Reactions of copper patina compounds—II. influence of sodium chloride in the presence of some air pollutants

The influence of NaCl on patina compounds occurring on outdoor copper and bronze objects was investigated. The laboratory study included cuprite (Cu2O), tenorite (CuO), brochantite (Cu4(OH)6SO4), and antlerite (Cu3 (OH)4SO4), pretreated with 10 wt % NaCl. The pure compounds were exposed in humid air containing combinations of trace amounts of SO2, O3 and NO2. Phase transformations were characterized by X-ray diffraction (XRD) after four weeks’ exposure. Deposition of SO2 and consumption of O3 were studied using on-line gas analysis. Mechanisms are suggested and reactions are discussed in relation to outdoor conditions. Tenorite did not react with NaCl in humid air. However, when SO2 was added to the air, reaction was rapid, copper hydroxy chlorides and hydroxy sulfates forming. In addition, an unknown phase appeared in this environment. Cuprite pretreated with NaCl formed copper hydroxy chlorides in humid air. When SO2 was added to the air, the mixture absorbed all SO2 supplied. The rapid sulfation is suggested to be caused by the increased basicity due to oxidation of cuprite. Treating brochantite or antlerite with NaCl(aq) resulted in a remarkably fast conversion to copper hydroxy chlorides. The experiments demonstrate that copper hydroxy chlorides are expected to form on outdoor patina even in rain-exposed area. The lack of occurrence of hydroxy chlorides in these areas is suggested to be due to a progressing cyclic weathering; including the formation of hydroxy chlorides by salt deposition, the formation of soluble copper compounds by dry deposition of SO2, and subsequent wash-out by the rain.