JAIC 2005, Volume 44, Number 2, Article 4 (pp. 103 to 116)
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Journal of the American Institute for Conservation
JAIC 2005, Volume 44, Number 2, Article 4 (pp. 103 to 116)

PLUTARCH'S REPORT ON THE BLUE PATINA OF BRONZE STATUES AT DELPHI: A SCIENTIFIC EXPLANATION

WALTER A. FRANKE, & MAGDA MIRCEA

ABSTRACT—Plutarch reported that the Spartan Monument from Delphi was coated with an unusual blue and glossy patina, due to peculiarities of the air inside the sanctuary. This bronze statuary group has actually vanished. The last remaining bronze sculpture from Delphi, the Charioteer, exhibited in burial a similar bluish appearance, which after a century of indoor exposure turned greenish. On visual inspection the lower torso still preserves a blue coloration. Professional conservators have not yet published any study on the Charioteer's patina. The present article integrates a new study of classical literary sources with knowledge of the natural sciences in order to find a chemical explanation for the blue patina, as well as the circumstances under which it might have been formed at Delphi in ancient times. The authors suggest that the the location of the sanctuary of Delphi above two active faults that break through limestone bedrock favored the formation of azurite on the surface of bronze statues in burial due to elevated concentrations of calcium hydrogen carbonate (e. g., the Charioteer), as well as in the open-air environment, under peculiar conditions (e. g., the Spartan Monument).

Le rapport de Plutarque sur la patine bleue des statues de bronze à Delphes: une explication scientifique. RÉSUMÉ—Selon Plutarque, le monument de Sparte à Delphes était recouvert d'une brillante patine bleue inhabituelle, ceci en raison des particularités de l'air à l'in-térieur du sanctuaire. Ce groupe statuaire en bronze est maintenant disparu. La seule sculpture en bronze qui y a été retrouvée, l'aurige de Delphes, démontrait lors de son retrait du sol une apparence bleutée semblable, qui après un siècle d'exposition est maintenant devenue verdâtre. Toutefois, la section du bas du torse garde encore une certaine coloration bleue. Les restaurateurs professionnels n'ont pas encore publié d'étude sur la patine de cette statue. Cet article présente les résultats d'une étude des sources littéraires classiques combinée à une connaissance des sciences naturelles dans le but d'expliquer la chimie derrière cette patine bleue, aussi bien que les circon-stances dans lesquelles elle pourrait s'être formée à Delphes dans le passé. Le site de Delphes se situe au-dessus de deux failles actives qui fracturent la roche mère en calcaire. Les auteurs suggèrent que ceci aurait causé la formation d'azurite sur la surface des statues en bronze, à la fois lors de leur ensevelissement, en raison des concentrations élevées de carbonate d'hydrogène de calcium (par ex. l'aurige de Delphes), et lors de leur exposition à ciel ouvert, sous des conditions particulières (par ex. le monument de Sparte).

El informe de Plutarco sobre la pátina azul de las estatuas de bronce de Delfos: una explicación científica. RESUMEN—Plutarco informó que el Monumento Espartano de Delfos estaba cubierto con una pátina azul y brillante poco común, debido a las peculiaridades del aire dentro del santuario. Este grupo de esculturas de bronce ha desaparecido. La última escul-tura existente de bronce de Delfos, el Auriga, mostraba cuando estaba enterrada una apariencia azulosa seme-jante, que luego de un siglo de exposición al ambiente interior se volvió verdoso. La parte inferior del torso al ser examinado visualmente todavía muestra una coloración azulosa. Los conservadores profesionales no ha publicado aún ningún estudio sobre la pátina del Auriga. Este artículo integra un nuevo estudio de fuentes literarias clásicas con conocimientos de ciencias naturales para encontrar una explicación química de la pátina azul, a la vez que las circunstancias bajo las cuales se pudo haber formado en Delfos en la antigüedad. Los autores sugieren que la ubicación del santuario de Delfos sobre dos fallas geológicas activas con grietas sobre una base de roca caliza favorecieron la formacion de azurita en la superficie de las estatuas de bronce al estar enterradas debido a elevadas concentraciones de bicarbonato de calcio (por ejemplo, en el Auriga), lo mismo que en el ambiente al aire libre, bajo condiciones peculiares (por ejemplo, en el Monumento Espartano).

