 4 196

M UR R 10 5016 256 47 1 22 II I 67 5 10 1 5 171

lHomce ~ l e,. Berkeley 12 9568 321 530 17 4 I 87 3 142 4 236

M URR 13 5945 4 12 422 21 13 I 73 5 11 7 5 205

~I ulc Creek Berkeley 21 9619 385 246 15 16 3 42 4 122 7 27 i(AnlelOpe Creek)

I- MURR 54 6441 380 206 10 2 1 3 37 4 106 5 23

IMuie Creck (N.Sawmill )

Berkelcy 34 8423 564 41 1 15 6 2 7 1 2 118 7 120

L MURR 26 5007 300 358 13 12 2 63 4 98 7 101

Mule Creek Berkeley 15 ~M u le

7936 297 184 7 10 2 25 2 120 5 32

Mounta in<;)

L MURR 13 4861 22 1 163 6 13 2 24 2 11 6 5 28

Gwynn Canyon Berkeley 20 85 12 539 226 \3 19 2 3 1 2 156 7 22

I---- MURR 12 6079 266 205 5 21 2 30 2 156 12 22

melope Well ... Berkeley 25 22521 19 14 348 22 7 I 128 6 1255 47 97

M URR 3 18 178 900 271 8 8 I 115 6 1069 50 82

Fewer samples with Iron concentrations.

Table 12.2:

Mean and standard deviations (in ppm by XRF) for some of the major sources In New Mexico for both the Berkeley Lab and MURR. Berkeley Lab concentrations taken from the laboratory

webSite (http://www.swxrfiab.netlswobsrcs.htm).

!I.d.

2

4

4

3

I

3

5

3

7

10

7

12

3

2

4

5

2

2

3

2

5

9

41 2 Jeffrey R. Ferguson

Maximizing the potential of XRF for obsidian

The goal of XRF analysis is to maximize the nuorcscence of, and thus sensitivity

lO, clements best able to discriminate geologic sources of obsidian. The mOst

useful group of elements that occur in sufficient concentrations for accurate

quantification by EO-XRF are the elements Rb through Nb (atomic numbe"

37-41). These clements generally occur in concentrations of tens to hundreds of

parts per million and the K01

nuorescence energies for each of these elements

are roughly in the middle of the spectrum of energies produced by most handheld

XRF instruments operating at 40kV.

The probability of nuorescing a particular atom is related to the cxcitarion energy of the atom relative to the energy of the incident X-ray. The probability of

fluorescence is greatest when using incident X-rays with energies just above the

exc itation energy of the atom in the sample. For example. iron has a Ka , of 6.4

keY. and there is no chance of K-shell electron nuorescence from incoming X-rays

with energies less than or equal to 6.4 keY, while X-rays lightly higher than 6.4

have the highest probability of fluorescence. The probability of fluorescence

decreases exponentially with higher energy. Thus, the goal should be to produce an

abundance of X-rays with energies just above the absorption edge of the elements

of greatest interest. The elements Rb-Nb have Ka, energies from 13.40 - 16.62

keY. An ideal incoming array would contain X-rays primarily above 17 keY and

tapering off at much higher energies.

Bruce Kaiser (with Bruker Scientific) and R. Jeffrey Speakman (wi th the

Museum Conservation Institute at the Smithsonian) have developed a filter that

acts like a secondary target to absorb the incoming X-rays below about 18 keY

in order to both reduce the background of scattering X-rays at the same energy as

Rb-Nb and to remove thc X-rays of low energy that do not significantly contribute

to the fluorescence of Rb-Nb. The filter consists of thin foils of copper ( 150 Iill')'

aluminum (50 1lJ11) and titanium (300 11m). The copper absorbs the X-ray'

below 18 keY, but the fluorescence of copper produces a large spike of X-ray'

at approximately 8-9 keY. The titanium layer absorbs the 8 .05 keY X-rays. but

produces another peak between 4 .5 and 5 keY as a result of titanium fluorescence.

