Electrochemical Age Determinations of Metallic Specimens

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Electrochemical Age Determinations of Metallic SpecimensUtilization of the Corrosion Clock Antonio Domeń ech-Carbo*́ ,† and Fritz Scholz*,‡ †

Departament de Química Analítica, Universitat de València, Dr. Moliner 50, 46100 Burjassot, València, Spain Institut für Biochemie, Universität Greifswald, Felix-Hausdorff Straße 4, 17487 Greifswald, Germany

Acc. Chem. Res. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/08/19. For personal use only.

‡

CONSPECTUS: Dating needs an age-dependent phenomenon (a “clock”), a procedure for monitoring the advance of time by measuring a physicochemical quantity, and, in the case of archeological artifacts, a sampling procedure that guarantees the representativity and integrity of the dated objects. Metal corrosion in an aerobic atmosphere is a phenomenon whose advance can in principle be used as a clock that depends on the environmental conditions. In spite of the limitation imposed by differences in local conditions of corrosion, a new approach for age determinations has been developed and applied as a feasible tool for age determinations of metallic specimens studied by archeologists and historians. These techniques allow the recording of specific electrochemical features characterizing the state of growth of corrosion patinas, i.e., they are based on corrosion clocks. The application of corrosion clocks for age determination is possible in favorable cases where the corrosion happened to proceed uniformly and continuously. The proposed methods for dating of lead, copper/bronze, leaded bronze, and gold are mainly based on the voltammetry of immobilized particles (VIMP). This technique is exceptionally useful in the archeological domain because it requires only submicrogram sample amounts and permits sampling of different locations on the object, thus yielding representative data collected essentially noninvasively. Reported methods for dating of metals include lead, copper/bronze, and gold, obviously in all cases assuming uniform conditions of corrosion in a moderately aggressive environment. In the case of lead, age markers are porous PbO and PbO2 formed in the secondary patina. In the case of copper/bronze, aging is accompanied by a rise in the tenorite-to-cuprite ratio in the secondary patina. These changes in the composition of the patina can be monitored electrochemically using VIMP. The case of gold is different, as no “true” corrosion patina is formed. Here the age marker is the increase in electrochemically active gold sites, which is ultimately related to the adsorption of oxygen species and its diffusion/interchange/spillover through the external layers of the metal surface. Conjointly considered, such methods provide a new research line intersecting electrochemistry and cultural heritage that can be expanded via improvements in calibration and analysis to become an operative tool in the archeological domain.

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of the earth, the fauna and flora, and finally the cosmos. Dating means establishing a chronology of events, and two different approaches have to be distinguished: indirect (or relative) dating and absolute dating. In indirect dating, a relative chronology is established by comparing time-dependent features, be they cosmological, geological, or cultural. These methods, e.g., stratigraphy and typology, order events on a time scale but do not provide absolute time data, i.e., no absolute ages. Absolute dating involves determining numbers of time units for certain events on a well-defined time scale. These methods need clocks, such as radioactive decay (for radiocarbon dating), defect accumulation (for thermoluminescence dating), or tree ring growth (for dendrological dating). Although metals and metal artifacts are of crucial importance for archeology, anthropology, etc., their dating has remained a serious problem.3,4 Absolute dating is an analytical task, not only

INTRODUCTION 1

Aristotle (384−322 BC): It is clear, then, that time is “number of movement in respect of the before and after”, and is continuous since it is an attribute of what is continuous. Augustine of Hippo (354−430):2 What, then, is time? If no one asks me, I know what it is. If I wish to explain it to him who asks me, I do not know. Yet I say with confidence that I know that if nothing passed away, there would be no past time; and if nothing were still coming, there would be no future time; and if there were nothing at all, there would be no present time. The question of when time entered human consciousness must be reserved for anthropologists and psychologists. It is a dating problem of its own. However, we may assume that humans became conscious of time and started to contemplate the chronology of events when they began to reflect on their own histories (probably first on their individual histories (their lives) and later those of their family, tribe, etc.) and the history © XXXX American Chemical Society

Received: September 18, 2018

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DOI: 10.1021/acs.accounts.8b00472 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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of the corrosion layers with depth, and to discuss possible future developments as complementary analytical tools for archeologists, conservators, and restorers.