Relatório de Plutarco Sobre Pátina Azulem Estátuas de Bronze em Delphi: Uma Explica Ção Científica. RESUMO—Plutarco relatou que o Spartan Monument (Monumento Espartano) de Delphi foi coberto com uma incomum e brilhante pátina azul, devido às peculiaridades do ar dentro do santuário. Este grupo de estátuas de bronze, na verdade, desapareceu. A última escultura de bronze reminiscente de Delphi, o Charioteer (o Cocheiro), apresentava, enterrada, uma aparência azulada semelhante, a qual, depois de séculos de exposição em ambiente fechado, tornou-se verde. No exame visual, o torso inferior ainda preserva a coloração azul. Conservadores profis-sionais ainda não publicaram nenhum estudo sobre a pátina do Charioteer (o Cocheiro). Este artigo integra um novo estudo das fontes da literatura clássica com o conhecimento das ciências naturais a fim de encontrar uma explicação química para a pátina azul, bem como as circunstâncias sob as quais ela pode ter-se formado em Delphi nos tempos antigos. Os autores sugerem que a localização do santuário de Delphi sobre duas falhas ativas, as quais geravam uma camada de pedras calcárias, favoreceram a formação de azurita na super-fície das estátuas de bronze enterradas, através das elevadas concentrações de carbonato de cálcio hidro-genado (por exemplo, o Charioteer—o Cocheiro), bem como, num ambiente aberto, sob condições pecu-liares (por exemplo, o Spartan Monument— Monumento Espartano).


1 INTRODUCTION

Around A. D. 120–125 the ancient Greek writer Plutarch (1935) related that the Spartan Monument of the Admirals from Delphi was coated with an unusual blue and glossy patina, due to peculiarities of the air inside the sanctuary. A few decades later Pausanias (1979) provided a more detailed description of the whole statuary group, composed of 37 life-size bronze statues representing the Spartan general Lysander, as well as the gods and the masters of ships who helped him to defeat the Athenian fleet at Aigos Potamoi. The Spartan Monument of the Admirals was created between 405 and 395 B. C. and has vanished without a trace, presumably after A. D. 392, when the Oracle at Delphi was closed by order of the Christian emperor Theodosius (Pouilloux and Roux 1963; Bommelaer 1971).

At Delphi little escaped destruction except a few small bronze objects that mostly had a grayish black appearance (e. g., griffin heads used as cauldron attachments, an incense-burner, Daedalic figurines), and one life-size bronze statue, the one now called the Charioteer. Polyzalos, the tyrant from Gela, dedicated a statue of a chariot with a charioteer and four horses to Apollo after he won the chariot race at the Pythian Games in 478 B. C. The Charioteer of Delphi was placed in the upper part of the precinct and, during the earthquake of 373 B. C. or later, was destroyed by rock falls and buried by a huge landslide. After 2, 000 years of burial, the statue was brought to light at the end of the 19th century by the excavations of the French School at Athens (Bourguet 1914; Pouilloux and Roux 1963).

The French archaeologist Emile Bourguet (1914) suggested that some of the bronze artifacts found at Delphi, mainly the Charioteer, might have the famous blue patina reported by Plutarch on the Spartan Monument of the Admirals. These bronze fragments of the Charioteer's group exhibited an astonishing bluish patina when just recovered from the sanctuary's soil. Bourguet, who was an eyewitness to the discovery between April 28 and May 1, 1896, described the tinge sometimes as “greenish-blue (240) and other times as “bluish-green (44, 239). Since 1902 this masterpiece of early classical art, one of the best preserved examples of classical bronze casts, has resided at the Archaeological Museum of Delphi (Andronicos 2001). This bronze was cast by lost-wax process (Mattusch 1988) and was preserved in three fragments: torso with head (acc. no. 3520), lower torso from waist to feet (acc. no. 3485), and right arm (acc. no. 3540). Other bronze fragments belonging to the group include fragments of the horses, harnesses, and chariot (Boardman 1985; Rolley 1990; Stewart 1990).

