The conservation of stonework exposed to the weather is a matter of growing concern worldwide. A common cause of decay is chemical pollution of the atmosphere, but investigation of deterioration in Trajan's Column, one of the world's most famous monuments, indicates that a biological factor is also involved - colonization by lichens. Paradoxically, this kind of erosion has now stopped as lichens have long ceased to grow in Rome's polluted atmosphere. It must, however, be recognised as a potential threat to marble and limestone structuring in clean areas elsewhere. © 1991 Pergamon Press plc.
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