IIT Bombay researchers have developed an optimised thermal weathering protocol and a crucial correlation to safeguard India's iconic Makrana marble structures.

The beautiful, pristine white Makrana marble, found in the Makrana region of Rajasthan, not only adorns the Taj Mahal, Dilwara Temple, and Jaswant Thada, but also makes these structures stand strong for centuries. An IUGS Heritage Stone, Makrana marble has excellent mechanical strength and is resistant to seepage. However, extreme temperature variations, rain and wind tend to degrade the majestic stone. Sadly, the increasing pollution aggravates the damage. Restoration efforts to preserve the heritage Makrana marble structures are ongoing, but assessing the extent and type of damage is crucial for choosing preservation and restoration methods.

In a study published in the journal npj Heritage Science, Ms Anupama Ghimire and Prof Swathy Manohar of the Indian Institute of Technology Bombay have proposed methods to accurately estimate and simulate the extent and type of damage in Makrana marble. They have described a detailed protocol for simulating the effects of natural heating and cooling cycles, specifically in Makrana marble. Their research addresses the challenge of the lack of standardised, optimised protocols to achieve desired porosity levels in laboratory marble samples that match those found in field-weathered specimens.

Additionally, Ghimire and Manohar have made on-field testing of Makrana marble more accurate and reliable. A commonly used method for assessing internal quality, structural integrity, and damage in marble blocks or installed slabs without damaging them (non-destructive testing) is Ultrasonic Pulse Velocity (UPV) testing. In UPV, ultrasonic sound waves are passed through marble samples or slabs, and the velocity of the wave in the sample is measured. A reduction in wave velocity indicates degradation of the marble. Ghimire and Manohar have established a mapping between open porosity—a parameter quantifying the amount of damage—and UPV.

Pristine Makrana marble, freshly quarried, is extremely dense. Over time, the marble starts flaking, chipping off and staining. Its surface becomes rough and undulating and develops pits and cracks. The damage is linked to many factors such as air pollution, acid rain, and tourist activity. Interestingly, the blue staining on the marble surfaces of the Taj Mahal was caused by the excrement of mosquitoes breeding in nearby water! The researchers posit that while different environmental factors induce and accelerate damage in the marble, the root cause is an increase in the marble's open porosity, many times driven by temperature variation.

Open porosity refers to the voids or pores in the marble that are accessible from the surface. If more voids open at the surface, water, air, and pollutants can easily penetrate the marble. Persistent thermal variations increase the marble's open porosity, allowing water and other impurities to enter, affecting its structural integrity and aesthetic appearance, and leading to its degradation.

“Given that freshly quarried Makrana marble has very low porosity, understanding how thermal weathering affects this property is critical for assessing and mitigating damage in heritage structures,” say the researchers. Creating laboratory samples that match the open porosity of the degrading marble in the field helps explore and experiment with restoration strategies. However, there is currently no documented, optimised laboratory process to simulate the natural weathering of Makrana marble.

Ghimire and Manohar designed an experiment to accelerate thermal weathering in the laboratory by using very high temperatures. Heating 5 cm cubes of marble in a furnace for 1 hour, followed immediately by cooling the sample by dipping it in deionised water at room temperature, constitutes one cycle. The furnace is designed to prevent exposure to the heat source and combustion byproducts, thereby avoiding contamination. The water is deionised, that is, all dissolved minerals are removed from the water, so that the damage to the marble will be only due to the heating, and not any impurities in the water.

The researchers conducted different sets of experiments with various temperatures (100°C, 200°C, 300°C and 400°C) and varying number of cycles at each temperature, aiming to replicate the porosity levels observed in naturally weathered heritage marble “The study was limited to heating temperatures of 400 °C, as marble begins decomposing at this threshold, with decomposition accelerating beyond 500 °C,” report the researchers.

After each cycle, the researchers monitored changes in the marble's physical and mechanical properties. They measured open porosity, assessed visual changes, performed detailed micro-structural analysis using Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) and Energy Dispersive Spectroscopy (EDS) to identify chemical alterations, and conducted Ultrasonic Pulse Velocity (UPV) tests.

Based on their experiments, the researchers observe that five to seven thermal cycles are optimal for achieving the target porosity in the laboratory, and additional cycles are unlikely to increase the porosity significantly. The researchers have also noted the approximate open porosity values obtained for thermal cycling at different temperatures.

Current approaches to assessing damage to heritage structures include both destructive techniques that require removing small samples from the stone, and non-destructive techniques such as ultrasonic pulse velocity, infrared thermography, moisture meters, and 3D scanning that allow testing without causing any material damage. For non-destructive testing (NDT) methods, on-field measurements need to be correlated with, or mapped to, deterioration of actual material parameters, such as porosity or material strength. “Because each stone type behaves differently, NDT values alone do not indicate damage unless such correlations are established. Correlated NDT datasets are limited for materials such as bricks and stones, but are available for conventional materials in modern construction, such as concrete,” says Prof Manohar.

The researchers have also come up with an equation that correlates UPV measurements with open porosity values based on a graph of ultrasonic pulse velocity versus open porosity. “The established correlation allows UPV to act as a non-destructive test for estimating degradation in similar marble used in real structures,” explains Prof Manohar. Heritage conservators can now use a non-invasive UPV device on-site to estimate the marble's porosity, providing a quick, practical way to assess its condition without causing further damage. However, the researchers found that when open porosity exceeded a threshold, the samples showed significant variation in UPV measurements, indicating that UPV testing is unreliable for estimating open porosity in highly damaged marble.

“The study also opens possibilities such as defining threshold UPV values that indicate when intervention is needed, developing predictive models for long-term deterioration, creating deterioration maps of heritage structures using portable UPV tools, and integrating NDT-driven data into 3D digital twins for continuous heritage monitoring. This work therefore supports future digital, science-based conservation practices,” concludes Prof Manohar.

Funding Information :

This work was supported by DMart Pvt Ltd., India, through IIT Bombay (DMart Fellowship, DO/2023-FLSP002-001).

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