Unfortunately, after more than a century of indoor exposure, the Charioteer's patina became less blue and more an uneven greenish color. However, the belt, the harnesses, and the folds of the robe (and even other fragments of the statuary group) still preserve bluer shades that are visible even to the naked eye.1 Professional conservators have not yet published any study aiming to solve the mystery of the blue patina of bronze statues at Delphi.


2 BACKGROUND RESEARCH

Plutarch was an important official in the hierarchy of Apollo's temple at Delphi for 30 years, and he played a notable part in the revival of the shrine in Trajanic and Hadrianic times (Flacelière 1943, 1987; Barrow 1967). A recent groundbreaking discovery made by an American multidisciplinary team provided evidence that he was indeed a reliable eyewitness and a most valuable ancient source on the Delphic Oracle. The “sweet smelling exhalation” that he mentioned really existed and was an emission of light hydrocarbon gases generated in the underlying strata of the bituminous limestone of Delphi (De Boer and Hale 2000; De Boer et al. 2001; Spiller et al. 2002).

Plutarch (1935) began his work The Oracles at Delphi No Longer Given in Verses by noting the impressions of a young man, Diogenianus, who made a tour of the Delphic sanctuary: “The appearance and technique of the statues had only a moderate attraction for the foreign visitor, who, apparently, was a connoisseur in works of art. He did, however, admire the patina of the bronze, for it bore no resemblance to verdigris or rust, but the bronze was smooth and shining with a deep blue tinge, so that it gave an added touch to the sea-captains [he had begun his sightseeing with them], as they stood there with the true complexion of the sea and its deepest depths” (260–61). Another speaker, named Theon, suggested an explanation for this uneven color of the bronze: “It is air alone; we have most reason to believe that the air occasions it and from its constant presence and contact the bronze here gets its exceptional quality” (Plutarch 1918). Theon proceeded to describe the air from Delphi, which has intriguing features, as

dense and compact, possessing a certain vigour because of the repulsion and resistance that it encounters from the lofty hills; and it is also tenuous and keen … so the air, by reason of its tenuity, works its way into the bronze and cuts it, disengaging from it a great quantity of rust like dust, but this it retains and holds fast, inasmuch as its density does not allow a passage for this. The rust gathers and, because of its great abundance, it effloresces and acquires a brilliance and lustre on its surface. (Plutarch 1935, 267)

In the last decade of the 19th century, archaeologists noticed that there might be a link between Plutarch's report and the surface coloration of some archaeological bronzes discovered in Delphi and elsewhere in Greece. Villenoisy (1896, 70) suggested this blue patina could have been produced by “oxidation, ” since the atmosphere at Delphi is “ozone-rich.” Chaplet (1936) agreed that Plutarch's description of the bronzes at Delphi was consistent with the favorable mountain climate of this region. Moreover, Bourguet (1914) believed that it was the arid atmosphere of Delphi (“l'air sec de Delphes”) that turned the Charioteer's color to green-blue within only a few days of outdoor exposure.

The archaeological controversy that has centered on this passage concerns the question of whether all the bronze statues in the sanctuary had the same peculiar patina or only the statues of the Spartan Monument (Pouilloux 1965; Bommelaer 1971; Jouanna 1975). In the past decade a comprehensive review of the studies related to Plutarch and his connection with Delphi has concluded that almost nothing remained to be said on the blue patina of bronze statues at Delphi: “Il semble qu'il n'y ait plus rien à ajouter sur la patine du bronze à Delphes” (Zagdoun 1995, 589).

The present study aims to combine a new study of classical literary sources with the developments of natural science in a theoretical attempt to find a scientific explanation for a blue bright patina on the bronze statues, as well as to determine the circumstances under which it might have been formed at Delphi in ancient times.

The problem resembles an equation with two vari-ables: the nature of the blue compound of the patina and the specific environmental conditions that could produce and maintain it on the surface of the bronze statues.

To solve the problem, it is necessary to analyze the general environmental conditions at Delphi; review the colors of possible copper patina minerals to select the blue ones; and test whether the selected blue patina mineral is thermodynamically stable under the environmental conditions inferred for ancient Delphi.


