The Red Fort, one of India’s most iconic UNESCO World Heritage Sites, is being damaged by Delhi’s toxic air according to a new Indo–Italian study.
The research has revealed that the Fort’s red sandstone is being corroded by black crusts made of gypsum, heavy metals and pollutants from vehicle exhaust, construction and cement factories.
The study, the first to closely examine what pollution is doing to the Fort’s stone, focused on both the Vindhyan red sandstone that makes up much of the Fort and the black crusts that have formed on its walls – the chemical products of air pollution that eat away at the stone.
The team collected samples and studied them with advanced tools that reveal the materials’ composition at a microscopic level.
The red sandstone, they found, is mostly made of quartz and microcline with trace amounts of calcite. On its own, this stone is quite durable. But the black crusts were found to contain gypsum, described as ‘a common product in stone degradation’ as well as bassanite, weddellite and traces of heavy metals.
Calcium is one of the primary ‘building blocks’ of gypsum – to form gypsum, calcium has to combine with sulfate and water. The conclusion drawn here is that if gypsum is found on the building’s surface but the original stone has very little calcium in it, the calcium must have come from outside sources, such as construction dust, cement particles or road debris, which then reacts with sulfur in the polluted air to create the gypsum deposits.
Data from Delhi’s air quality monitors confirm the scale of the problem. Between 2021 and 2023, PM2.5 levels near the Fort were more than double the safe limit, while PM10 levels were often three times higher. Nitrogen dioxide, a byproduct of traffic and industry, also accelerates the chemical reactions that damage stone, even at relatively low concentrations.
By identifying the chemical makeup of the crusts and tracing their origins, the research provides a foundation for better conservation strategies. It also shows just how urgent the problem is: if pollution remains unchecked, the Fort – and many other monuments in India -will continue to deteriorate.
The findings suggest practical steps to limit the damage: ‘The formation of a black crust is a progressive phenomenon that usually begins with a thin black layer or deposit, which can be removed without significantly damaging the stone’s integrity, at least in the early stages.
‘In addition, a maintenance cleaning program for the most affected areas and the application of stone protectives could prevent or at least slow down the formation of a black crust.’
The full research can be read here.
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