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