Performance codes are now in use internationally after a twenty-year eriod of gestation. The performance equation and its philosophy is introduced and its relationship with the current performance concepts described. Its original use as a generic framework for research into the performance of the external fabric is outlined.
Standards lend themselves readily to numerically defined phenomena. The ubjective nature of some weathering causal factors and the difficulties of measuring these are highlighted. The decision to restore the appearance of architectural buildings falls into this category of preservation. Visual changes lack definitive descriptors, but the paper reviews those available causal factors that would be useful in modeling the phenomena. Each major descriptor such as precipitation and pollution, and their components are considered in turn. Recognized patterns of surface soiling are presented.
It is held that Preservation standards related to changes in appearance begin with studies of rainfall and particulate distributions over buildings. Measures of aesthetic or visual failure are required because these processes have to eventually form a valid part of a preservation philosophy; and need to be covered by performance codes. By their longevity and example, historic buildings provide a laboratory of case studies. The paper suggests that standardization of visual change will involve standardization of interactive modeling techniques rather than direct testing.