Past and future colouring patterns of historic stone buildings

The patterns of colour on stone buildings change with time. Buildings were blackened by coal soot, but now mostly diesel particles and in future, wind-driven rain may alter the patterns and oxidation of surface organics. Colouring patterns that outline or shadow architectural elements are publicly more acceptable than those that cut across them e.g. rain streaking. Thresholds of acceptability for blackening can be related to soot loading. When soot loading of the ambient air is high, e.g. near busy roads, buildings are dark and typically viewed as unacceptable. However at lower concentrations (2-3 μg m-3 elemental carbon), building appearance is typically seen as more acceptable. The future offers a potential for variation in building colour, arising through different biological growth under changing climates or the presence of different pollutants. In future urban atmospheres more dominated by organic pollutants a yellowing process may be more important. Diesel soot has many organic compounds that can oxidise to brownish-coloured humic-like (HULIS) materials. We illustrate the development of colour patterns on the Cathedral Church of St John the Baptist, Norwich UK from its opening (beginning of the 20th century) to the end of the 21st century derived from historic photographs, observations and predictions for the future appearance. The latter has to consider removal of deposits by changed rainwater flow and enhanced biological activity. Blackening from coal smoke may have reached a peak around 1960s and 1970s, but diesel soot predominates now.