3 ATMOSPHERIC AND ENVIRONMENTAL CONDITIONS AT DELPHI

This study presumes that the known changes of climate from the days of Plutarch in the early 2nd century A. D. until today have not drastically affected the atmospheric conditions at Delphi. Meteorological statistics for the last 30 years show an average of approximately 60 days per year with fog in the region of the archaeological ruins, which is enough humidity for the slow formation of a patina. Delphi is situated approximately 650 m above sea level, the shortest distance to the sea being 11 km to the east. The influence of salt-containing aerosols cannot be totally excluded, but any such effect was probably reduced by the fact that the statues stood in the open or under a small open porticus and were washed by the rain from time to time. According to Sikka et al. (1991), no basic copper chlorides are formed at concentrations of chloride ion lower than 10-3 mol/liter.

The mean carbon dioxide (CO2) content of the atmosphere today is 0. 036 vol% owing to heavy combustion of coal and oil in the last 150 years; the amount 1, 900 years ago was presumably 0. 029 vol%, similar to the value in the mid-19th century (Stumm and Morgan 1981). For sulfur dioxide (SO2) we presume a value in the range of 10 ppb, like today's level in regions not polluted by industry.

The surface of any bronze statue placed in the sacred precinct of Delphi more than 2, 000 years ago would have been altered by the influence of the atmosphere, and a patina would have formed. Oxidation would have produced a layer of cuprite (Cu2O). Further oxidation of the Cu2O would lead to at least a monomolecular layer of cupric oxide (CuO). Fitzgerald et al. (1998) showed that in a very pure environment, even much thicker layers can be formed. Any further reactions were due to the reaction of this CuO with anions in the adsorbed thin water film. The majority of such reactions presumably happened at humidities of 80% and higher (Stoch et al. 2001). The water layer is assumed to have had a thickness of 1 μm at 99% relative humidity and approximately 10 μm in case of dew (Fitzgerald et al. 1998). Rain and fog precipitation may cause thicker layers, but on inclined surfaces any layer with a thickness of more than approximately 25 μm will cause a runoff (Franke 2001).

The solubility of gases like CO2, oxygen (O2), and SO2 in water depends on the partial pressure, as well as on the temperature. In our case the equilibrium concentration is reached within a very short time because extremely thin water films have a very great ratio of surface to volume. A 10 ppb content of SO2 in the ancient atmosphere would have caused a 10-8 M concentration of sulfurous acid (H2SO3); its reaction with CuO causes the formation of copper sulfite (CuSO3), which in turn is very quickly oxidized to copper sulfate (CuSO4) (Strandberg 1998). Since the sulfurous acid vanishes in this way from the liquid phase, again SO2 can be absorbed from the gas phase, and concentrations of CuSO4 in the 10-5–10-6 M range may have resulted in the adhering fluid layer. No nucleation of basic copper sulfates occurs at SO42concentrations lower than 10-4 M, according to equilibrium diagrams calculated by Sikka et al. (1991).

Climatic conditions at Delphi today are similar to those in ancient times. The assumed low SO2 value may have facilitated the formation of basic copper carbonates (malachite and azurite). The malachite, however, is green, and therefore cannot be entirely held responsible for a blue patina. The blue azurite is formed only from near-neutral or weak acid solutions with high concentrations of the hydrogen carbonate ion (HCO3-).

Recent tectonic data show Delphi to have a very special geological setting, being located above the intersection of two major faults (named Kerna and Delphi), which broke through bituminous limestone. All over the world faults usually provide pathways for gases that rise, especially in periods after seismic and tectonic agitation. Dominant among the gases that surface in the eastern Mediterranean and Near East are hydrocarbons and carbon dioxide. Very recent investigations have shown conclusively that exhalations of saturated and unsaturated hydrocarbons at Delphi were formerly present, from either tectonic vents or springs (De Boer and Hale 2000; Piccardi 2000; De Boer et al. 2001). Moreover, other geological studies have shown that the limestone bedrock might have produced discharges of CO2 at Delphi (Higgins and Higgins 1986; De Boer et al. 2000; Piccardi 2000).

Thus the question arises: Could exhalations of CO2 have produced a sufficiently high concentration of this gas at the site of the bronze statues to cause the formation of azurite?

