Abstract: An important component that has been missing from image synthesis is the effect of weathering. In this paper, we present an approach for the modeling and rendering of one type of weathering - metallic patinas. A patina is a film or incrustation on a surface that is produced by the removal of material, the addition of material, or the chemical alteration of a surface. Oxidation, sulphidization, and painting are examples of phenomena that produce patinas. We represent a surface as a series of layers. Patinas are simulated with a collection of operators, such as "coat," "erode," and "polish," which are applied to the layered structure. The development of patinas is modulated according to an object's geometry and local environmental factors. We introduce a technique to model the reflectance and transmission of light through the layered structure using the Kubelka-Munk model. This representation yields a model that can simulate many aspects of the time-dependent appearance of metals as they are exposed to the atmosphere or treated chemically. We demonstrate the approach with a collection of copper models.