Over the long term, the loss of patina from copper and bronze outdoor sculpture is dominated by the dissolution rate of its constituent minerals, rather than the oxidation rate of the metal. Thus it is appropriate to treat the problem as a mineral weathering process. The development of a modified Pourbaix diagram reveals that the stable copper mineral phase should be the basic copper sulfate, brochantite. This can dissolve incongruently or by neutralization of acid rain. Runoff studies of bronze sculpture at Gettysburg National Park indicated that copper dissolution rate is not correlated with the incident rain pH. This implies that the loss rate is controlled by incongruent dissolution and possibly by ammonium sulfate particulate deposition, but not by acid rain. The estimated section loss at Gettysburg is 2.9 [plus or minus] 1.6 [micro]m/yr. This is in very good agreement with the 100-year average rate of 1.3 [plus or minus] 0.2 [micro]m/yr measured on the Statue of Liberty. Another implication is that the application of artificial patinas for conservation of bronze outdoor sculpture will be ineffective in the long run.