Will Red Fort turn black? A new study warns about future of India’s iconic building

A study published in the peer-reviewed open access journal Heritage warns that Delhi’s air pollution is darkening the Red Fort, putting India’s famous UNESCO-listed monument at risk. Conducted between 2021 and 2023 by researchers from India and Italy, it is the first study to closely examine the materials and damage affecting the historic fort.

The Red Fort, built in 1648 under Mughal Emperor Shahjahan, spreads across about 1 km² and is surrounded by 2.41 km of walls. Its buildings and walls are mainly made of red Vindhyan sandstone. Laboratory tests show that this sandstone is mostly quartz and microcline (K-feldspar), with very little calcite. Elements like Silicon (Si), Potassium (K), and Aluminium (Al) dominate the stone.

Why are the walls turning black?

Researchers found black crusts forming on the stone. Particulate matter settles and accumulates, causing visible discoloration and eventually forming thick, dendritic crusts that lead to the stone exfoliating, the study explains.

These crusts, between 55 and 500 µm thick, contain gypsum, bassanite, and weddellite. Since the original sandstone has very little calcium, the gypsum must have come from outside sources.

Pollution from construction work, road dust, and cement factories contributes to the problem. The crusts also contain heavy metals such as Titanium, Vanadium, Chromium, Manganese, Nickel, Copper, Zinc, Barium, and Lead. These metals are linked to fossil fuel burning, vehicle traffic, and wear from roads and vehicles.

How bad is Delhi’s air?

Air quality data from 2021 to 2023 shows that PM2.5 and PM10 levels near the Red Fort often exceed national limits. PM2.5 levels were over 100 µg/m³ on average—more than twice the national standard of 40 µg/m³.

Nitrogen Dioxide (NO₂) slightly exceeded safe limits in 2021 and 2023. While Ammonia (NH₃) and Sulfur Dioxide (SO₂) remained low, long-term exposure still matters. “SO₂ contributes to sulfuric acid formation, and NO₂ acts as an oxidizing agent, accelerating degradation,” the study notes.

Can anything be done?

Although the damage happens slowly, the study provides ways to protect the Red Fort. Cleaning the stone regularly and applying protective coatings to the most affected areas could slow down black crust formation. Reducing Delhi’s pollution is a bigger challenge, but these measures could help preserve the iconic monument for future generations.