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Article

Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India

by
Gaurav Kumar
1,†,
Lucia Rusin
2,†,
Pavan Kumar Nagar
3,
Sanjay Kumar Manjul
4,
Michele Back
2,
Alvise Benedetti
2,
Bhola Ram Gurjar
1,
Chandra Shekhar Prasad Ojha
1,
Mukesh Sharma
3,* and
Eleonora Balliana
5,*
1
Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India
2
Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Via Torino, 155b, 30170 Venice, Italy
3
Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
4
Archaeological Survey of India (ASI), Delhi 110001, India
5
Department of Environmental Sciences, Informatics and Statistics, Ca’ Foscari University of Venice, Via Torino, 155b, 30170 Venice, Italy
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Heritage 2025, 8(6), 236; https://doi.org/10.3390/heritage8060236
Submission received: 21 April 2025 / Revised: 7 June 2025 / Accepted: 15 June 2025 / Published: 19 June 2025
(This article belongs to the Special Issue Deterioration and Conservation of Materials in Built Heritage)
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Abstract

:
Urban air pollution poses significant risks to cultural heritage buildings, particularly in polluted megacities like Delhi, India. The Red Fort, a UNESCO World Heritage Site and a symbol of India’s rich history, is highly susceptible to degradation caused by air pollutants. Despite its great importance as an Indian and world heritage site, no studies have focused on characterizing its constituent materials or the degradation phenomena taking place. 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. To understand their composition and degradation, Vindhyan sandstone and black crust samples were studied. Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) indicated that the red sandstone predominantly consisted of quartz and microcline, while the black crusts mainly comprised gypsum, bassanite, weddellite, quartz, and microcline. The analysis attributed the formation of gypsum to exogenous sources, such as construction activities and cement factory emissions. This pioneering study provides a basis for further research into the impacts of air pollution on Indian patrimony and promotes conservation strategies.

1. Introduction

Urban air pollution poses a significant threat to health, property, and the environment in both developed and developing countries. Rapid urbanization has led to increased emissions of air pollutants from transportation, power generation, and industrial activities, particularly in densely populated areas. New Delhi, the capital of India, stands out as one of the most polluted megacities in the world. By 2030, it is expected to become the most populous megacity globally, with an estimated population of approximately 37 million and an area of about 1484 km2 [1]. Given the alarming nature of this scenario, numerous studies have been conducted to assess air pollution in India, particularly in Delhi. Several emission sources have been identified in and around the city, including burning fossil fuels, vehicular emissions, construction activities, and the incineration of plastic and biomass. These activities have led to pollutant concentrations that exceed the safe limits established by the National Ambient Air Quality Standard (NAAQS, India) [2,3,4,5,6,7].
Air pollution is a recognized threat to human health and stone material conservation. While many studies focus on its effects on humans, there is a lack of research dedicated to Indian monuments. A review article by Natarajan et al. [8] offers, for example, an overview of air pollution’s impacts on India’s indoor and outdoor cultural heritage sites. The authors highlight that notable damages observed on monuments (i.e., exfoliation, efflorescence, crust formation, and corrosion) are closely linked to pollution sources, including fossil fuel combustion, iron foundries, power plants, and oil refineries. The article also emphasizes the need for further research, as much of the existing literature is outdated, dating back to the previous century and the early 2000s. For instance, there is a significant lack of scientific studies concerning the Red Fort of Delhi. To the best of the authors’ knowledge, up to now, only one study has addressed the characterization of pigments in the decorations of the Chatta Chowk [9], and no research has examined the impact of pollution on the monument itself.
Since ancient times, Delhi has held a significant role in India. Many emperors who ruled the city built numerous structures and complexes, including the Red Fort, Humayun’s Tomb, Purana Quila, and Safdarjung’s Tomb. The primary materials for these Cultural Heritage Buildings (CHBs) were Vindhyan sandstone and Makrana marble, selected for their local availability, durability, and aesthetic appeal [10].
The Red Fort was built after Mughal Emperor Shahjahan relocated the capital of his empire from Agra to Delhi in 1638. The construction began in 1639 and was completed in 1648. However, subsequent Mughal emperors made various additions and demolitions until 1660. After 1857, significant modifications were made by the British during their occupation. Today, approximately 30% of the Red Fort’s complex showcases Mughal architecture, beautifully illustrating influences from Persian, Timurid Saffavid, and Hindu design. The British made later modifications to the Red Fort, primarily to meet military needs, which as the site primarily served as a cantonment area. The Indian army added some additional structures during its occupation. These unique architectural examples and technologies from different cultures, along with the rich traditions, beliefs, and historical events associated with them, have solidified the status of the Red Fort as one of India’s most important symbols. In 2007, it was inscribed in the UNESCO World Heritage list [11]. In addition to its historical and aesthetic values, the site also provides economic benefits by boosting tourism, contributing to the country’s gross domestic product [12].
The Red Fort owes its name to the red Vindhyan sandstone used to build its fortification walls and several buildings within the complex [10]. Originally, the perimeter of the Fort was protected by a 9 m moat on three sides, while its eastern side faced the Yamuna River. However, the river’s course has since been altered, and the area is now occupied by the Inner Delhi Ring Road, which experiences heavy traffic. The Red Fort complex covers an area of about 1 km2 and is surrounded by 2.41 km of fortification walls. The walls are 20–23 m high, with a thickness of 14 m at the base and 10 m at the top [10]. A map of the Red Fort complex is shown in Figure 1.
Among the various degradation mechanisms that undermine the integrity of CHBs, this article focuses on black crusts that are highly present around the walls of the Red Fort.
Black crusts are often considered one of the most threatening threats to the degradation of outdoor CHBs. Pollutants easily deposit on architectural surfaces, creating not only blacking but also reacting with the substrate, particularly when carbonate materials are involved [14]. The deposition of pollutants initiates a sulfation process on the stone’s surface, where calcite (CaCO3) reacts with sulfate ions (SOx−) present in the atmosphere, forming a gypsum layer (CaSO4·2H2O). This newly formed gypsum layer is rapidly washed away during rainfall, and the presence of metal oxides in the atmosphere accelerates the sulphation processes [15,16,17]. Recent research has shown that NOx species may act as a catalyst for the oxidation of SO₂, further exacerbating the stone degradation processes. Additionally, it has been observed that soluble nitrate can infiltrate the stone during crystallization, potentially causing internal stress and leading to cracks within the stone material [18]. In a study by Pozo-Antonio et al., thick black crusts were observed also on silicate stones, despite these having minimal calcium composition. In this case, the calcium contributing to black crust formation is thought to derive from the dissolution of surrounding cement and mortar materials [14]. Black crusts form when amorphous carbon and various heavy metals, responsible for the characteristic black color, are found in the atmospheric particulate matter and accumulate stone’s surface, often embedded in gypsum layers [19]. Black crusts, heavy metals, and organic species are well-known indicators of air pollution [6,20].
Spezzano et al. [21] discussed how stone materials exposed to the environment are influenced by several variables, including temperature variations, sunlight, wind, rain, humidity, crystallization of soluble salts, microbial colony growth, vascular plant growth, etc. Collectively, these factors contribute to what is known as natural weathering. When combined with intense air pollution, these elements accelerate and intensify the natural aging processes of the stone materials. In BC samples, oxalates are found and frequently acknowledged as biological weathering agents. These oxalates typically form on stone substrate due to the presence of oxalic acid which is often excreted by lichens or fungi growing on the stone [16,22,23].
The work was realized as part of the Italy–India joint sciences and technology cooperation within 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.
The primary objective of this study was to analyze the chemical composition of the back crusts affecting the conservation of the Red Fort complex and to investigate their possible origin. A preliminary overview of the air quality surrounding the Red Ford complex is presented based on the data from the Central Pollution Control Board (CPCB). Subsequently, selected degraded and non-degraded stone samples are examined, applying a range of multi-analytical techniques, including Field Emission Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy (FE-SEM-EDX), X-ray diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Finally, the composition of these samples and the air quality data are discussed to explore possible links and impacts on the conservation of the Red Fort complex.

