London Tower Will Become Yellow Because of Pollution

Pollution takes its toll on historical monuments, more than ever. Natural causes affect all manmade constructions, like the Statue of Liberty that turned green through the galvanic corrosion of copper and the pyramids lost much of the outer layer because of weather conditions over the millennia.

Artificial causes affect buildings much quicker and the London Tower suffers severe consequences of pollution over only a few centuries.

The historic monument in central London, England, on the north bank of the River Thames, has known many uses throughout history, as it was a fortress, a royal palace and a prison (particularly for high status and royal prisoners, such as the Princes in the Tower and the future Queen Elizabeth I), but also a place of execution and torture, an armory, a treasury, a zoo, a mint, a public records office, an observatory and since 1303, the home of the Crown Jewels of the United Kingdom.

A new study by Peter Brimblecombe and colleagues from England and Italy looked at the origin and transformation of the elemental carbon and the organic carbon found in basic organic compounds (EC and OC). These compounds are found in the black crust that formed on the outside of the stone walls over almost six centuries.

Since the late 13th century, when coal smoke from human applications started expanding in the atmosphere, the walls have endured a continuous blackening. But with the latest air pollution control measures, the amounts of sulfur dioxide from the coal are being reduced, causing more microbes to grow on buildings.

They transform other chemical compounds and contribute to a color change. "In particular, one should note that modern deposits have taken on a slightly different color and now appear more brownish," said the study. "These changes may arise from oxidation processes in the organic rich materials. The color change is particularly evident here at the Tower of London, where yellowing may become of greater concern than the habitual blackening in the near future."