Toxic Delhi is eating away Lal Quila walls: What’s the ‘Black Fort’ mystery?

Red Fort walls turning black

A grim new scientific diagnosis has been delivered for one of India’s most cherished monuments. The 17th-century Red Fort in Delhi is being actively damaged by layers of black crust, a direct result of the city’s severe air pollution.

An Indo-Italian study reveals that these crusts are composed of gypsum and heavy metals. They are accelerating irreversible damage, including cracks and flaking, on the Unesco World Heritage site.

How is Delhi pollution attacking the Red Fort?



The study, titled ‘Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India’, published in the peer-reviewed journal Heritage, is the first to comprehensively examine the effects of Delhi's toxic air on the fort. It identifies a dangerous chemical interaction.

Particulate matter (PM2.5 and PM10) and other pollutants in the atmosphere settle on the fort’s red sandstone. Over time, they react with the stone to form hard, dark deposits.

These black crusts, measuring between 0.05mm and 0.5mm thick, are not merely a cosmetic issue. They trap moisture and pollutants, leading to physical deterioration like blistering and the loss of intricate carvings.

What did the international study find?



Conducted between 2021 and 2023 by a team of Indian and Italian researchers, the study combined air quality data with direct analysis of the fort's walls. The scientists scraped samples from various locations for laboratory testing.

Their findings confirmed that the crusts are actively damaging architectural elements across the site. This includes delicate stonework, vaults and arches.

The phenomenon is progressive. It starts as a thin, removable layer but hardens into a destructive crust that compromises the structural fabric of the monument.

Why is the Red Fort so vulnerable?



The Red Fort's plight is deeply connected to its urban environment. When Mughal Emperor Shah Jahan built it in the mid-17th century, its eastern walls met the waters of the Yamuna river.

Today the fort overlooks the congested Inner Ring Road, choked with traffic. This dramatic change in surroundings has placed the historic structure on the front line of Delhi's pollution crisis.

The fort itself has witnessed centuries of history, from Mughal additions to British demolitions after the 1857 rebellion. Its current battle against environmental decay is a new, insidious chapter.

Is this a problem for other Indian monuments?



The Red Fort is not an isolated case. Conservationists have long warned about the impact of pollution on India's heritage. The findings from the Red Fort study echo concerns raised about other iconic sites.

In 2018, India's Supreme Court intervened over the discolouration of the Taj Mahal. The white marble mausoleum had turned yellow and greenish-brown due to air and water pollution, prompting urgent conservation orders.

The situation at the Red Fort, a symbol of national pride where prime ministers deliver the annual Independence Day address, underscores a widespread threat to the country's historical treasures.

Can the damage be stopped?



The study calls for the timely implementation of conservation strategies. While the crust becomes permanent over time, in its early stages it can be removed.

The researchers suggest applying specific stone protectives or sealants to highly-affected areas. This could slow down or prevent further formation of the destructive black crusts.

However, without concerted action to address the root cause - Delhi's chronically poor air quality - this cleaning may prove to be a constant and ultimately losing, battle against the elements.

The study was conducted by scientists from IIT Roorkee, IIT Kanpur, Ca’ Foscari University of Venice and the Archaeological Survey of India, as part of a collaboration between India's Department of Science and Technology and Italy's Ministry of Foreign Affairs (MAECI).