because it makes use of the chemical and physical methods that are known for chemical analysis but also because it relies on the same metrological basis.5 Physicochemical methods of dating require the measurement of time-dependent specific physical quantities. The physical or chemical “clock” starts when a given event (e.g., the end of the manufacturing of a copper pot) occurs. This defines an apparent age of the object, which is not necessarily coincident with the age of its manufacturing. For example, in case of potassium-containing minerals dated by the argon method,3,4 the clock starts at the last cooling below the so-called closing temperature, after which the argon cannot anymore leave the mineral phase. Similarly, all dating methods produce age data that are related to the start of the clock, which, as explained above, is not necessarily defined by the formation of the object. Ideally, the physicochemical clock should be universal (independent of location and historical period), uniform (without discontinuities), and insensitive to “local” environmental conditions. However, these conditions are not easily accomplished by the majority of available dating methods.6 Dating of metals is particularly difficult because radioactive decay does not start at the production date of metal objects (with one exception, gold7,8) and no radiation defect formation occurs inside the metal phase, as happens in insulators.3,4 Therefore, methods based on radioactive decay and defect accumulation cannot be applied for dating of metals. A limited number of methods have been proposed for dating of metal objects. In the case of bronze and brass alloys, the Zn/Cu and Sn/Cu ratios in the patina and the base metal have been proposed as chronological indicators.9,10 The supporting phenomena are the decuprification, destannification, and dezincation undergone by bronze and brass alloys,11,12 but no calibrations for age determination are currently available. In the case of lead, Reich et al.13 proposed a dating method based on the estimate of the corrosion advance determined from the Meissner response of the superconducting state of the metal at temperatures below 7 K. This method requires samples having masses of about 100 to 150 mg punched out of the object. A method based on He, U, and Th analysis was recently described by Eugster et al.7,8 Their analysis needs milligram amounts of samples and a special mass spectrometer as well as ICP-MS instruments. The scope of available methods for metal dating is currently expanding through the application of solid-state electrochemical techniques, especially the voltammetry of immobilized particles (VIMP).14−16 In this technique, the voltammetric response of a micro- or nanoparticulate deposit is recorded following its mechanical transfer to an inert electrode, usually graphite, in contact with a suitable electrolyte. Because of the possibility of using amounts of sample at the nanogram level, VIMP is directly applicable in the archaeometric domain, with reported applications for identification, quantification, authentication, provenance tracing, etc. of pigments, organic matter, glass and ceramic materials, and metals.17−20 The first application of VIMP for dating purposes was proposed relative to ceramic materials.21 This philosophy has been further expanded for dating of lead,22 copper/bronze,23 leaded bronze,24 and gold.25 Several of these methods also make use of electrochemical impedance spectroscopy (EIS) complementing VIMP.26,27 This Account aims to present the characteristics and scope of these methods, reexamining published data with a focus on the influence of the manufacturing procedure and the variation of the composition

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THE CORROSION CLOCK Metal corrosion is a very general phenomenon that mainly operates “electrochemically”, i.e., with local galvanic elements: at the surface of a metal, some regions act as anodes, producing oxidized metal species, whereas other regions act as cathodes, where atmospheric oxygen (usually dissolved in water or a water film) is reduced to water. High humidity in the environment and the presence of metal-complexing species (e.g., chloride ions) facilitate this kind of corrosion. Often metal corrosion results in the formation of metal oxides, but depending on the environmental conditions, a variety of other solid phases (sulfides, carbonates, sulfates, chlorides, etc.) or dissolved salts are formed. In general, the oxidation of metals to such compounds is thermodynamically favored, but the kinetics of the corrosion process is considerably variable. Empirical studies on long-term metal corrosion under reasonably uniform conditions have shown that the growth of the thickness of the corrosion patina can be often modeled as a diffusive expansion.28,29 The corrosion clock of a metallic specimen may start when the fabrication of the metal object, which could involve different thermal and mechanical treatments, ends. Generally it starts when the corrosion conditions become constant, e.g., when the metallic specimen is buried in soil, seawater, or other environments. When corrosion is produced under “smooth” atmospheric conditions, a primary “impermeable” patina of metal oxide is formed in the case of lead and copper. This primary patina subsequently expands by the formation of a secondary (or even tertiary) “permeable” patina characterized by lower crystallinity.30 Figure 1 depicts a scheme of the corrosion layers typically formed on copper (similar schemes apply to lead and other

Figure 1. Pictorial representations of the corrosion clocks operating for (a) copper and (b) gold.

metals) and a second scheme describing the aging of gold. The corrosion clock operates as a result of the advance of the corrosion through the secondary patina. In the case of copper and bronze, the aging process determines the oxidation of cuprite (Cu2O) to tenorite (CuO),23 whereas in the case of lead, oxidation of litharge to lead(IV) oxide forms and B


