PLUTARCH'S REPORT ON THE BLUE PATINA OF BRONZE STATUES AT DELPHI: A SCIENTIFIC EXPLANATION
WALTER A. FRANKE, & MAGDA MIRCEA
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).