Algal and Lichen Growth Following Chemical Stone Cleaning

Abstract

Chemical cleaning methods have commonly been used to remove soiling from sandstone building facades. There has been anecdotal evidence suggesting that algal regrowth on facades can be increased following cleaning. It is shown here that both algal and lichen growths on sandstones may be substantially increased following chemical cleaning if the cleaning agents leave residues of phosphate (a nutrient normally in limited supply) in the sandstone. The growth of algae and lichens on samples of chemically-cleaned sandstones has been followed over five years. The effects of chemical residues with respect to biological growths were found to vary substantially depending on the characteristics of the sandstone. Phosphate may be chemically bound to iron compounds in sandstones. Iron-rich sandstones can therefore retain more residual phosphate than iron-poor sandstones. The duration of increased algal growth was found to vary from two years (iron-poor sandstone) to over five years (iron-rich sandstone). Where the sandstone was of low porosity and algal growth was consequently slow to become established, increased algal growth could be delayed until three years following cleaning. Lichens appeared to be stimulated by lower amounts of phosphate, and increased lichen growth was found to be of longer duration than increased algal growth. Biological growths can be disfiguring, may encourage soiling and some are capable of causing damage to stone. These results indicate that phosphate-bearing chemical cleaning methods should be used with caution on porous stone types in situations where biological growths could cause problems.