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Accounts of Chemical Research conversion of “impermeable” PbO to “porous” PbO22 can be taken as age markers. The case of gold is different because no formation of a “true” oxide patina occurs. Instead, aging is marked by the formation of electroactive gold sites as a result of adsorption of oxygen species followed by a place-exchange process in which oxygen surface atoms penetrate deeper layers.25,31,32 The use of the corrosion clock for dating purposes has the inherent advantage of the “universal” character of the phenomenon but also the disadvantage that the corrosion pathways and rates are significantly dependent on the local conditions that a given object experiences (in the following, this is called the “corrosion history” of the artifact). The corrosion history determines the structure of the corrosion layer, which generally has a depth-dependent composition and/or crystallinity and/or degree of hydration. The depth dependence, however, can be used to some extent for dating, as will be detailed below. In all of the electrochemical dating work that has been published to date, the sampling procedure consisted of pressing the surface of the graphite bar (diameter between 0.5 and 2.0 mm), i.e., the later electrode, for 5 to 15 s onto the surface of the metal specimen. This protocol is called the “one-touch technique”.22−25

sarcophagus (450 CE; recovered from the Roman archeological site of Gadara, Jordan) and (b) a leaded bronze sculpture. The voltammetric response was recorded in an aqueous acetate buffer at pH 4.75. The sample of the sarcophagus (Figure 2a) displays cathodic peaks of the reduction of “impermeable” litharge (PbO) to lead metal (C1) accompanied by the peaks of the reduction of “porous” litharge (C2) and plattnerite-type (PbO2) compounds (C3). In previous work in this laboratory, it was observed that the C2 peak increased monotonically with age. If other studies were to be carried out with the same result, the last two peaks may become recognized as age markers.22 If it is assumed that the growth of the “porous” litharge follows a potential law, the i(C2)/i(C1) ratio varies with the corrosion time t according to the following equation: i(C2) i(C1)

= K1 + K 2[(1 + β)t ]1/(1 + β)

(1)

where β is part of the exponent of the potential rate law and K1 and K2 are two constants that depend only on the corrosion conditions. Figure 3 depicts the corresponding calibration

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VIMP DATING OF LEAD AND COPPER/BRONZE SPECIMENS Although the voltammetric response depends on the composition and metallographic structure of the base metal and the corrosion history of the object, several of the voltammetric signals of submicrogram samples of the patinas can be used as age markers. Figure 2 compares typical squarewave voltammograms of microparticulate samples of (a) a lead

Figure 3. Calibration plots for dating of lead and leaded bronze objects based on the variation of the peak current ratio i(C2)/i(C1) for lead (open rectangles) and the logarithm of the i(C4)/i(C1) ratio for leaded bronze objects (solid rectangles) with the nominal time of corrosion (t). The curves were obtained from VIMP data such as those shown in Figure 2 using literature data for lead22 and leaded bronze,24 respectively. Continuous lines represent the fits to potential laws such as eq 1 with β = 0.07.

graph obtained for a series of objects with ages known with reasonable accuracy that were corroded under burial conditions in calcareous soils.22 The experimental data are in agreement with eq 1 for β = 0.07, a value in accordance with that obtained by Reich et al.13 using measurements of the Meissner effect of the superconducting state of lead based on well-dated lead samples from Tel-Dor, the Persian period, Caesarea, the Byzantine, and the Crusader periods buried in soils at pH > 6.5. In the case of leaded bronze (Figure 2b), the voltammogram contains a relatively weak signal due to the reduction of copper corrosion products (mainly cuprite, Cu2O) to copper metal (C4) preceding the sharp cathodic peak for the reduction of lead corrosion products (C1). This voltammetric profile, which corresponds to an artificial patination frequently used for Roman sculptures,33,34 permits the ratio of the signals C4 and C1 to be used as an age marker. The corresponding calibration graph obtained from data of sculptures and other objects made

Figure 2. Square-wave voltammograms of submicrogram-samples of (a) a Roman sarcophagus (lead) from Gadara, Jordan, dated to ca. 450 CE and (b) a sculpture (leaded copper) from the Roman site of Valeria, Spain, dated to the second half of the first century CE. Sample traces were attached to graphite bars. Electrochemical measurements were performed following immersion of the graphite electrode into 0.25 M HAc/NaAc aqueous buffer solution (pH 4.75). Starting potential, +1.25 V vs Ag/AgCl; potential scan in negative direction; potential step increment, 4 mV; square-wave amplitude, 25 mV; frequency, 5 Hz. The inset shows a photograph of the sarcophagus from Gadara, Jordan (courtesy of Prof. W. Al-Sekhaneh, Yarmouk University). C