2. Materials and Methodology

Sample Collection Sites/Points

To minimize further damage, a limited number of selected samples were collected under the supervision of the conservators from the Archaeological Survey India (ASI). The sampling focused on the fortification wall (indicated with number 1 in Figure 1), adjacent to the heavily trafficked Mahatma Gandhi Road (indicated with number 11 in Figure 1), and the Zafar Mahal building within the Red Fort complex (indicated with number 2 in Figure 1). Table 1 briefly describes the samples, while Figure 2 illustrates the sampling sites.
The samples were analyzed in two ways to gain a comprehensive understanding of the relationship between the stone and its environment. First, the samples were examined under a microscope as they were, and, then, the black crust layers were carefully separated with a stainless-steel scalpel. Second, part of the samples were embedded in resin to obtain cross-sections for further analysis. This dual approach allowed us to investigate the black crust’s components and understand its formation in relation to the unaltered red sandstone [24].
All samples were examined using a ZEISS Stereo Discovery V8 optical microscope (OM) equipped with a microscope camera ZEISS Axiocam 208 color (ZEISS, Oberkochen, Germany). This setup facilitates the initial observation and classification of the variability in black crusts, including factors such as thickness, adhesion to the substrate, and evidence of exfoliation. Subsequently, as mentioned above, representative and significant samples were embedded in resin to obtain cross-sections.
The representative sandstone samples were carefully selected from a location well shielded from direct exposure to rain, wind, and air pollution, ensuring that their intrinsic properties remained intact and unaltered. In contrast, the representative BC samples were selected considering multiple factors, including the level of visible contamination and the thickness and aspect of the black crust, favoring unsheltered areas suitable for air pollutant accumulation. Additionally, it was also taken into consideration that the orientation of the BC sample was toward high-traffic zones, and other anthropogenic pollutants were more common [14].
To comprehensively understand all possible stratifications, the samples were separated with a scalpel and subjected to X-ray diffraction for mineralogical identification [17]. Powder X-ray diffraction (PXRD) was carried out with a Bruker D8-Advance, X-ray source 2.2 kW Cu anode, 40 kV/40 mA. The range of 2-theta was taken as 5–90°, using a step size of 0.02° and a step-time of 2 s/step. Data were analyzed with proprietary software and further elaborated with Origin 9.5.
Infrared Spectroscopy was performed using a Bruker TENSOR II FT-IR spectrometer (KBr mode) (Bruker, Billerica, MA, USA). The samples were analyzed as KBr pellets, in the spectral range from 4000 cm−1 to 400 cm−1, with 4 cm−1 spectral resolution, and 64 scans per analysis. Data were analyzed with proprietary software and further elaborated with Origin 9.5.
The SEM analysis was performed using a Zeiss Gemini 300 equipped with an EDX Bruker Quantax 200 probe to gather morphological and elemental identification of the unaltered stone and the black crusts. The analysis was performed on the samples without preparation and on the obtained cross-section to better explore the formation of the black crust.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was performed using an Agilent, 8900 ICP-MS Triple Quad (Agilent, Santa Clara, CA, USA), with the following specifications: detection range: Li to U, Frequency: 40 MHz, free running, Torch: Concentric Quartz Tube, Spray Chamber: Scott Type, Nebulizer: Cross flow design with Gem Tips. Before ICP-MS analysis, samples were digested using a wet digestion process. A total of 0.5 g of the samples 1RS and 2BC, considered representative of the unaltered stone and the black crust, were placed in two separate digestion tubes, and 3.5 mL of concentrated H2SO4 was added to each tube. After 30 min, 3.5 mL of H2O2 (30% wt) was added to the mixture, and the tubes were heated on a hot plate at 250 °C for 30 min. The digestion tubes were then removed from the hot plates and allowed to cool down to room temperature. Once cooled, an additional 1 mL H2O2 (30% wt) was added drop by drop to the mixer until the digest was clear upon cooling. The solutions were then filtered using a 0.45 µm filter paper. A 5 mL aliquot of filtrate was transferred to a 100 mL volumetric flask, and distilled water was added to create a total volume of 100 mL solution. The diluted samples were analyzed to investigate trace elements [25,26]. Due to the unavailability of certified reference materials with compositions closely resembling red sandstone, five identical subsamples of sandstone and black crust were independently digested and analyzed to assess the consistency and reproducibility of the trace element measurements. The average values obtained provide an approximate estimation of the trace elemental composition [27].

3. Results and Discussion

3.1. Red Fort’s Conservation State

The buildings within the Red Fort complex were constructed using various materials and adorned with diverse decorative techniques, leading to the observation of different degradation phenomena based on their characteristics. Blistering is evident on the walls and vaults covered with plaster, likely caused by humidity and salts (Figure 3A,B). The elements made of red sandstone exhibit noticeable flaking (Figure 3C,D), resulting in the loss of details in carved decorations. This flaking could be a consequence of salt crystallization, as well as thermal and mechanical stress. Another clear example of salt crystallization appears on the vaults (Figure 3F), where a white crust has formed. This phenomenon is more common in the inner part of the arches, possibly indicating that water infiltrations are to blame rather than rising damp. Nevertheless, rising damp is a well-known degradation factor in building conservation and can create substantial damage, such as stone exfoliation and salt crystallization. Rising damp is, in this case, for example, to blame for the pulverization and degradation phenomena affecting the lower part of one of the entrance doors (Figure 3C,E,F), which has led to the development of colored patinas likely of biological origin. In contrast, the elements made of Makrana marble, mainly used to create decorative motifs inside the buildings and located in the sheltered areas (Figure 3G), appear to be less damaged compared to those made of sandstone. A recent study [28] suggests that this is likely due to the coarse interlocking grains of calcite in the marble, which provide increased strength and weather resistance.