Based on the results of archaeological findings, figure 1 shows a graphic reconstruction of the sacred precinct of Delphi at the beginning of the 2nd century A. D.

The whole area is situated on a slope, enclosed by a high wall. The Spartan Monument was located just beyond the entrance, on the left, in the very lowest part of the sacred precinct. It is highly probable that the gates were usually closed because the Oracle was open only on certain days of the year. In Roman times it must have been totally closed for longer periods (Parke 1943; Parke and Wormell 1953). Carbon dioxide, which is a gas 1. 5 times heavier than air, would therefore be trapped inside the precinct area and would flow like a liquid to the lowest point unless it was dissipated by diffusion, air convection, or wind. We assume a maximum content of no more than 6 vol% CO2. Human beings exposed to a higher level for any length of time would have risked severe injury to their health (CO2 MSDS 2002, 2).


4 THE BLUE COPPER COMPOUNDS OF PATINA

According to Pausanias (1979), nine different artisans from all over the Greek world contributed to the Spartan Monument. It seems therefore extremely improbable that all the 37 bronze statues had the same unusual bronze composition unknown in Greek sculptural art or had been prepared artificially with a rare agent to produce a unique patination. In addition, recent studies usually deny that artificially applied patina ever existed on large classical bronzes (Born 1993).

These conclusions would suggest that the outdoor statues of the Spartan Monument were coated with a natural blue patina due to deterioration

Fig. 1. Drawing of the sacred precinct of Delphi at the beginning of the 2nd century A. D. based on a map of the results of archaeological findings that was scanned and then modified electronically
and changes at the surface of the bronze while exposed to the same open-air environment. In present times a green patina, not a blue one, characterizes the surface of bronze in an open-air environment. Usually the green patina consists essentially of basic copper sulfates. Basic copper chlorides predominate only in a purely marine atmosphere. Where urban and marine conditions coincide, however, the basic sulfates in the product greatly predominate over basic chlorides (Vernon and Whitby 1929;Vernon 1933). Contrary to general belief, basic copper carbonates are only minor components of contemporary patinas.

Bronze is an alloy of copper and tin, and in ancient times usually had only trace amounts of lead, but only copper compounds can be responsible for blue or green patina on the bronze. A full inventory of the copper corrosion products occurring in bronze patinas reveals that the majority of the some 30 known patina minerals are green, gray, or black, while only a few are blue or show at least a green with a bluish tint (see table 1).

All the bronze patina minerals cited in table 1 are formed by the influence of soil solutions or by artificial treatment, or they are formed extremely rarely and in vestigial amounts as transient phases. Natural blue patinas are usually formed in the course of long burial in the earth, and not in an open-air environment. In the days of Plutarch, however, these statue offerings dedicated by Lysander had already been in place for 500 years. To make a further choice about the source of the patina, we can look to Plutarch's text to provide clues.

The statues of the admirals were described as looking “like the sea” and “they stood there with the true complexion of the sea and its deepest depths” (Plutarch 1935, 260-61). Their patina was “shining with a deep blue tinge” (kyanou) (Plutarch 1935, 261). Kyanos is the Greek name for a dark blue enamel used to adorn armor in Homeric times. Kyanos also means “blue copper carbonate” or azurite (Liddell and Scott 1996). The use of the mineral azurite as a pigment after it had been crushed and ground into powder must have been known by Plutarch, since it was already recorded by Plinius the Elder in the first century A. D. (Gettens and FitzHugh 1993).

The blue patina was also described as “agreeable and brilliant by blending light and luster with the blue” (Plutarch 1935, 269), “pleasant to the eye and

Table . Funori
Table .
brighter mingling lustre and shine with the azure of the blue” (Plutarch 1918), “smooth and shining with a deep blue tinge” (Plutarch 1935, 261). The patina is shiny (stílbontos) and exhibits a bright hue and a glassy shine (augén kaì gánoma) (Plutarch 1935, 270). Thus, in addition to the deep blue color, an astonishing feature of this patina was the brightness.