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Accounts of Chemical Research of leaded bronze is also depicted in Figure 3. Again, the experimental data are consistent with the proposed theoretical curve.24 VIMP dating of copper/bronze artifacts is illustrated in the cover figure, where the square-wave voltammograms for a coin minted in 1709 (catalogue no. ADF16, Prehistory Museum of Valencia, Spain) immersed in air-saturated 0.25 M aqueous HAc/NaAc (pH 4.75) is depicted. The voltammograms are dominated by a cathodic peak at −0.10 V vs Ag/AgCl (I), which corresponds to the reduction of cuprite to copper metal, followed by a shoulder near −0.45 V (II) preceding the broad cathodic wave for the reduction of dissolved oxygen (no deaeration was performed in order to test the use of portable equipment for field analysis), which can be assigned to the reduction of tenorite, whose formation by aerobic oxidation of cuprite is thermodynamically spontaneous.23 For dating purposes, the essential idea is that the tenorite/ cuprite ratio in the secondary patina is proportional to the current ratio i(II)/i(I) and thus increases with the age of the object.12 However, two important problems have to be considered: The first one is that the tenorite/cuprite ratio varies with the depth in the secondary patina, so the average value given by the i(II)/i(I) ratio differs depending on the depth reached during the sampling. The second problem is that the tenorite/cuprite ratio is quite sensitive not only to the corrosion history of the object under study but also to the composition and metallographic structure of the base metal.35,36 The first of the above problems is reflected in the fact that for a given metal object the value of the i(II)/i(I) ratio decreases when the absolute value of i(I) (or i(II)) increases, as can be seen in Figure 4a, which depicts the variations of the i(II)/i(I) ratio with i(I) for three sets of archeological coins: Iberian coins from the site of Iltirta, Spain, minted between the first and second centuries BCE (green triangles), maravediś minted between 1661 and 1664 in 10 different Spanish mints (orange squares), and ten cash coins minted by the last Chinese emperors between 1889 and 1911 and the Republic of China from 1911 to ca. 1925 in different provinces (yellow squares). Since, as previously noted, the absolute values of the currents depend on the net amount of sample transferred from the corrosion layers of the artifact to the graphite electrode, the above variation can be interpreted as the result of the decrease of the tenorite/cuprite ratio with depth, as schematically depicted in Figure 1, which can be modeled consistently with voltammetric data.35,36 Examination of the data shown in Figure 4a suggests that in spite of large data dispersion, each set of coins of different age can be grouped into a trend band whose position in the diagram shifts vertically with increasing age. It is pertinent to note that data in Figure 4a correspond to sets of coins from different mints and degrees of corrosion. Consistent with the above considerations, when restricted sets of samples are taken, i.e., coins from a selected coinage (age, mint) and similar corrosion degree, the voltammetric data fit to well-defined trend lines, as can be seen in Figure 4b for maravediś minted in Segovia, Spain, in 1661 and 1662, ten cash coins minted in the Chinese province of Hun Nan ca. 1901, and Spanish Eurocents minted in 2007. In order to use the above data for dating purposes, the calibration graph can be obtained using i(II)/i(I) ratios corrected to compensate for their variation with depth denoted by the representations in Figure 4. If it is assumed that

Figure 4. Variation of the i(II)/i(I) ratio with i(I) for (a) Iberian coins from the site of Iltirta, Spain, minted between the first and second centuries BCE (green triangles), maravediś minted between 1661 and 1664 in 10 different Spanish mints (orange squares), and ten cash coins minted by the last Chinese emperors between 1889 and 1911 and the Republic of China from 1911 to ca. 1925 in different provinces (yellow squares) and (b) maravediś minted in Segovia, Spain, in 1661 and 1662 (green triangles), ten cash coins minted in Hun Nan (yellow squares), and Spanish Eurocents (blue diamonds) minted in 2007. Data were taken from refs 35 and 36. Continuous lines are fits of the experimental data to potential functions.

potential rate laws apply for the growth of Cu2O and CuO in the secondary patina, the time variation of the i(II)/i(I) current ratio is given by23 i(II) At a =G+ i(I) B + tb

(2)

The calibration graph in Figure 5, constructed from voltammetric data for copper and bronze coins presumed to be smoothly corroded under atmospheric conditions, uses the i(II)/i(I) ratios interpolated at i(I) = 10 μA, in the following [i(II)/i(I)]corr. The experimental data are consistent with eq 2 using a = 0.03 and b = 0.07.

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DATING OF GOLD SPECIMENS Gold can be considered as particularly useful for dating purposes because, as a result of its noble character, the conservation of gold artifacts is very insensitive to local conditions of aging. Unfortunately, a satisfactory description of D


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performed using 0.10 M HCl as the electrolyte. Under these conditions, gold is oxidized to Au(III)−chloride complexes at potentials of about 1.0 V,43 and the peak currents are clearly larger than those recorded in H2SO4 or HClO4 electrolytes. Figure 6 depicts the initial anodic linear potential scan

Figure 5. Calibration graphs of [i(II)/i(I)]corr vs time applicable for age determinations of copper/bronze objects and [i(A*(2))/ i(AAu1(1))]corr vs time constructed from voltammetric data for coins and other objects that have undergone uniform, smooth corrosion in an atmospheric environment. The data were taken from refs 23 and 25. The continuous lines correspond to the theoretical variations predicted by eq 2 (copper/bronze) and eq 5 (gold) with the parameter values described in the text.