3.2. Air Quality at the Red Fort

Air pollutant data were retrieved from the Central Pollution Control Board (CPCB) according to standard operating procedures and quality assurance guidelines [29,30]. Air pollution monitoring stations are located within a 10 km radius of the Red Fort, specifically from Anand Vihar, Ashok Vihar, Vivek Vihar, Pusa, Patpadganj, ITO, and Dilshad Garden, as illustrated in Figure 4. The data obtained from these stations reflect the levels of air pollutants near the Red Fort.
In this study, data were organized on annual average concentrations of PM2.5, PM10, NO2, NH3, and SO2 around the Red Fort from 2021 to 2023, provided by the CPCB. Monitoring these pollutants is crucial due to their known impact on Cultural Heritage Buildings (CHBs) [5,20,31]. At present, only three years of reliable air pollution data are available from the aforementioned monitoring stations, providing insight into potential pollution sources near the Fort. However, the formation of black crust may have begun prior to this data collection.
PM2.5 and PM10 are widely recognized as significant contributors to the soiling of surfaces exposed to ambient air. This phenomenon occurs when particulate matter settles and accumulates over time, leading to visible discoloration and blackening of these surfaces. The effect is particularly evident on intricate carvings and architectural details, where particulate matter not only causes visible damage but also diminishes the cultural and aesthetic value of these sites [19,21,32].
Table 2 summarizes the average values of PM10, PM2.5, NO2, NH3, and SO2 for 2021, 2022, and 2023. These values are based on the data collected from various monitoring stations located nearby and compared to the corresponding limits indicated by the National Ambient Air Quality Standards (NAAQS). Figure 5 and Figure 6 report the monthly concentrations of PM2.5 and PM10 around the Red Fort in the considered period against the suggested limits. In the years considered, both PM2.5 and PM10 are largely above the NAAQS limits. For instance, the annual average concentration of PM2.5 was always found above 100 µg/m3 in the considered years (Table 2), which is nearly 2.5 times higher than the NAAQS limits (40 µg/m3). Similarly, the annual average concentration of PM10 often exceeds three times the NAAQS indicated limit (60 µg/m3) for three consecutive years, as shown in Table 2. Hence, higher annual average concentrations of PM10 pose ongoing threats to the Fort, which has already endured decades of atmospheric pollution.
PM2.5 and PM10 (Figure 5 and Figure 6) also show considerable annual variation with a relatively high associated error (Table 2). This variability is most probably due to the intense seasonal variations, with maximum values occurring in the winter (November and December) and minimum values nearing the NAAAQS standards in summer (June and July) [33,34].
The situation regarding the annual average concentration of NH3 and SO2 appears to be improving. In 2023, the concentration of NH3 was recorded at 46 ± 26 μg/m3, remaining relatively constant over the three years analyzed and well below the recommended NAAQS year limit of 100 μg/m3. Similarly, in 2023, the average SO2 concentration in 2023 was 10 ± 9 μg/m3 in 2023) with annual values always way below the indicated NAAQS year limits of 50 μg/m3. Despite their low levels, the role of NH3 and SO2 in atmospheric processes that lead to the formation of acidic compounds should not be underestimated. Even at levels below the NAAQS thresholds, SO2 can still contribute to the formation of sulfuric acid when combined with water, accelerating the degradation of stone and metal structures [15]. The observed decrease in SO2 levels—from 14 ± 9 µg/m3 in 2021 to 10 ± 6 µg/m3 in 2023—is encouraging. In contrast, the annual average concentrations of NO2 showed a slight increase, with levels recorded at 44 ± 25 μg/m3 in 2021 and 45 ± 24 μg/m3 in 2023, both slightly exceeding the annual NAAQS threshold limits of 40 μg/m3.
In summary, PM2.5, PM10, and NO2 levels exceeded the indicated NAAQS limits. In contrast, NH3 and SO2 levels were significantly below the national indicated thresholds within the considered period. However, it is essential to recognize that the effect of air pollutants on CHBs occurs over the long term. Therefore, even though NH3 and SO2 are below the limit, their long-term impacts on the Red Fort should not be overlooked. For example, even at low levels, the presence of NO2 in the atmosphere accelerates the sulfation phenomenon as it acts as an oxidizing agent in the degradation process of the CHBs [18].
Figure 7 shows the micrographs, SEM-EDX, and XRD analysis results obtained for sample 1RS, which are here reported as representative of the collected stone samples without a visible crust (7RS, 9RS, and 10RS). The close examination of the sample 1RS cross-section under optical microscopy (Figure 7A) reveals small crystals ranging from pale pink to dark red. Additionally, some areas show white spots due to conglomerates of white crystals, usually associated with silicon oxide. This characteristic is often linked to a natural variant of Vindhyan sandstone [10,35].
The SEM image of the cross-section of sample 1RS (Figure 7C) evidenced the typical characteristics of Vindhyan stone, featuring distinct crystals of similar dimensions embedded in an amorphous region (point 2) surrounded by silica-based cement (points 1, 3, and 4). The voids (indicated with yellow arrows in Figure 7C) present between the silica-based cement correspond to the porosity of the stone, with Vindhyan sandstone being classified as having low to moderate porosity [35]. EDX elemental analysis of the samples was performed on four different points, as shown in Figure 7C, giving a similar elemental profile. Si was identified as the main element, alongside less abundant elements such as Al, K, Mg, O, and Fe, which are likely derived from feldspar minerals [36]. Figure 7D shows the XRD spectrum obtained for sample 1RS, confirming the presence of silicon oxide in the form of quartz (SiO2) and feldspar in the form of microcline, i.e., K-feldspar (KAlSi3O8). The XRD spectrum also displayed minor peaks corresponding to calcite (CaCO3) and dolomite (CaMg(CO3)2), although Ca was not detected in the SEM-EDX analyses. These findings align with other studies on Vindhyan sandstone, which classifies it as a quartz arenite. Quartz is the dominant mineral, followed by feldspars, with a matrix primarily composed of silt and clay and a ferruginous-siliceous cement [35].
The FTIR spectra of the unaltered red sandstone samples (Figure 8) show consistent patterns, with most peaks corresponding to silicate materials and minimal evidence of other components: 3620 and 3417 cm−1 O-H stretching [37,38], 1081 and 1034 cm−1 Si-O asymmetric and planar stretching [37,39], 799 and 780 cm−1 Si-O symmetric stretching [39], 695 cm−1 Si-O stretching [40], and 519 and 421 cm−1 Si-O asymmetric bending [41,42]. The three bands at 2927 and 2857 cm−1 (more evident in the spectrum of sample 10RS, Figure 8B) can be attributed to asymmetric and symmetric C-H stretching in the methylene group [43,44]. Meanwhile, the band at 2959 cm−1 may correspond to the asymmetric C-H stretch of the methyl group [43,44]. These bands are typical of aliphatic hydrocarbons and possibly linked to fossil fuel or large-scale waste-burning [42,45], rather than being a component of the stone itself or resulting from previous conservation treatment.
