Why is Delhi’s historic Red Fort turning ‘black’ after centuries?

Delhi’s iconic Red Fort is turning “black”, a new study has found. The city’s toxic air quality has led to the formation of “black crusts” on the walls of the monument.

India’s national capital is among the world’s most polluted cities. The study has shed light on the severe pollution crisis that plagues Delhi every year, particularly during the winter months.

Let’s take a closer look.

How is the Red Fort turning ‘black’?

Researchers found thick black crusts forming on parts of the Red Fort’s sandstone walls.

The study, titled Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India, revealed that “black crusts” on the fort’s wall were between 0.05mm and 0.5mm thick.

It warned that if steps were not taken, the intricate carvings on the Mughal-era monument could be damaged.

This is the first such study to analyse the impact of air pollution on the 17th-century monument. The research was published in the peer-reviewed open access scientific journal Heritage in June.

The research was a collaboration between the Department of Science and Technology under the Union Science and Technology Ministry and Italy’s Ministry of Foreign Affairs.

It was carried out between 2021 and 2023 by scientists from IIT Roorkee, IIT Kanpur, Ca’ Foscari University in Italy’s Venice, and the Archaeological Survey of India (ASI). The researchers analysed Delhi’s air quality data during this period.

Delhi’s pollution is hurting Red Fort

The scientists collected samples from the Red Fort ’s fortification wall and Zafar Mahal. They also scraped off the black crust found on several walls of the monument to examine its composition.

The researchers then retrieved the Central Pollution Control Board’s (CPCB) air quality data from 2021 to 2023. During this period, the concentration of particulate matter was more than two and a half times the national limits.

According to the study, the particulate matter and other pollutants in the air had led to the formation of black sediment on the Red Fort’s walls. Due to the pollution, other architectural elements such as vaults, arches and delicate stone carvings were also damaged.

The researchers also found blistering and flaking on the walls of the fort.

“PM2.5 and PM10 [types of particulate matter] are widely recognised as significant contributors to the soiling of surfaces exposed to ambient air. This phenomenon occurs when particulate matter settles and accumulates over time, leading to visible discolouration and blackening of these surfaces,” the study noted.

While the red sandstone mainly consisted of quartz and microcline, the black crusts comprised gypsum, bassanite, weddellite, quartz, and microcline, as per the study.

“The authors correlate the substantial presence of gypsum in black crust to construction activities, road dust and emissions from cement factories,” the study said.

During a visit to the site, researchers noticed vehicular traffic, industries and factories near the Red Fort as well as high particulate matter levels, biomass burning and soil dust.

Researchers concluded the black crust “is probably a result of atmospheric deposition linked to pollution.”

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What’s the solution?

The study calls for implementing conservation efforts to protect the Red Fort. It noted that reducing air pollutants in densely populated areas was challenging, but the formation of black crust on stone surfaces can be minimised.

The scientists said that preventive conservation measures can help slow down the formation of the black crust. “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.”

The study recommended launching a maintenance cleaning programme for the most affected areas. It also said stone protectives can be applied to prevent or at least slow down the formation of the black crust.

Researchers said this study could help create an action plan for the restoration and preservation of cultural heritage buildings around the globe.

With inputs from agencies