The aluminum layer absorbs the titanium peak, and the resulting aluminum

fluorescence does not have sufficient energy to impact sample analysis. Figure

12.3 shows the difference in background and increased sensitivity for the same

obsidian sample analyzed under the same conditions with and without the filter.

-

X-ray fluorescence of obsidian 413

16000

14000

4j12000 c c 210000 v

~8000 tl § 6000 0

~ 4000 15 ~ 2000

0

0 S 10 IS 20 2S 30 3S 40 KeV

Figure 12.3: S~tra of the same large flake of obsidian showing the difference in background when using the coppeHitanium-aluminum filter (solid line) versus without the filter (dashed line). The filter reduces the total number ofX:7aYs hitting the sample so the filtered data have been multiplied eight times. Note the minimal background under the peaks between 13 and 17 keV that correspond to the elements rubidium to niobium.

Normalization and small/thin sample analysis

Samples selected for XRF analysis should ideally be infinitely thick fortheelements

of interest. Infinite thickness is defined as the thickness at which additional sample

thickness does not result in additional fluorescent X-rays . lnfinite thickness is

different for each element in direct correlation with the excitation energy and

varies between matrices. although sample matrix differences are not addressed

here because obsidians are relatively similar in their overall high-silica matrix.

For example, 99 percent of the iron fluorescent X-rays (6.4 keY) are from up to

0.1 mm from the surface of a pure silica matrix (obsidian is close to a pure silica

matrix). The X-rays with the highest probability of exciting iron have relatively

low energy and thus arc not capable of penetrating deep into a sample, and even if

they did fluoresce an iron atom deep in the sample, it is unlikely for the low energy

fluorescent X-ray to make it back through a thick sample and into the detector.

While iron peaks in a spectrum result primarily from nuorescence at the surface

of the sample. under the same conditions, zirconium Ka X-rays (15.8 keY) escape

from up to 2.2 mm deep, and barium Ka X -rays (32 .2 keY) from up to 17 mOl

deep.

414 Jeffrey R. Ferguson

The differences in infinite thicknesses are critical 10 an understanding of the

difficulties in analyzing thin samples (I will expand on the analysis of thin samples

in the following seclion), but normalization can help account for some of the issues

related to sample thickness variability. Normalization was initia lly developed as

a means to neutralize the effects of small fluctuations in the output of the X-ray

sources. The method involves selecting a region of the spectrum that does not have

peaks derived from elements of interest in the sample and that directly reneel the

amount and energy of X-rays hitting the sample. The Bruker Tracer lll-V PXRF

in use at MURR uses a rhodium source and thus produces a characteristic peak of

X-rays for Rh Ka at approximately 20.2 keY due to Raleigh scatter of the source

X-rays on the sample. This interaction of Rh X-rays with the sample produces

both elastic and inelastic scatter resulting in both the Raleigh (elastic scalter at -

20.2 keY as mentioned above) and Compton (inelastic scalter at approx imately

18.5-19.5 keY) peaks being produced in the spectrum based on infinitely thick

samples. In order to gel quantitative concentrations for a batch of sarnpl e~ it is

necessary to normalize the peak areas in the rhodium Compton peak and apply the

same correct ion to the other peaks in the spectra. This minimizes the differences

in the incoming X-ray profile.

While the Compton peak serves well for normalization in most obsidians. there

are certain sources with elevated niobium concentrations that have a sufficiently

large niobium K~ peak to interfere with the rhodium Compton peak area . In

these cases, the calibration interprets the additional peak area from niobium as

increased X-ray output requiring comparable reduction in all peaks in the spectra.

Obsidians with such elevated niobium concentrations are rare worldwide. but they

are common for the sources in the Rift Valley of Kenya. For Kenyan obsidian

a slightly modified version of the calibration that normalizes to a portion of the

backscatter in a region above obvious elemental interferences is used. This avoidS

the niobium effects on normalization. but it moves the normalization region

even farther from the peaks for the main elements of interest (Rb-Nb) making

correction for sample size even more difficult. XRF calibration requ ires a series

of compromises to arrive at the best possible data. There is no obvious one-size-

fits-all answer.