Figure 9A,D illustrate, for example, areas where samples of black crusts were collected (2BC, 3BC, 4BC, 5BC, 6BC, and 8BC), corresponding to the arc along the wall and a nearby decorative structure. The stone is heavily impacted by pollution, leading to the formation of a thin and well-attached black deposit (Figure 9B,C). Over time, this layer thickens and develops into dendritic crusts, which ultimately causes the stone to exfoliate (Figure 9E,F).
This black layer can range from 55 to 500 µm (Figure S1 in Supplementary Materials) and tends to grow following the stone’s profile, creating a strong connection between the crust and the underlying material. The cross-section examination highlighted how the black crust develops mainly on the surface, but the stone below appears unaffected.
To gain a comprehensive understanding of the morphology and formation of the black crusts, SEM-EDX analysis was performed on the same samples, focusing on the top of the crust and the corresponding cross-sections. Figure 10 reports two images obtained by SEM-EDX of the top of sample 2BC, here reported as representative of the collected stone samples with visible crust. The sample is covered by a thick black crust showing the typical acicular crystals of gypsum (Figure 10). Gypsum is a common product in stone degradation, often due to alteration in carbonate stone or atmospheric deposition [24].
The SEM-EDX of the cross-section obtained from sample 2BC (Figure 11) provides valuable insight into how the black gypsum-based layer is related to the stone. For this reason, four points were selected explicitly on the cross-section for elemental analysis to study: the composition of the crust (point 1), the interface crust-stone (point 2), the stone matrix where crystals are visible (point 3), and the stone cement or amorphous areas of the stone matrix (point 4).
The elemental analysis using EDX, shown in Table 3, confirmed that Si and Al are the main components of the stone matrix, with Mg, K, Ca, and Fe as traces detectable near the outer surface. Ca and S were primarily found in the crust, as evidenced by the analysis conducted on the top of the crust. Elemental maps obtained for the cross-section of sample 2BC (Figure 11) further confirmed that Si is the main component of the stone, with Al, K, and Mg appearing as trace elements. Notably, Ca and S were mainly concentrated on the black crust and combined with each other. Ca is also present within the thin stone cement in the unaltered stone, but only in traces. Therefore, the thick gypsum crust is unlikely to be related to stone degradation, but it is probably a result of atmospheric deposition linked to pollution [14,24].
The XRD analyses of the powdered black crust (Figure S2 in Supplementary Material), next to the stone component (quartz and microcline), confirmed the presence of gypsum (CaSO4·2H2O) and bassanite (CaSO4·½H2O). Next, calcium oxalate in the form of weddellite (Ca(C2O4)·2H2O) was also identified. Oxalates are often associated with the partial oxidation of organic carbon, present in organic protective compounds used in conservation treatments, metabolic activities of microorganisms, or atmospheric pollutants [16,22,23]. Since no restoration work has been recorded on the Red Fort’s fortification walls, it is more likely that biological activity and the deposition of environmental pollutants are the main contributors to oxalate formation, at least in this case.
The FTIR spectra of the black crust samples (Figure 12) showed consistent results. All spectra indicated the presence of absorption peaks related to silicate material: 1035 (Si-O planar stretching [37,39]), 797, 781 (Si-O symmetric stretching [39]), 520 and 468 cm−1 (Si-O asymmetric bending [41,42]). As already pointed out by SEM-EDX and XRD analyses, gypsum was detected in all samples in proportion to the peaks: at about 3546 and 3406 cm−1 (asymmetric and symmetric O-H stretching vibration of the crystallization water molecules [46]); at 1622 cm−1 (out-of-plane bending vibration -OH [47]); at 1150 and 1120 cm−1 (SO42− asymmetric stretching [47]); at 673 and 603 cm−1 (in-plane banding of the sulfate ion SO42− [47]).
The peaks between 2960 and 2850 cm−1 might be linked to CH vibrations of aliphatic hydrocarbons, as already observed for the RS samples (Figure 8). Aliphatic hydrocarbons may be due to air pollution, as no conservation interventions have been documented on the Red Fort [48]. Next to these, nitrate, at 1384 cm−1 (NO3− asymmetric stretching band [49,50]), and oxalate salts, at 1545 and 1318 cm−1 (COO− asymmetric and symmetric stretching [51]), were also depicted. These may again indicate connections to environmental pollution and biological activity.
In conclusion, the FTIR spectra confirmed that calcite is nearly absent in both the back crust and the stone. A review by Ruffolo et al. [24] suggests that minerals like plagioclase can release calcium during weathering processes like acid leaching in rocks with relatively low calcium content. However, the calcium content in the unaltered Vindhyan sandstone is, in this case, too low to account for the formation of the observed gypsum-based crust. As previously hypothesized based on SEM-EDX and XRD data, the gypsum characterizing the black crust is most likely of depositional origin, accumulated on the stone surface as particulate matter produced by construction activities, road dust, and emissions from cement factories [2,3,4,5,6,7,20,29,46,52,53].
To thoroughly investigate the elemental composition, ICP-MS analysis was performed on samples 1RS and 2BC. The identification of trace and selected elements helped to investigate possible existing air pollution sources in the vicinity of the Red Fort that are majorly responsible for the deposition of a black crust over the Fort’s red sandstone [19].
Specifically, V, Ni, Cu, Zn, Ba, and Pb were found in minor or trace amounts in the unaltered red sandstone (1RS). In contrast, Ti, Cr, Mn, and Zn were present in either the red stone or the black crust sample (2BC). Nevertheless, all elements exhibited high levels due to pollution accumulation in the back crust (Table 4).
Elements like V, Cr, Ni, Cu, Zn, Ba, and Pb can be associated with the emission of fossil fuels such as gasoline and diesel, wear and tear of the mechanical parts of the vehicles, and wear of asphalt [19,22,54]. Surprisingly, Ti, Cr, and Zn were present in the 1RS sample composition at higher levels than expected. The presence of Ti in the 1RS sample may be due to the existence of Ilmenite minerals, which are usually found in the sedimentary rocks of the Earth’s crust [55]. Chromite minerals can, nevertheless, be found in consistent amounts in sedimentary rocks [56], while Zn may be due to sphalerite minerals [57,58]. The high level of Ti in the BC sample can be traced back to the combustion of oil and coal, in addition to contributions from particulate matter (PM) [59,60]. Mn can be associated with emissions from vehicles and soil or dust emissions in Delhi [5,6,7,15].

