ALGAL GROWTH ON BUILDING SANDSTONES - EFFECTS OF CHEMICAL STONE CLEANING METHODS

Biological growths including algae, lichens, bacteria, fungi and mosse s are common on building sandstones wherever suitable conditions occur . Some growths, such as lichens, may be considered desirable since the y can give a mature appearance to a facade, but others are often consi dered undesirable as they can be disfiguring and may lead to, or exace rbate, alone decay. The disfiguring of buildings and monuments by biol ogical growths, particularly on recently cleaned buildings, is therefo re a cause of some concern. Following stone cleaning, re-soiling from inorganic sources may take many years, however, re-soiling in the form of green algal growths may occur within only a few months. This paper reviews the factors which influence algal growth on building sandston e and summarizes the results of research into the effects of chemical stone cleaning with respect to algal re-growth. Results indicate that residues of some phosphate-rich stone cleaning chemicals can act as nu trients accelerating algal growth on vulnerable building sandstones.