Biological growths including algae, lichens, bacteria, fungi and mosses are common on building sandstones wherever suitable conditions occur. Some growths, such as lichens, may be considered desirable since they can give a mature appearance to a facade, but others are often considered undesirable as they can be disfiguring and may lead to, or exacerbate, stone decay. The disfiguring of buildings and monuments by biological growths, particularly on recently cleaned buildings, is therefore 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 sandstone 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 nutrients accelerating algal growth on vulnerable building sandstones.

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