The same normalization procedure used to account for tube fluctuations can

also be used , with somewhat less success, to account for variabi lity in sample size.

particularly thickness. Many applications of XRF involve thin samples. such as

the analysis of thin films and filters. This type of thin sample is relatively easy to

correct for if the samples have consistent thicknesses and it is possible lO creat~ comparable calibration standards (Markowicz 2008; Piorek 2008). Although It

is possible to create thin slices of obsidian. artifacts have complex morphology

4

X·ray fluorescence of obsidian 415

and thicknesses are not only inconsistent between samples, but across a single

,ample. By understanding the physics behind the fluorescence of thin samples as

well as the potential errors inherent in normalization it is possible to confidently

determine the source of obsid ian artifacts that are on ly a couple millimeters in

diameter and less than 05mm thick. Figure 12.4 shows two overlaid spectra of a

largelthick flake and a smallithin flake fro m the same obsidian cobble.

700

600

~ 500 c ~

-'= ~ 400 [ ~300

'\' .. ~ 0 ~ 200 , g • , >-'- 100

,

•

. ! ~j

NI ., ~::i

0

0 5 10 15 20 25 30 35 40 KeV

~ igure 12.4: Comparison of large (dashed line) and small (solid line) flakes from the same cobble. The large flake covered the entire beam with a thickness of 7mm. and the small flake measured 4 by 3mm and O.4mm thick.

The current solution to small sample analysis at MURR involves a number of strategies. First of all , we use an instrument with a small beam cross-section .

Figure 125 is a briefly exposed dental X-ray of the Bruker Tracer III-V PXRF

compared to that of the benchtop ElvaX. The Bruker beam focu ses on an area of

approxi mately 2 x 3 mm . Although a narrow beam is more susceptible to var iat ion

in heterogenous samples. obsidian is usually homogenous enough to not create any

problems. The beam in the Tracer III-V is large compared to that of a micro-XRF

and small beam use has great potential for the analysis of obsidian microdebitage.

The small beam improves the analysis of samples with a small diameter by simply

Covering more of the beam. If samples were infirtitely thick, the small sample

diameter would easily be accounted for by the typical normalization process and

no data shift should occur.

416 Jeffrey R Ferguson

I I Smm

Figure 12.5: Dental X-ray film brieOy exposed to the Elva-X (left) and Tracer III-V (right) beams. Note the much smaller beam area of the Tracer.

The real problems arise with the analysis of samples of less than infinite thickness.

For the purposes of this discussion, consider an example in which we are only

concerned with obtaining concentration data for iron and niobium. The niobium

Ka, peak is between 16.3 and 16.9 keY, and this is relatively close to the rhodium

Compton peak used for normalization that is between 18.5 and 19.5 keY. The

closer the two peaks are in energy, the closer they are to having the same infinite

thickness value, thus the two peaks are absorbed at simi lar rates with increases in

sample thickness. When the calibration normalizes to the rhodium Compton peak,

and the same correction factor is applied to the niobium peak the quantitative

results are reasonable. But , iron has an infinite thickness in obsidian measured in

microns , and thus the size of the iron peak does not change as a function of sample

thickness until the samples are extremely thin. This may not seem like a problem,

but the normalization applies the same correction to all peak areas. whether it

is needed or not. Therefore, in a sample thin enough to generale only half the

rhodium Compton peak area. the niobium peak area (and thus the calcu lated

concentration) will be correctly increased by a factor of two, but the iron peak arca

will be significantly over-corrected. The primary elements of interest in obsidian

characterization, Rb-Nb, are all relatively close to the rhodium Compton peak. and

the normalization does a good job of correcting for sample size although there is an

increasing overcorrection moving down in energy. This overcorrection is extreme

for low-energy elements like iron. When analyzing assemblages that include small

samples, it is best to discount data from all elements with fluorescence energie~

below that of rubidium.