Figure 6. Linear potential scan voltammograms of a nanosample of a stater coin minted in Cartago in 310−290 BCE. Sample traces were attached to a graphite electrode, and measurements were performed in contact with 0.10 M HCl. The black lines are for the initial anodic scan, and the red lines are for the second anodic scan performed after a cathodic run. The potential scan rate was 50 mV s−1.

the aging of gold surfaces is not yet available. The loss of brightness of the surface has been attributed to the formation of oxides of impurities, especially silver and copper, but also to the physical adsorption of oxygen and hydrophobic recovery.37−39 Gold displays a complex electrochemistry that in the case of microcrystalline gold electrodes in contact with acidic noncomplexing media consists of an anodic signal at ca. 1.1 V vs Ag/AgCl. This process can be described in terms of the formation of an adsorbed primary oxide layer. For simplicity, the formation of the primary oxide monolayer can be formulated as follows:40 Au + 3H 2O → Au(OH)3 + 3H+(aq) + 3e−

voltammogram (black line) of a nanosample of a stater coin minted in Cartago in 310−290 BCE. A well-defined oxidation peak appears at 1.05 V (A1(1)) preceded by a shoulder at about 0.80 V (A*(1)) that can be attributed to the oxidation of gold active sites.31,32 After the application of an inverse cathodic scan, the second positive-going voltammogram (red line) yields a decreased signal A1(2) for the oxidation of bulk gold accompanied by an enhanced signal A*(2) for the oxidation of the active sites. In agreement with the above hypothesis, experiments for a series of gold objects of known age produced a general enhancement of the ratio of the signals of the active sites and the signals of bulk gold. After correction for the sample amount effect25 similar to that previously described for copper/bronze, a more satisfactory calibration was obtained by plotting the ratio i(A*(2))/i(A1(1)) versus time, as shown in Figure 5. The gold calibration graph in Figure 5 can be satisfactorily described by Lagergren’s model of adsorption kinetics, which corresponds to a pseudo-first order rate law of the form

(3)

This process is accompanied by weaker secondary signals at less positive potentials. These signals can be attributed to the oxidation of active gold sites31,32 (i.e., gold atoms that are coordinatively unsaturated or occupy defect lattice sites), which can be represented as (n + 3)H 2O −3e− Au* ⎯⎯⎯⎯⎯⎯→ Au 3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Au 3 +(n + 3)H 2O −3H+ ⎯⎯⎯⎯⎯⎯⎯→ Au(OH)3 ·nH 2O

(4)

i(A (2)) * = H − Q e−kt i(A1(1))

The formation of electroactive gold sites has been described as a result of a place-exchange process in which adsorbed oxygen species penetrate deeper layers, creating defect sites.31 This scheme is supported by Raman spectroscopy data that reveal the presence of different oxygen species, namely, adsorbed ·O, ·OH, ·OOH, and O2 as well as Au(OH)3 and Au2O3 forms in microcrystalline gold surfaces.41,42 The above considerations support the idea that gold aging is accompanied by an increase in the number of active sites that can be interrogated by electrochemical measurements.25 Accordingly, VIMP experiments were carried out with goldmodified graphite electrodes using the same one-touch protocol that was used for lead- and copper-based metals. In order to enhance the sensitivity, the VIMP experiments were

(5)

This equation satisfactorily reproduces the experimental data in Figure 5 with H = 0.71 ± 0.05, Q = 0.58 ± 0.08, and k = (5.8 ± 0.4) × 10−4 year−1.

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FUTURE DEVELOPMENTS AND CONCLUDING REMARKS The described results of electrochemical dating of lead, copper/bronze, leaded bronze, and gold clearly demonstrate that the surface corrosion of metals is in several cases a reliable clock for age determinations. The sampling technique of the described dating is practically noninvasive, since the “oneE


