Delhi’s air quality is threatening Red Fort as it is facing gradual and accelerating damage. A study, titled Characterisation of red sandstone and black crust to analyse air pollution impacts on a cultural heritage building: Red Fort, Delhi, has revealed that black crusts of pollutants are steadily forming on the red sandstone walls of the 17th-century monument, threatening its structural and aesthetic integrity.
Strangely, no studies have focused on characterising its constituent materials or the degradation phenomena taking place. According to the researchers, the black crusts primarily consist of gypsum, bassanite, weddellite, and trace amounts of heavy metals such as lead, zinc, chromium, and copper. These pollutants are not inherent to the sandstone but are deposited from external sources, including vehicular emissions, cement factories, and construction activities in the city.
The study highlighted how chemical reactions between pollutants like sulphur dioxide and nitrogen oxides lead to the formation of gypsum layers, which eventually erode during rainfall. Moreover, nitrates and oxalates were found to infiltrate the sandstone, causing internal stress, cracks, and biological growth. The team also observed blistering, salt crystallisation and dampness-related crumbling near entrances, while interior Makrana marble decorations showed comparatively less damage due to their stronger grain structure.
This study was developed in the framework of the MAECI (Italian Ministry of Foreign Affairs) and the Department of Science and Technology under the Ministry of Science and Technology, India, project: Indo-Italian Centre of Excellence for Restoration and Assessment of Environmental Impacts on Cultural Heritage Monuments. This study is the first comprehensive scientific investigation into how urban air pollution is impacting the historic monument built by Emperor Shahjahan between 1639 and 1648.
The team analysed samples of sandstone and black crust collected from different areas of the Red Fort complex, including Zafar Mahal. The findings revealed that black crusts varied from thin deposits of approximately 0.05 millimetres in sheltered areas to thick layers of up to 0.5 millimetres on walls facing high-traffic zones.
These thick layers are strongly bonded to the stone surface, posing a risk of surface flaking and loss of intricate carvings. “The higher concentrations of heavy metals in the black crust sample may arise from various sources, including the burning of fossil fuels, wear and tear of vehicles' mechanical parts, vehicular traffic, particulate matter, and soil, dust,” the report noted.
Accessing air quality data from the Central Pollution Control Board for the years 2021 to 2023, the researchers confirmed that fine particulate matter concentrations remained more than two and a half times above national limits, with coarser particles exceeding the threshold by over three times.
Nitrogen dioxide levels also crossed safe limits, contributing to the stone’s gradual decay, even though ammonia and sulphur dioxide levels were within limits. “This is a wake-up call. Regular cleaning, protective coatings, and stricter pollution control measures are essential if the Red Fort and other heritage sites are to be preserved for future generations,” the study concluded.