4. Conclusions

The Red Fort and other important monuments in India are severely impacted by degradation, primarily due to heavy air pollution. Numerous researchers have been working on collecting air data and understanding the sources and quality of air pollutants, particularly in Delhi. Despite many efforts, few specific studies focus on characterizing and comprehending the effects of pollution on Indian monuments, such as the Red Fort complex in Delhi.
This paper investigates Vindhyan red sandstone (RS) and black crust (BC) samples collected from the Red Fort to analyze their composition and explain their relationship to the surrounding environment. The elemental analyses indicated that the RS samples primarily consist of quartz and K-feldspar; with Si, K, and Al as the dominating elements. Calcite was present only in a minimum amount. The XRD data showed that the main components of the BC were gypsum (CaSO4∙2H2O), bassanite (CaSO4∙½H2O), and weddellite (C2CaO4∙2H2O), along with quartz and K-feldspar in the form of microcline. Since Ca was present only in minimal amounts in the RS sample from the Fort, the gypsum must be exogenous and likely deposited on the stone. This hypothesis aligns with characterization studies of the atmospheric PM in Delhi, as reported in recent research [20]. Therefore, the authors correlate the substantial presence of gypsum in BC to construction activities, road dust, and emissions from cement factories. The higher concentrations of heavy metals such as Ti, V, Cr, Mn, Ni, Cu, Zn, Ba, and Pb 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 [5,6,7,19,20,22,54]. Hence, it was also evident that the Red Fort was surrounded by heavy vehicular traffic, nearby industries and factories, high PM levels, biomass burning, and soil dust during the site visit.
Given the alarming air quality situation in Delhi, studying important monuments like the Red Fort is crucial for promoting effective conservation policies and interventions. This is the first research article addressing the impact of air pollution on the monumental complex of the Red Fort. Hopefully, this case study will enhance our understanding of the degradation processes affecting this monument and help evaluate the conservation state in relation to the outdoor environment of other Indian CHBS.
The degradation of CHBs is a slow and long-term process. Long-range air pollution datasets, combined with real case studies applications, are therefore crucial to analyze the impact of air pollution on CHBs. This work represents just the first step in initiating a monitoring survey related to stone conservation in India. The goal is to integrate the collected data with other available datasets in the region using simulation models such as the WRF (Weather Research Forecasting) model and the AERMOD (American Meteorological Society/Environmental Protection Agency Regulatory Model) air pollution datasets, as longer-term datasets are not available for Delhi, India. This research can help develop an action plan for the restoration and preservation of CHBs worldwide.
As for practical and feasible conservation recommendations, while reducing air pollutants in heavily populated areas is challenging, it is still possible to minimize the formation of a black crust on stone surfaces. The formation of a black crust is a progressive phenomenon that usually begins with a thin black layer or deposit, which can be removed without significantly damaging the stone’s integrity, at least in the early stages. In addition, a maintenance cleaning program for the most affected areas and the application of stone protectives could prevent or at least slow down the formation of a black crust.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/heritage8060236/s1, Figure S1: Micrographs of the cross-section of sample 2BC, magnification: 6X (A), sample 3BC, magnification 3.75X (B), sample 5BC, magnification: 3X (C) and sample 8BC, magnification 12X (D).; Figure S2: XRD pattern of sample 4BC representing a black crust present on the degraded red sandstone.