The luster of a material depends on its index of light refraction. All the blue minerals listed above will show a glassy, vitreous luster on a flat and even surface. There are a few exceptions: covellite shows a metallic luster, chrysocolla an earthy one, and sampleite a pearly one. These minerals can therefore be excluded. All blue patina minerals form patinas

that consist of very tiny crystals, thus exhibiting a dull, lusterless surface. Only chalcanthite and azurite show a tendency to form bigger crystals that may show a vitreous to adamantine luster. Because chalcanthite is very easily dissolved and weathered in moist environments, excluding chalcanthite leads to the conclusion that the bright blue patina of the Spartan Monument at Delphi was probably due to an extensive coating with azurite. Since the ancient Greek name of the pigment azurite was kyanos, it might suggest a very correct but unconscious identification of the patina mineral by Plutarch himself, who used exactly this word to describe the color of the bronze statues.


5 CONDITIONS FOR THE FORMATION OF AZURITE

Azurite is formed only from solutions with high concentrations of the hydrogen carbonate ion (HCO3-). The presence of azurite indicates that the artifact corroded in the presence of elevated hydrogen carbonate activity (Neil and Little 1992).

Is it possible, then, that increased carbon dioxide concentrations favored the formation of a blue azurite patina on the bronze statues at Delphi?

In a very pure system, only the very low concen-3-tration of dissolved Cu2+ ions and the ion HCO must be considered. Even assuming a concentration of 6 vol% carbon dioxide at ancient Delphi, this would apply to a concentration of 2 x 10-3 mol CO2 per liter at 25°C and 4. 4 x 10-3 M at 1°C, calculated with the tabulated absorption coefficient for CO2 in water according to Henry's law (Plummer and Busenberg 1982). Unfortunately less than 1% of the dissolved CO2 reacts to carbonic acid (Stumm and Morgan 1981); thus the concentration of HCO3at a pH of 6 only reaches values of 8 x 10-4 and 1. 4 x 10-3 mol per liter for 25° and 1°C, respectively. These values were calculated by using the apparent dissoci-+-ation constant K1 equals; (H) x (HCO3) / (H2CO3*) equals; 4 x 10-7. The concentration of Cu2+ was assumed to be 10-4 M and lower according to experimental results by Fitzgerald et al. (1998). The following equilibrium diagram by Sikka et al. (1991) and Zheru Zhang (1994) shows clearly that malachite would have been formed at pH values of 6 and higher under such conditions (see fig. 2). The formation of azurite can, therefore, be excluded.

This conclusion is corroborated by a multitude of observations on patinas. Azurite as a patina mineral has been observed only on copper alloys that have been excavated from soil. Such intergranular soil solutions may have elevated concentrations of the hydrogen carbonate ion, due to considerable concentrations of calcium hydrogen carbonate (Riederer 2003).

An experiment one of the authors performed also showed that no azurite is formed in the pure system Cu-O-CO2-H2O. In this experiment a sheet of pure copper was immersed in distilled water saturated with 0. 1 MPa CO2 at 5°C. Some hydrogen peroxide was added to ensure slow oxidation, and the closed vessel was stored at 5°C. In a very sluggish reaction, only malachite was formed; the first visible green tarnish was observed only after some months.

In another experiment a slurry of freshly prepared Cu(OH)2 was bubbled with CO2 0. 1 MPa at 20°C; within a few hours the blue copper hydroxide had converted to green malachite (Krings 1996). Similar Cu(OH)2 species can be assumed to be present as a very thin, perhaps monomolecular layer at the surface of CuO.

It can therefore be concluded that an azurite patina could not be formed in a pure system Cu-O-CO2-H2O, even at greatly increased CO2 concentrations.

Nevertheless, the occurrence of an azurite patina cannot be totally excluded at Delphi because azurite can form readily in natural environments if very diluted CuSO4 solutions react with Ca(HCO3)2solutions, which usually result from the dissolution of limestone by CO2-rich waters.