It should now be clear why the calibration discussed previously that account,

for the elevated niobium levels in some East African obsidians is an example of

X~ray fluorescence of obsidian 41 7

a difficult compromise. The East African calibration sacrifices accuracy of small

sample analysis because of the higher energy of the normalization region in order

to avoid the negative effects of high niobium concentrations. In other areas of the

world. namely the western United States, the niobium levels are low enough that

sample size is more important and one should use the calibration that normalizes

to the rhodium Compton region.

The combination of the variably erroneous normalization correction in thin

samples and the overall reduced counts for small/ thin samples causes a slight

shift in the data the correlates with sample size. It can make source assignment

by whatever means (i.e. bivariate plots, cluster analysis, Mahalanobis distance

projection) more difficult. Bivariate plots can show the correlation in the scattered

data that should co-vary with the correct source data across all plots for clements

Rb-Nb. It is very important to confirm the source assignment with direct spectral

overlay, keeping in mind the expected shifts resulting from small samples.

A number of recenr-projects on obsidian from a number of si tes in central and

southern California has recently been conducted at MURR to demonmate that

XRF is effective for small sample source assignment. Of the more than 1,000

artifacts analyzed by XRF, 40 of the smallest and most difficult to assign samples

were selected for additional analysis by short-irradiation NAA . The advantages of

the short NAA are that the samples become only sl ightly radioactive and can be

safely handled after on ly a brief decay period , the technique is a bulk technique that

is not subjected to variations in sample morphology because the samplc rotated ,

and it can produce precise data for about six elements on samples of only a few

milligrams. Of the forty samples, XRF source ass ignment was confirmed for 38

of them . One sample was not assigned to any source by XRF but was assigned by

NAA, and one sample was incorrectly assigned by XRF to a source with similar

composition. This error rate of five percent is higher than one would like, but it

occurs on the smallest and most challenging samples. The error rate for the entire

study, including the larger artifacts, would likely be much lower. Similar tests have

been done with smaller assemblages from the American Southwcst with similarly

encouraging resu lts.

The current approach to small sample analysis by XRF is not yet an ideal

solution. Attempts have been made to create an element-specific correction

factor that uses the rhodium Compton peak as an indicator of sample mass. One

possibil ity is to lise a two-element standard behind the artifact that provides a

good approximation of the thickness of the sample in the beam. The standard

would contain sufficient concentrations of two elements, one with a peak below

the elements of interest and one above. In some preliminary tests using a pressed

pellet containing gallium and indium in which the lower energy element (in this

418 Jeffrey R. Ferguson

case gallium) would have a peak area reduced in relation to the higher energy

e lement (indium) in a ratio reOecting sample mass. This method has some promise.

but it is time consuming to keep the standard in proper position and this would

also greatly increase the time required to interpret the final data. It is important to

verify source assignments. particularly with small samples. using other analytical

techniques such as NAA or LA-ICP-MS, but the ultimate solution to small artifact

analysis may involve micro-XRF.

Conclusions

XRF has held a prominent position in the provenance study of archaeological

obsidian for decades, and all indications are that it wil l continue to be a major

analytical technique. Handheld XRF instruments provide some logistical

advantages over lab-based instruments, but aside from a few, relatively minor,

limjtations the two groups of instruments pose the same analytical potential as

well as limitations. Handheld XRF has been the unfair subject of recent skepticism

in the literature (i.e. Shackley 20 I 0). The problems arise not from the portability of

a particular instrument. but from the lower cost of most portable instruments that

places the technology in the hands of researchers with insufficient understanding

of the physics, calibration methods , and analytical limitations. This chapter begi ns

to address some of these problems .

Notes

I . A completely new set of calibration Mandards is in preparation that will cover the kno\\n concentration ranges in obsidian for the elements iron , rubidium , strontium , yttrium. zirconium, niobium and barium. This new calibration set wi ll include 40 obsidian samples from around the world. The concentrations wi ll be derived from a combination NAA, microwave digestion ICP-MS. and XRF analysis of the individual rocks. A publication detailing the concentrations of the originaJ set of caJibrat ion standards in currently in preparation , and considering it will soon be obsolete upon the completion of the new set of calibration standards the values are not reported here.

X-ray fiuorescence of obsidian 41 9

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