Author Contributions

G.K.: Conceptualization, Investigation, and data curation, Methodology, Validation, Formal analysis, Writing—original draft, Writing—Review and Editing. L.R.: Conceptualization, Investigation and data curation, Methodology, Validation, Formal analysis, Writing—original draft, Writing—Review and Editing. P.K.N.: Data curation, Validation, Review and Editing, Supervision. S.K.M.: Resources, Supervision. M.B.: Conceptualization, Methodology, Writing—Review and Editing, Supervision. A.B.: Conceptualization, Methodology, Writing—Review and Editing, Supervision. B.R.G.: Conceptualization, Validation, Writing—Review and Editing, Supervision. C.S.P.O.: Conceptualization, Writing—Review and Editing, Supervision. M.S.: Conceptualization, Methodology, Writing—Review and Editing, Supervision E.B.: Conceptualization, Investigation and data curation, Methodology, Validation, Formal analysis, Writing—original draft, Writing—Review and Editing, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Italian Ministry of Foreign Affairs, grant number CUP H73C21000120005, and the Department of Science and Technology under the Ministry of Science and Technology, India, grant number INT/Italy/P-23/2022 (NE) (G). The APC was funded by E.B. (Department of Environmental Sciences, Informatics and Statistics, Ca Foscari University of Venice), Department of Civil Engineering, Indian Institute of Technology Kanpur, India, and Indian Institute of Technology Roorkee, India.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors would like to thank the Italian Ministry of Foreign Affairs and the Department of Science and Technology under the Ministry of Science and Technology, India for the financial support, and the Italian Embassy in India for promoting the collaboration within Italian and Indian partners. A special thanks goes to all the Italian and Indian institutes involved in the project: Ca’ Foscari University of Venice, Soprintendenza Archeologia, belle arti e paesaggio per il comune di Venezia e laguna and Istituto Centrale per il Restauro of Rome, Indian Institute of Technology Kanpur, Indian Institute of Technology Roorkee, and the Archaeological Survey of India.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Ram, M.; Gulagi, A.; Aghahosseini, A.; Bogdanov, D.; Breyer, C. Energy Transition in Megacities towards 100% Renewable Energy: A Case for Delhi. Renew. Energy 2022, 195, 578–589. [Google Scholar] [CrossRef]
  2. Bhandari, S.; Gani, S.; Patel, K.; Wang, D.S.; Soni, P.; Arub, Z.; Habib, G.; Apte, J.S.; Hildebrandt Ruiz, L. Sources and Atmospheric Dynamics of Organic Aerosol in New Delhi, India: Insights from Receptor Modeling. Atmos. Chem. Phys. 2020, 20, 735–752. [Google Scholar] [CrossRef]
  3. Gurjar, B.R.; Ravindra, K.; Nagpure, A.S. Air Pollution Trends over Indian Megacities and Their Local-to-Global Implications. Atmos. Environ. 2016, 142, 475–495. [Google Scholar] [CrossRef]
  4. Guttikunda, S.K.; Jawahar, P. Atmospheric Emissions and Pollution from the Coal-Fired Thermal Power Plants in India. Atmos. Environ. 2014, 92, 449–460. [Google Scholar] [CrossRef]
  5. Hama, S.; Kumar, P.; Alam, M.S.; Rooney, D.J.; Bloss, W.J.; Shi, Z.; Harrison, R.M.; Crilley, L.R.; Khare, M.; Gupta, S.K. Chemical Source Profiles of Fine Particles for Five Different Sources in Delhi. Chemosphere 2021, 274, 129913. [Google Scholar] [CrossRef]
  6. Jain, S.; Sharma, S.K.; Choudhary, N.; Masiwal, R.; Saxena, M.; Sharma, A.; Mandal, T.K.; Gupta, A.; Gupta, N.C.; Sharma, C. Chemical Characteristics and Source Apportionment of PM2.5 Using PCA/APCS, UNMIX, and PMF at an Urban Site of Delhi, India. Environ. Sci. Pollut. Res. 2017, 24, 14637–14656. [Google Scholar] [CrossRef] [PubMed]
  7. Rai, P.; Furger, M.; El Haddad, I.; Kumar, V.; Wang, L.; Singh, A.; Dixit, K.; Bhattu, D.; Petit, J.-E.; Ganguly, D.; et al. Real-Time Measurement and Source Apportionment of Elements in Delhi’s Atmosphere. Sci. Total Environ. 2020, 742, 140332. [Google Scholar] [CrossRef]
  8. Natarajan, N.; Vasudevan, M.; Dineshkumar, S.K.; Nandhini, S.S.; Balaganesh, P. Effects of Air Pollution on Monumental Buildings in India: An Overview. Environ. Sci. Pollut. Res. 2022, 29, 29399–29408. [Google Scholar] [CrossRef]
  9. Sharma, A.; Singh, M.R. Execution Technique and Pigment Characteristics of the Decorative Wall from Seventeenth-Century CE Chatta Chowk at Red Fort Complex, New Delhi, India. Period. Mineral. 2021, 90, 43–56. [Google Scholar] [CrossRef]
  10. Kaur, G.; Singh, S.; Ahuja, A.; Singh, N.D. Natural Stone and World Heritage: Delhi-Agra, India; CRC Press: Boca Raton, FL, USA, 2020; Volume 11, ISBN 9780429286391. [Google Scholar]
  11. UNESCO. The Red Fort Complex; UNESCO: New Delhi, India, 2007. [Google Scholar]
  12. Patil, S.M.; Kasthurba, A.K. Weathering of Stone Monuments: Damage Assessment of Basalt and Laterite. Mater. Today Proc. 2021, 43, 1647–1658. [Google Scholar] [CrossRef]
  13. Google Earth. Aerial View of the Red Fort Complex in Delhi. Google Earth Pro Version:7.3.6.10201. Available online: https://earth.google.com/web/@28.65470184,77.241131,239.25963838a,2312.76729565d,35y,5.61031677h,6.02848352t,-0r/data=CgRCAggBOgMKATBCAggASg0I____________ARAA (accessed on 10 November 2024).
  14. Pozo-Antonio, J.S.; Cardell, C.; Comite, V.; Fermo, P. Characterization of Black Crusts Developed on Historic Stones with Diverse Mineralogy under Different Air Quality Environments. Environ. Sci. Pollut. Res. 2022, 29, 29438–29454. [Google Scholar] [CrossRef] [PubMed]
  15. Comite, V.; Álvarez de Buergo, M.; Barca, D.; Belfiore, C.M.; Bonazza, A.; La Russa, M.F.; Pezzino, A.; Randazzo, L.; Ruffolo, S.A. Damage Monitoring on Carbonate Stones: Field Exposure Tests Contributing to Pollution Impact Evaluation in Two Italian Sites. Constr. Build. Mater. 2017, 152, 907–922. [Google Scholar] [CrossRef]
  16. Lamhasni, T.; El-Marjaoui, H.; El Bakkali, A.; Lyazidi, S.A.; Haddad, M.; Ben-Ncer, A.; Benyaich, F.; Bonazza, A.; Tahri, M. Air Pollution Impact on Architectural Heritage of Morocco: Combination of Synchronous Fluorescence and ATR-FTIR Spectroscopies for the Analyses of Black Crusts Deposits. Chemosphere 2019, 225, 517–523. [Google Scholar] [CrossRef]
  17. Rabl, A.; Spadaro, J.V.; Holland, M. Impacts of Air Pollution on Building Materials. In How Much Is Clean Air Worth? Cambridge University Press: Cambridge, UK, 2014; pp. 131–159. [Google Scholar] [CrossRef]
  18. Gibeaux, S.; Thomachot-Schneider, C.; Eyssautier-Chuine, S.; Marin, B.; Vazquez, P. Simulation of Acid Weathering on Natural and Artificial Building Stones According to the Current Atmospheric SO2/NOx Rate. Environ. Earth Sci. 2018, 77, 327. [Google Scholar] [CrossRef]
