doi:10.1038/nindia.2014.170 Published online 12 December 2014
The discolouration of Taj Mahal’s white marble facade has long been a cause of worry for India's tourism and archaeological conservation professionals. The root cause of this discolouration has now been explained by an international research team.
Researchers from Indian Institute of Technology, Kanpur, Archaeological Survey of India and US-based Georgia Institute of Technology and University of Wisconsin have found that deposition of air-borne light-absorbing fine particles on the white marble surface results in the discolouration of Taj Mahal1. They say black carbon, brown carbon and dust particles the key culprits that change the ability of the marble surfaces to reflect light.
Recent studies have shown high levels of air pollutants throughout the Indo-Gangetic plain, including relatively high concentrations of fine particles in Agra. Fine particles in this region mainly consist of light-absorbing components such as black carbon, organic carbon and dust. To date, no studies had probed the direct effects of these light-absorbing particles on the white marble surfaces of Taj Mahal.
The researchers measured the concentration of fine particles, total suspended fine particles, anions, organic carbon, elemental carbon and trace elements in the air around Taj Mahal. In addition, they securely placed pre-cleaned cuboids of white marbles roughly 300 metres away from the main dome of Taj Mahal. After a two-month exposure to pollutant-laden air, the marbles were analysed for surface deposits.
Using sophisticated imaging techniques and analytical methods, the researchers detected that daily concentration of fine particles and total suspended fine particles were much higher than the permissible limits stipulated by World Health Organisation. Besides dust, they found that light-absorbing elemental carbon and organic carbon contributed to the mass of total suspended fine particles.
Analyses revealed that particles having diameter less than 2 micrometre accounted for 30 per cent of the surface-covering fine particles. They observed that roughly half of the fine particles were carbon-based and carbon particles having diameter less than 3 micromtre dominated the surface of the marble pieces. These fine particles mainly come from burning of biomass, crop residues and trash. These particles are then airborne and get deposited on the outer surfaces of Taj Mahal causing the discolouration of white marbles.
Next, the researchers measured how the deposited fine particles changed the ability of the white marble surfaces to reflect light. Results indicated that separately each component of fine particles contributes to the colour change.
Black carbon changed the colour of the surface to greyish. Both brown carbon and dust influenced colour with preferential absorption at ultraviolet wavelengths resulting in yellowish-brown hues. When the light-absorbing effects of all types of fine particles were combined, the perceived colour of the surface shifted towards darker shades of yellow-brown.
The researchers say that deposition of light-absorbing fine particles not only damage cultural monuments but also the aesthetics of the environment by modifying the surfaces.
1. Bergin, M. H. et al. The discoloration of the Taj Mahal due to particulate carbon and dust deposition. Environ. Sci. Technol. (2014) doi: 10.1021/es504005q