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Accounts of Chemical Research touch” protocol transfers only minute amounts of the surface material to a suitable electrode. The following electrochemical measurement benefits from the advantages of the voltammetry of immobilized particles, which allows the measurement of electrochemical signals that are specif ic for the solid immobilized particles. The inherent sensitivity of voltammetry is the basis for the ability to measure distinct current versus potential signals even for absolute amounts of samples down to 10−12 mol.14−16 Electrochemical impedance spectroscopy and atomic force microscopy forcefully supplement the voltammetric techniques.17−19 The corrosion clocks underlying the dating procedures can be modeled on the basis of corrosion mechanism schemes and some simplified boundary conditions. Current research is focused on applying these dating techniques to trace provenances, characterize manufacturing techniques, and authenticate archeological metals.18−20 One promising line of research in this context is the use of the catalytic effect exerted by several tested materials on the electrochemical response of different redox probes, including the hydrogen evolution reaction, the oxygen evolution reaction, and others. Positive results have been reported for lead22 and leaded bronze.24 We envisage that the hitherto developed electrochemical age determinations can be expanded by (i) studying other metals (silver, iron, etc.) and alloys (brass, etc.) under different aging conditions (e.g., under aquatic corrosion), (ii) applying the methods to other directly related areas such as geology44 and dating of archeological strata,45 (iii) refining calibration data by improving the representativity of sampling and the precision in time estimates, and (iv) applying the methods to nonmetallic archeological materials.46,47 At the beginning we referred to Augustine of Hippo. Still, about 1600 years later we do not know what time is. However, we handle measurements of time and time durations in a more and more sophisticated way. Since the beginning of scientific electrochemistry, time has been considered for developing the kinetics of electrochemical reactions. Now we come back to time by measuring its effects in corrosion for the purpose of age determinations.

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research is focused on the electrochemistry of three-phase systems, thermodynamics of insertion electrochemical reactions of solids and droplets, solid-state electroanalysis, and the electrochemistry of liposomes. In 1989 he introduced the voltammetry of immobilized microparticles. He is the founder and Editor-in-Chief of the Journal of Solid State Electrochemistry and ChemTextsThe Textbook Journal of Chemistry as well as the editor of the series Monographs in Electrochemistry.

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ACKNOWLEDGMENTS A.D.-C. gratefully acknowledges financial support from MICIN Project CTQ2017-85317-C2-1-P, supported with funds from MINECO, ERDF, and Agencia Estatal de Investigación (AEI). F.S. is very thankful to the Deutsche Forschungsgemeinschaft (DFG) for support of several projects concerning the development of the voltammetry of immobilized microparticles.

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REFERENCES

(1) The Works of Aristotle; Ross, W. D., Ed. and Transl.; Clarendon Press: Oxford, U.K., 1930; Vol. II, Physica, Book IV, 10, p 220. (2) Augustine of Hippo. Confessions; Outler, A. C., Ed. and Transl.; Westminster Press: Philadelphia, 1955; Vol. I, Book XI, Chapter XIV. (3) Aitken, M. J. Science-Based Dating in Archaeology; Longman: New York, 1990; Chapter 1. (4) Geyh, M. A.; Schleicher, H. Absolute Age Determination: Physical and Chemical Dating Methods and Their Application; Springer: Berlin, 1990. (5) Doménech-Carbó, A. Dating: an analytical task. ChemTexts 2015, 1, 5. (6) Doménech-Carbó, A. Electrochemical dating: a review. J. Solid State Electrochem. 2017, 21, 1987−1998. (7) Eugster, O. Dating Gold Artifacts Applications for Noble Gas Analysis of Gold. Gold Bull. 1996, 29, 101−104. (8) Eugster, O.; Kramers, J.; Krähenbühl, U. Detecting forgeries among ancient gold objects using the U, Th-4He dating method. Archaeometry 2009, 51, 672−681. (9) Robbiola, L.; Hurtel, L.-P. Standard nature of the passive layers of buried archaeological bronze - The example of two Roman halflength portraits. In METAL 95; MacLeod, I., Pennec, S., Robbiola, L., Eds.; James & James Science Publishing: London, 1997; pp 109−117. (10) Welter, J.-M. The zinc content of brass: a chronological indicator. Techné 2003, 18, 27−36. (11) Meeks, N. D. Tin-rich surfaces on bronze − Some experimental and archaeological considerations. Archaeometry 1986, 28, 133−162. (12) Chiavari, C.; Rahmouni, K.; Takenouti, H.; Joiret, S.; Vermaut, P.; Robbiola, L. Composition and electrochemical properties of natural patinas of outdoor bronze monuments. Electrochim. Acta 2007, 52, 7760−7769. (13) Reich, S.; Leitus, G.; Shalev, S. Measurement of corrosion content of archaeological lead artifacts by their Meissner response in the superconducting state; a new dating method. New J. Phys. 2003, 5, 99. (14) Scholz, F.; Meyer, B. Voltammetry of Solid Microparticles Immobilized on Electrode Surfaces. Electroanal. Chem. 1998, 20, 1− 86. (15) Scholz, F.; Schröder, U.; Gulaboski, R.; Doménech-Carbó, A. Electrochemistry of Immobilized Particles and Droplets, 2nd ed.; Springer, Berlin, 2014. (16) Doménech-Carbó, A.; Labuda, J.; Scholz, F. Electroanalytical chemistry for the analysis of solids: characterization and classification (IUPAC Technical Report). Pure Appl. Chem. 2012, 85, 609−631. (17) Doménech-Carbó, A.; Doménech-Carbó, M. T.; Costa, V. Electrochemical Methods Applied to Archaeometry, Conservation and Restoration; Monographs in Electrochemistry (Scholz, F., Series Ed.); Springer: Berlin, 2009.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Antonio Doménech-Carbó: 0000-0002-5284-2811 Notes