  19. Ruffolo, S.A.; Comite, V.; La Russa, M.F.; Belfiore, C.M.; Barca, D.; Bonazza, A.; Crisci, G.M.; Pezzino, A.; Sabbioni, C. An Analysis of the Black Crusts from the Seville Cathedral: A Challenge to Deepen the Understanding of the Relationships among Microstructure, Microchemical Features and Pollution Sources. Sci. Total Environ. 2015, 502, 157–166. [Google Scholar] [CrossRef]
  20. Jangirh, R.; Ahlawat, S.; Arya, R.; Mondal, A.; Yadav, L.; Kotnala, G.; Yadav, P.; Choudhary, N.; Rani, M.; Banoo, R.; et al. Gridded Distribution of Total Suspended Particulate Matter (TSP) and Their Chemical Characterization over Delhi during Winter. Environ. Sci. Pollut. Res. 2022, 29, 17892–17918. [Google Scholar] [CrossRef] [PubMed]
  21. Spezzano, P. Mapping the Susceptibility of UNESCO World Cultural Heritage Sites in Europe to Ambient (Outdoor) Air Pollution. Sci. Total Environ. 2021, 754, 142345. [Google Scholar] [CrossRef]
  22. La Russa, M.F.; Comite, V.; Aly, N.; Barca, D.; Fermo, P.; Rovella, N.; Antonelli, F.; Tesser, E.; Aquino, M.; Ruffolo, S.A. Black Crusts on Venetian Built Heritage, Investigation on the Impact of Pollution Sources on Their Composition. Eur. Phys. J. Plus 2018, 133, 370. [Google Scholar] [CrossRef]
  23. La Russa, M.F.; Fermo, P.; Comite, V.; Belfiore, C.M.; Barca, D.; Cerioni, A.; De Santis, M.; Barbagallo, L.F.; Ricca, M.; Ruffolo, S.A. The Oceanus Statue of the Fontana Di Trevi (Rome): The Analysis of Black Crust as a Tool to Investigate the Urban Air Pollution and Its Impact on the Stone Degradation. Sci. Total Environ. 2017, 593–594, 297–309. [Google Scholar] [CrossRef]
  24. Ruffolo, S.A.; La Russa, M.F.; Rovella, N.; Ricca, M. The Impact of Air Pollution on Stone Materials. Environments 2023, 10, 119. [Google Scholar] [CrossRef]
  25. Hseu, Z.Y. Evaluating Heavy Metal Contents in Nine Composts Using Four Digestion Methods. Bioresour. Technol. 2004, 95, 53–59. [Google Scholar] [CrossRef]
  26. Jones, J.B. Chapter 15 Sampling, Handling, and Analyzing Plant Tissue Samples. In Soil Testing and Plant Analysis; Soil Science Society of America: Madison, WI, USA, 1990. [Google Scholar]
  27. Bulska, E.; Wagner, B. Quantitative Aspects of Inductively Coupled Plasma Mass Spectrometry. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 2016, 374, 20150369. [Google Scholar] [CrossRef] [PubMed]
  28. Garg, S.; Kaur, P.; Pandit, M.; Fareeduddin; Kaur, G.; Kamboj, A.; Thakur, S.N. Makrana Marble: A Popular Heritage Stone Resource from NW India. Geoheritage 2019, 11, 909–925. [Google Scholar] [CrossRef]
  29. Guttikunda, S.K.; Goel, R. Health Impacts of Particulate Pollution in a Megacity—Delhi, India. Environ. Dev. 2013, 6, 8–20. [Google Scholar] [CrossRef]
  30. CPCB Air Quality Monitoring Station’s Data, Delhi, India. Available online: https://airquality.cpcb.gov.in/ccr/#/caaqm-dashboard-all/caaqm-landing/caaqm-data-repository (accessed on 9 January 2025).
  31. Pandey, A.K.; Kumar, V. Impact of Environmental Pollution on Historical Monuments of India: Conservation Problems and Remedial Measures. J. Appl. Geochem. 2015, 17, 50–55. [Google Scholar]
  32. Rovella, N.; Aly, N.; Comite, V.; Randazzo, L.; Fermo, P.; Barca, D.; Alvarez de Buergo, M.; La Russa, M.F. The Environmental Impact of Air Pollution on the Built Heritage of Historic Cairo (Egypt). Sci. Total Environ. 2021, 764, 142905. [Google Scholar] [CrossRef]
  33. Yadav, A.; Behera, S.N.; Nagar, P.K.; Sharma, M. Spatio-Seasonal Concentrations, Source Apportionment and Assessment of Associated Human Health Risks of Pm2.5-Bound Polycyclic Aromatic Hydrocarbons in Delhi, India. Aerosol Air Qual. Res. 2020, 20, 2805–2825. [Google Scholar] [CrossRef]
  34. Nagar, P.K.; Sharma, M.; Das, D. A New Method for Trend Analyses in PM10 and Impact of Crop Residue Burning in Delhi, Kanpur and Jaipur, India. Urban. Clim. 2019, 27, 193–203. [Google Scholar] [CrossRef]
  35. Kaur, G.; Singh, S.; Kaur, P.; Garg, S.; Fareeduddin; Pandit, M.K.; Agrawal, P.; Acharya, K.; Ahuja, A. Vindhyan Sandstone: A Crowning Glory of Architectonic Heritage from India. Geoheritage 2019, 11, 1771–1783. [Google Scholar] [CrossRef]
  36. Astuti, T.R.P.; Aditiyo, R.; Oktavioni, A.; Supriyanto, S. SEM-EDX Study on Authigenic Clay Minerals in Sandstone of Jatiluhur Formation. IOP Conf. Ser. Earth Environ. Sci. 2020, 538, 012041. [Google Scholar] [CrossRef]
  37. Davarcioglu, B.; Ciftci, E. The Clay Minerals Observed in the Building Stones of Aksaray-Guzelyurt Area (Central Anatolia-Turkey) and Their Effects. Int. J. Phys. Sci. 2010, 5, 1734–1743. [Google Scholar]
  38. Nuevo, M.; Sandford, S.A.; Flynn, G.J.; Wirick, S. Mid-infrared Study of Stones from the Sutter’s Mill Meteorite. Meteorit. Planet. Sci. 2014, 49, 2017–2026. [Google Scholar] [CrossRef]
  39. Volpi, F.; Vagnini, M.; Vivani, R.; Malagodi, M.; Fiocco, G. Non-Invasive Identification of Red and Yellow Oxide and Sulfide Pigments in Wall-Paintings with Portable ER-FTIR Spectroscopy. J. Cult. Herit. 2023, 63, 158–168. [Google Scholar] [CrossRef]
  40. Friolo, K.H.; Stuart, B.; Ray, A. Characterisation of Weathering of Sydney Sandstones in Heritage Buildings. J. Cult. Herit. 2003, 4, 211–220. [Google Scholar] [CrossRef]
  41. El Omari, M.; El Amraoui, M.; Fikri, I.; Bejjit, L.; Haddad, M.; Ait Lyazdi, S.; El Amrani, A. Structural Characterization of Stone Flooring “Calçada Marble”, in Lisbon, Portugal. Mater. Res. Proc. 2024, 40, 294–302. [Google Scholar]
  42. Stewart, G.J.; Nelson, B.S.; Drysdale, W.S.; Acton, W.J.F.; Vaughan, A.R.; Hopkins, J.R.; Dunmore, R.E.; Hewitt, C.N.; Nemitz, E.; Mullinger, N.; et al. Sources of Non-Methane Hydrocarbons in Surface Air in Delhi, India. Faraday Discuss. 2021, 226, 409–431. [Google Scholar] [CrossRef]
  43. Derrick, M.R.; Stulik, D.; Landry, J.M. Infrared Spectroscopy in Conservation Science. In Handbook of Near-Infrared Analysis, 4th ed.; CRC Press: Boca Raton, FL, USA; Taylor and Francis: Abingdon, UK, 2021; pp. 45–67. ISBN 9798891132245. [Google Scholar]
  44. Invernizzi, C.; Rovetta, T.; Licchelli, M.; Malagodi, M. Mid and Near-Infrared Reflection Spectral Database of Natural Organic Materials in the Cultural Heritage Field. Int. J. Anal. Chem. 2018, 2018, 7823248. [Google Scholar] [CrossRef] [PubMed]
  45. Ambade, B.; Sankar, T.K.; Kumar, A.; Sethi, S.S. Characterization of PAHs and N-Alkanes in Atmospheric Aerosol of Jamshedpur City, India. J. Hazard. Toxic. Radioact. Waste 2020, 24, 04020003. [Google Scholar] [CrossRef]
