Taj Mahal conservation and the scope of electromagnetic sensing

Abstract One of the seven wonders of the world and a UNESCO heritage site, the Taj Mahal, built in the 17th century in the northern Indian city of Agra, is known for its sublime beauty in white marble with exquisite inlay works in the style of ‘pietra dura’. Due to structural damage and surface discoloration, stemming partly from anthropogenic causes, a restoration process is ongoing. Conventional means of damage evaluation in heritage structures such as the Taj Mahal are based mostly on visual assessments by experts, which is highly time-consuming and subjective. The resulting restoration therefore becomes exhaustive and expensive. This topical review describes our ongoing work of combining various electromagnetic sensing techniques, such as micro-Raman spectroscopy, terahertz (THz) time domain spectroscopic imaging, THz quantum cascade laser-based laser feedback interferometry, energy-dispersive x-ray analysis and others as an alternative approach for detection of both surface and sub-surface damage in marble structures. In our work, white slabs or coasters bearing similar ornamental colourful motifs to those in the Taj Mahal were probed by the above techniques and minute changes in surface quality due to the introduction of sulphur and organic materials, as well as water damage were mapped with high accuracy. This electromagnetic sensing approach paves the way to providing a fast and reliable damage evaluation technique for cultural heritage architecture made of natural stone such as marble and sandstone for the next generation.