A similar carbonate system was apparently present at the surface of the copper artifacts at Delphi. Indeed, Delphi is situated in a landscape of limestone bedrock; the building blocks were also made of porous limestone brought from the neighboring deposit at Kastri (De Boer and Hale 2000). Moreover, there is historical evidence of a vast rebuilding and restoration of the sanctuary during the reign of the Roman emperors Domitian, Trajan, and Hadrian (A. D. 90–135) (Bourguet 1914; Hammond and Scullard 1970). This is just the time when Plutarch was working at Delphi (A. D. 95–125) and when he wrote the dialogue De Pythiae Oraculis (A. D. 125). It is in this work that he mentions the blue patina of the Spartan Monument and also alludes to “so great, so vast a change” of the sanctuary, as well as the “many buildings added which were not here formerly, many restored which were ruinous or destroyed” (Plutarch 1918). Airborne calcite dust could well have been present, especially during restoration works within the temple precinct, since the stonecutters were working mostly on marble and calcite-bearing rocks.

The additional calcite dust may dissociate to increase the hydrogen carbonate ion (HCO3-) concentration, as shown by:

In this equation, the partial pressure of CO2determines the amount of dissolved calcium hydrogen carbonate (Ca[HCO3]2), as well as the amount of additionally dissolved CO2 in the system CaCO3-CO2-H2O (Stumm and Morgan 1981;Plummer and Busenberg 1982). This additionally dissolved CO2 has an important influence on the pH value of the resulting solutions. The solution is alkaline with a pH of "8 at very low partial pressures applying to approximately 0. 1vol% CO2, but higher values of CO2 in the atmosphere gradually shift the pH value toward the neutral and weak acid region. Moreover, the solubility of calcite, and thus the hydrogen carbonate concentration as well as the amount of additionally dissolved CO2, are enhanced at lower temperatures. A drop from 25°–1°C would cause an increase of approximately 50%. Figure 2 is calculated for 25°C, but the phase boundaries of this diagram would shift only very slightly for temperatures in the 25°–1°C range.

Looking at figure 2 again, we can expect any azurite crystallization only in the 5. 5–6. 5 pH range 3-and HCO concentrations in the 10-3 M range, at Cu2+ concentrations in the range of 10-5–10-6 M. According to Karlen et al. (2002), such Cu2+concentrations and pH values in the range of 5. 5–6 are prevalent in adhering water layers of Cu2O-coated copper alloys on contact with urban air. Experimental data of Plummer and Busenberg (1982) show the following values for an atmosphere containing 3 vol% CO in equilibrium with water 2 and CaCO3:

Azurite growth can be expected on contact of such a solution adhering to calcite particles with solutions reported by Karlen et al. (2002) in the 5. 5–6 pH range and Cu2+ in the range of 10-5 M and lower. An optimal pH of approximately 6. 5 is approached in this way. So, we can expect azurite formation at CO2 concentrations in the 1–4 vol% range, the lower concentrations applying to low temperatures. Any formation of copper sulfates at such low SO42- concentrations can be excluded on the basis of calculations by Sikka et al. (1991). This is again corroborated by experiments of Krings (1996), who obtained azurite plus gypsum by the slow reaction of a 0. 004 M CuSO4 solution with a calcite crystal in the presence of an atmosphere with 50% CO2 at 4. 5°C. The SO42- concentration cannot exceed 1. 2 x 10-2 M in the presence of calcite due to the low solubility of gypsum.

Fig. 2. Equilibrium diagram CuO-H2O-CO2; log M HCO3- versus pH at 25°C and 0. 1 MPa, according to Sikka et al. (1991) and Zheru Zhang (1994)


6 DISCUSSION

As our study has shown, the formation of azurite was due to an increased local CO2 concentration, as well as the coincidental presence of calcite dust. The CO2 concentration must have reached values of 1–4 vol% CO2, the lower values applying to lower temperatures. Most probably this CO2 was released intermittently by diffuse exhalations, tectonic vents, and spring water discharges. Because carbon dioxide is heavier than air, it was partly accumulated at the lowest part of the walled temple precinct, in the entrance area, which is precisely where the bronze statues of the Spartan Monument were erected.

The formation of azurite would have stopped if one or both of the following conditions had ceased: a local CO2 concentration of at least 1 vol%, or the deposition of calcite dust on the bronze surface due to stonecutting work.

