A field exposure program has been implemented in a rural atmosphere with the primary aim of studying the corrosion mechanisms of sheltered copper. Emphasis has been placed upon the initial corrosion behavior after days, weeks, and months. Two starting dates, one in October and one in April, with different environmental characteristics have been studied. By using a multianalytical approach combined with environmental characterizations, a picture of the corrosion processes has emerged. Initially, a film of cuprite (Cu2O) is formed. The rate and growth depends mainly on the relative humidity levels. Due to deposition of sulfur containing species in the atmosphere, mainly ammonium sulfate, noncrystalline copper sulfate precipitates after days or weeks of exposure which gradually transforms into crystalline posnjakite(Cu4SO4(OH)(6) . H2O) within months of exposure.