For almost 800 years, the Tower of London has had an iconic status in British history, but now, according to a new study, it is facing a major pollution threat.
The entire complex is turning yellow from the exhaust of cars and trucks, and the discoloration is most noticeable at the complex's White Tower, the original square fortress built by William the Conqueror in 1078, says study co-author Peter Brimblecombe, an atmospheric chemist at the University of East Anglia in the United Kingdom.
The results are detailed in this month's issue of the journal Environmental Science and Technology.
In the next 20 years, Brimblecombe has warned that the famous tower will turn a more brownish yellow.
To measure the color change, Brimblecombe and his colleagues used colorimeters to record the color of stone stored in the tower's basements—and away from airborne pollution. Placed there through the ages during repairs, the stone provided the team with a timeline of discoloration.
Yet finding the source of the problem led Brimblecombe and his colleagues to another track record of pollution: Patches of black crust found all over the complex's walls, reports Live Science.
Resembling burnt residue in an oven, the crusts are made of gypsum formed when acid in rainwater and pollution reacts with the limestone and mortar. Over time, the gypsum soaks up atmospheric pollution like a stone sponge. Since the advent of coal-burning industry in the late 13th century, the crusts turned black with soot and sulfur from coal smoke.
Brimblecombe explained that too much washing could damage the tower's stonework.
"Our aim is to be aware of coming changes, prepare for them and keep them at a publicly-acceptable level," he said.