Moreover, in such a case the blue azurite would have slowly but permanently deteriorated to green malachite. The borderline between the azurite field and the malachite field in figure 2 describes the equilibrium reaction:

Whenever the water layer totally evaporates on hot summer days, the hydrogen carbonate ion and the proton recombine according to the formula HCO3-+ H+ YCO2 + H2O, and the carbon dioxide leaves the equilibrium system. This process means that the equilibrium is shifted toward the malachite side and subsequently an amount of malachite proportional to the lost amount of carbon dioxide remains behind. Even if the conditions for azurite formation were fulfilled again, and azurite grew once more, this slight amount of malachite formed by the drying process would not have been converted to azurite, because, once formed, malachite is metastable within the azurite field (Franke 1997).

These facts lead to the conclusion that a blue patina consisting of much blue azurite and small amounts of green malachite prevailed only at the time of intensive restoration work at Delphi. This is the very time when Plutarch served at Delphi and took part in the management of this restoration. When the constant impact of calcite dust stopped, owing to the end of stonecutting activities, we have to assume a slow but constant conversion of the azurite to malachite. That means the color of the patina would have turned green within a few decades. Perhaps this sequence explains why no other ancient author before or after Plutarch has mentioned the unusual blue patina.


7 CONCLUSION

The Spartan Monument has not come down to our time. The same is true of all other life-size bronze statues from Delphi. Only the Charioteer survived because it was buried by a landslide more than 2, 000 years ago and was excavated in 1896. The Charioteer was rescued from the debris accumulated behind Ischegaon, an ancient retaining wall north of Apollo's temple (see fig. 1). According to recent geological analyses, the masonry of this wall shows a relatively thick coating of travertine (calcite) formed in situ over a period of time when spring waters ran downhill and splashed across the wall (De Boer and Hale 2000; De Boer et al. 2001). The presence of travertine is the main geological evidence for Ca-rich springs and enhanced CO2 degassing. In such geo-archaeo-logical conditions, the formation of azurite and malachite might be favored by intergranular soil solutions that may have elevated concentrations of the hydrogen carbonate ion, due to considerable concentrations of calcium hydrogen carbonate (Riederer 2003). The residue of the blue patina of this bronze is therefore presumably azurite that was developed during the burial time. It is unlikely that this statue had a blue patina before its burial because a considerably increased carbon dioxide concentration can hardly be assumed for its place in the upper part of the temple precinct (unless the statuary group was also enclosed by a wall). A thorough technical investigation of a cross section of the blue parts of the Charioteer would decide if the bluer shades are indeed made of azurite.


ACKNOWLEDGEMENTS

Thanks are due to Eva Logemann and Eveline Siegmann (Berlin) for their able help in preparing the figures. We would like also to thank Bea Hopkinson, prehistorian/researcher at University of California– Los Angeles, for having the kindness to proofread the English version, and for sharing with us her experience in resolving age-old artifactual evidence, or lack of it where materials do not survive time.


NOTES

1. See the color slide “Bronze statue of the Charioteer” (S. Stournaras, Delphoi, no. 4. Athens: Ministry of Culture, n. d., slides). We also examined the digital image (D12 jpg) of the same artifact at high zoom level (Archaiomania: Ilektroniko Mouseio tis archaias Ellinikis technis. CD-ROM. Internet ROM Multimedia: Athens, 1998). The same photo is available at www.rom.gr/rom6/museum/images/d/d12.jpg (accessed 06/05/05).



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AUTHOR INFORMATION

WALTER A. FRANKE, Diplomchemiker, Dr. rerum naturalium, is a retired professor of mineralogy at Freie Universität in Berlin, Germany, where he studied chemistry and mineralogy and has worked since 1965 in the field of experimental mineralogy with special interest in crystal dissolution, crystal growth, and morphology. He still gives regular lectures on interdisciplinary topics. Address: Freie Universität Berlin, Fachbereich Geowissenschaften, Fachrichtung Mineralogie, Malteserstr. 74-100, D-12249 Berlin, Germany. e-mail: wafranke@chemie.fu-berlin.de

MAGDA MIRCEA has a BA and MA in classical philology and is an assistant lecturer at Al. I. Cuza University of Iasi, Romania. Her main interests include application of natural science for explaining phenomena referred to by ancient texts. Address: Al. I. Cuza University of Iasi, Faculty of Letters, Department of Classical Languages, Italian and Spanish, Blvd. Carol I, Nr. 11, 700506 Iasi, Romania. e-mail: mmagda@uaic.ro

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