The authors declare no competing financial interest. Biographies Antonio Doménech-Carbó studied chemistry at the University of Valencia, where he was born in 1953. Currently he is a professor in the Department of Analytical Chemistry at the same university. His research is focused on solid-state electrochemistry and, in particular, its application in the fields of archaeometry, conservation, and restoration of cultural heritage. He is member of the editorial board of ChemTexts and a topical editor of the Journal of Solid State Electrochemistry. Fritz Scholz, born in 1955, studied chemistry at Humboldt University in Berlin and currently occupies the chair of Analytical and Environmental Chemistry at the University of Greifswald. His F


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Accounts of Chemical Research (18) Doménech-Carbó, A. Voltammetric methods applied to identification, speciation and quantification of analytes from works of art: an overview. J. Solid State Electrochem. 2010, 14, 363−369. (19) Doménech, A. Tracing, authentifying and dating archaeological metal using the voltammetry of microparticles. Anal. Methods 2011, 3, 2181−2188. (20) Doménech-Carbó, A.; Doménech-Carbó, M. T. Electroanalytical techniques in archaeological and art conservation. Pure Appl. Chem. 2018, 90, 447−462. (21) Scholz, F.; Schröder, U.; Meyer, S.; Brainina, Kh. Z.; Zakhachuk, N. F.; Sobolev, N. V.; Kozmenko, O. A. The electrochemical response of radiation defects of non-conducting materials An electrochemical access to age determinations. J. Electroanal. Chem. 1995, 385, 139−142. (22) Doménech-Carbó, A.; Doménech-Carbó, M. T.; Peiró-Ronda, M. A. Dating archaeological lead artifacts from measurement of the corrosion content using the voltammetry of microparticles. Anal. Chem. 2011, 83, 5639−5644. (23) Doménech-Carbó, A.; Doménech-Carbó, M. T.; Capelo, S.; Pasíes, T.; Martínez-Lázaro, I. Dating archaeological copper/bronze artifacts using the voltammetry of microparticles. Angew. Chem., Int. Ed. 2014, 53, 9262−9266. (24) Doménech-Carbó, A.; Doménech-Carbó, M. T.; RedondoMarugán, J.; Osete-Cortina, L.; Barrio, J.; Fuentes, A.; VivancosRamón, M. V.; Al Sekhaneh, W.; Martínez, B.; Martínez-Lázaro, I.; Pasíes, T. Electrochemical characterization and dating of archeological leaded bronze objects using the voltammetry of immobilized particles. Archaeometry 2018, 60, 308−324. (25) Doménech-Carbó, A.; Scholz, F.; Doménech-Carbó, M. T.; Piquero-Cilla, J.; Montoya, N.; Pasíes-Oviedo, T.; Gozalbes, M.; Melchor-Montserrat, J. M.; Oliver, A. Dating of archaeological gold by means of solid state electrochemistry. ChemElectroChem 2018, 5, 2113−2117. (26) Doménech-Carbó, A.; Doménech-Carbó, M. T.; Peiró-Ronda, M. A.; Martínez-Lázaro, I.; Barrio-Martín, J. Application of the voltammetry of microparticles for dating archaeological lead using polarization curves and electrochemical impedance spectroscopy. J. Solid State Electrochem. 2012, 16, 2349−2356. (27) Doménech-Carbó, A.; Capelo, S.; Piquero, J.; DoménechCarbó, M. T.; Barrio, J.; Fuentes, A.; Al Sekhaneh, W. Dating archaeological copper using electrochemical impedance spectroscopy. Comparison with voltammetry of microparticles dating. Mater. Corros. 2016, 67, 120−129. (28) Kobus, J. Long-term atmospheric corrosion monitoring. Mater. Corros. 2000, 51, 104−108. (29) Balasubramaniam, R.; Laha, T.; Srivastava, A. Long term corrosion behaviour of copper in soil: A study of archaeological analogues. Mater. Corros. 2004, 55, 194−202. (30) Robbiola, L.; Blengino, J. M.; Fiaud, C. Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys. Corros. Sci. 1998, 40, 2083−2111. (31) Declan Burke, L.; O’Mullane, A. P. Generation of active surface states of gold and the role of such states in electrocatalysis. J. Solid State Electrochem. 2000, 4, 285−297. (32) Nowicka, A.; Hasse, U.; Sievers, G.; Donten, M.; Stojek, Z.; Fletcher, S.; Scholz, F. Selective Knockout of Gold Active Sites. Angew. Chem., Int. Ed. 2010, 49, 3006−3009. (33) Formigli, E.; Ferro, D.; Bovani, S. La patina artificiale antica dei grandi bronzi di Ercolano. Kermes: La Rivista del Restauro 2006, 64, 29−34. (34) Donate, I.; Medina, M. C.; Faieta, R.; Barrio, J.; DoménechCarbó, A.; Fuentes, A.; Pardo, A. I.; Martínez, O. Estudio interdisciplinar de pátinas en esculturas romanas de bronce procedentes del yacimiento de Valeria (Cuenca). In Actas del II Congreso de Conservación y Restauración del Patrimonio Metálico (MetalEspaña 2015); Cano, E., Barrio, J., Eds.; Universidad Autónoma de Madrid: Madrid, Spain, 2015. (35) Doménech-Carbó, A.; Doménech-Carbó, M. T.; Montagna, E.; Á lvarez-Romero, C.; Lee, Y. Electrochemical discrimination of mints:

the last Chinese emperors Kuang Hsü and Hsüan T’ung monetary unification. Talanta 2017, 169, 50−56. (36) Doménech-Carbó, A.; Doménech-Carbó, M. T.; Á lvarezRomero, C.; Montoya, N.; Pasíes-Oviedo, T.; Buendía, M. Electrochemical characterization of coinage techniques the 17th century: The maravediś case. Electroanalysis 2017, 29, 2008−2018. (37) Plumb, R. C.; Thakkar, N. Volta Potential Studies of the Aging of Gold Surfaces. J. Phys. Chem. 1965, 69, 439−441. (38) Antes, L. L.; Hackerman, N. Contact Potential Variations on Freshly Condensed Metal Films at Low Pressures. J. Appl. Phys. 1951, 22, 1395−1398. (39) Prysiazhnyi, V.; Slavicek, P.; Cernak, M. Aging of plasmaactivated copper and gold surfaces and its hydrophilic recovery after water immersion. Thin Solid Films 2014, 550, 373−380. (40) Doyle, R. L.; Lyons, M. E. G. The mechanism of oxygen evolution at superactivated gold electrodes in aqueous alkaline solution. J. Solid State Electrochem. 2014, 18, 3271−3286. (41) Desilvestro, J.; Weaver, M. J. Surface structural changes during oxidation of gold electrodes in aqueous media as detected using surface-enhanced Raman spectroscopy. J. Electroanal. Chem. Interfacial Electrochem. 1986, 209, 377−386. (42) Yeo, B. S.; Klaus, S. L.; Ross, P. N.; Mathies, R. A.; Bell, A. T. Identification of Hydroperoxy Species as Reaction Intermediates in the Electrochemical Evolution of Oxygen on Gold. ChemPhysChem 2010, 11, 1854−1857. (43) Herrera Gallego, J.; Castellano, C. E.; Calandra, A.; Arvia, A. J. The electrochemistry of gold in acid aqueous solutions containing chloride ions. J. Electroanal. Chem. Interfacial Electrochem. 1975, 66, 207−230. (44) Doménech-Carbó, A.; Scholz, F.; Schmitt, R. T.; Usera, J.; García Forner, A. M.; De la Fuente-Arévalo, E.; Chinnaya, J.; PiqueroCilla, J.; Montoya, N. Electrochemical characterization of natural gold samples using the voltammetry of immobilized particles. Electrochem. Commun. 2017, 85, 23−26. (45) Di Turo, F.; Montoya, N.; Piquero-Cilla, J.; De Vito, C.; Coletti, F.; Favero, G.; Doménech-Carbó, M. T.; Doménech-Carbó, A. Dating Archaeological Strata in the Magna Mater Temple Using Solid-State Voltammetric Analysis of Leaded Bronze Coins. Electroanalysis 2018, 30, 361−370. (46) Doménech, A.; Doménech-Carbó, M. T.; Lee, Y.; OseteCortina, L. Potential application of voltammetry of microparticles for dating porcine blood-based binding media used in Taiwanese architectural polychromies. Chem. - Asian J. 2012, 7, 2268−2273. (47) Doménech-Carbó, A.; Da Silveira, G. D.; Medina-Alcaide, M. A.; Carmona, A. M.; López-Serrano, D.; Pasíes-Oviedo, T.; AlgarraPardo, V. M.; De Carvalho, L. M.; Montoya, N. Polythiophenes as markers of asphalt and archaeological tar pitch aging. Characterization using solid state electrochemistry. Electrochem. Commun. 2018, 87, 18−21.

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Electrochemical Age Determinations of Metallic Specimens

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