  46. Salvadori, B.; Errico, V.; Mauro, M.; Melnik, E.; Dei, L. Evaluation of Gypsum and Calcium Oxalates in Deteriorated Mural Paintings by Quantitative FTIR Spectroscopy. Spectrosc. Lett. 2003, 36, 501–513. [Google Scholar] [CrossRef]
  47. Anbalagan, G.; Mukundakumari, S.; Murugesan, K.S.; Gunasekaran, S. Infrared, Optical Absorption, and EPR Spectroscopic Studies on Natural Gypsum. Vib. Spectrosc. 2009, 50, 226–230. [Google Scholar] [CrossRef]
  48. UNESCO World Heritage Centre. UNESCO World Heritage Centre—Document—Advisory Body Evaluation (ICOMOS). Available online: https://whc.unesco.org/en/documents/153016/ (accessed on 21 August 2024).
  49. Liu, J.-H.; Zhang, Y.-H.; Wang, L.-Y.; Wei, Z.-F. Drawing out the Structural Information of the First Layer of Hydrated Ions: ATR-FTIR Spectroscopic Studies on Aqueous NH4NO3, NaNO3, and Mg(NO3)2 Solutions. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2005, 61, 893–899. [Google Scholar] [CrossRef] [PubMed]
  50. Pérez-Alonso, M.; Castro, K.; Álvarez, M.; Madariaga, J.M. Scientific Analysis versus Restorer’s Expertise for Diagnosis Prior to a Restoration Process: The Case of Santa Maria Church (Hermo, Asturias, North of Spain). Anal. Chim. Acta 2004, 524, 379–389. [Google Scholar] [CrossRef]
  51. Sotiropoulou, S.; Sciutto, G.; Tenorio, A.L.; Mazurek, J.; Bonaduce, I.; Prati, S.; Mazzeo, R.; Schilling, M.; Colombini, M.P. Advanced Analytical Investigation on Degradation Markers in Wall Paintings. Microchem. J. 2018, 139, 278–294. [Google Scholar] [CrossRef]
  52. Fatima, S.; Mishra, S.K.; Ahlawat, A.; Dimri, A.P. Physico-Chemical Properties and Deposition Potential of PM2.5 during Severe Smog Event in Delhi, India. Int. J. Environ. Res. Public Health 2022, 19, 15387. [Google Scholar] [CrossRef]
  53. Gurjar, B.R.; Butler, T.M.; Lawrence, M.G.; Lelieveld, J. Evaluation of Emissions and Air Quality in Megacities. Atmos. Environ. 2008, 42, 1593–1606. [Google Scholar] [CrossRef]
  54. Barca, D.; Belfiore, C.M.; Crisci, G.M.; La Russa, M.F.; Pezzino, A.; Ruffolo, S.A. A New Methodological Approach for the Chemical Characterization of Black Crusts on Building Stones: A Case Study from the Catania City Centre (Sicily, Italy). J. Anal. At. Spectrom. 2011, 26, 1000–1011. [Google Scholar] [CrossRef]
  55. El Khalloufi, M.; Drevelle, O.; Soucy, G. Titanium: An Overview of Resources and Production Methods. Minerals 2021, 11, 1425. [Google Scholar] [CrossRef]
  56. Bertolo, R.; Bourotte, C.; Hirata, R.; Marcolan, L.; Sracek, O. Geochemistry of Natural Chromium Occurrence in a Sandstone Aquifer in Bauru Basin, São Paulo State, Brazil. Appl. Geochem. 2011, 26, 1353–1363. [Google Scholar] [CrossRef]
  57. Pękala, A.; Koszelnik, P.; Musiał, M.; Galek, T. Trace Elements Anomalous Concentrations in Building Materials—The Impact of Secondary Mineralisation Processes. Materials 2024, 17, 3909. [Google Scholar] [CrossRef]
  58. Zhu, C.; Liao, S.; Wang, W.; Zhang, Y.; Yang, T.; Fan, H.; Wen, H. Variations in Zn and S Isotope Chemistry of Sedimentary Sphalerite, Wusihe Zn-Pb Deposit, Sichuan Province, China. Ore Geol. Rev. 2018, 95, 639–648. [Google Scholar] [CrossRef]
  59. Environmental Health Criteria 24. Titanium: Published under the joint sponsorship of UNEP, ILO and WHO. World Health Organization, Geneva, 1982. pp. 68. Sw.fr. 11.00 (available in the UK through HMSO). Food Chem. Toxicol. 1984, 22, 76. [CrossRef]
  60. La Maestra, S.; D’agostini, F.; Sanguineti, E.; González, A.Y.; Annis, S.; Militello, G.M.; Parisi, G.; Scuderi, A.; Gaggero, L. Dispersion of Natural Airborne TiO2 Fibres in Excavation Activity as a Potential Environmental and Human Health Risk. Int. J. Environ. Res. Public Health 2021, 18, 6587. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. Aerial view of the Red Fort Complex in Delhi: 1 Fortification wall; 2 Zafar Mahal; 3 Moti Masjid, 4 Diwan-i-Khaas; 5 Diwan-i-Am; 6 Delhi gate; 7 Lahori gate; 8 Salimgarh Fort; 9 Railway track; 10 Yamuna River; 11 Mahatma Gandhi marg; 12 Shantivana marg; 13 Netaji Subhash marg [13].
Figure 1. Aerial view of the Red Fort Complex in Delhi: 1 Fortification wall; 2 Zafar Mahal; 3 Moti Masjid, 4 Diwan-i-Khaas; 5 Diwan-i-Am; 6 Delhi gate; 7 Lahori gate; 8 Salimgarh Fort; 9 Railway track; 10 Yamuna River; 11 Mahatma Gandhi marg; 12 Shantivana marg; 13 Netaji Subhash marg [13].
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Figure 2. Sampling sites of the collected samples given in Table 1.
Figure 2. Sampling sites of the collected samples given in Table 1.
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Figure 3. Degradation phenomena visible at the Red Fort complex: plaster blistering (A,B), flaking (C,D), rising damp and patinas (E,F), salt efflorescences (F), and limited damage of Makrana marble decorated floor (G).
Figure 3. Degradation phenomena visible at the Red Fort complex: plaster blistering (A,B), flaking (C,D), rising damp and patinas (E,F), salt efflorescences (F), and limited damage of Makrana marble decorated floor (G).
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Figure 4. An aerial view of the area around the Red Fort, indicating the air quality monitoring stations [13].
Figure 4. An aerial view of the area around the Red Fort, indicating the air quality monitoring stations [13].
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Figure 5. Monthly concentration of PM2.5 in 2021, 2022, and 2023 near the Red Fort. Data retrieved from the CPCB database.
Figure 5. Monthly concentration of PM2.5 in 2021, 2022, and 2023 near the Red Fort. Data retrieved from the CPCB database.
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Figure 6. Monthly concentration of PM10 in 2021, 2022, and 2023 near the Red Fort. Data retrieved from the CPCB database.
Figure 6. Monthly concentration of PM10 in 2021, 2022, and 2023 near the Red Fort. Data retrieved from the CPCB database.
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Figure 7. Data obtained for the cross-section of sample 1RS: (A,B) micrographs of the cross-section of sample 1RS, at 1.65X and 3.75X; (C) SEM image of the highlighted red region at 250X and (D) XRD patterns.
Figure 7. Data obtained for the cross-section of sample 1RS: (A,B) micrographs of the cross-section of sample 1RS, at 1.65X and 3.75X; (C) SEM image of the highlighted red region at 250X and (D) XRD patterns.
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Figure 8. (A) FTIR spectra of samples 1RS, 7RS, 9RS, and 10RS corresponding to the unaltered Vindhyan stone and (B) zoom of the spectrum of sample 10RS of the region 4000–2000 cm−1.
Figure 8. (A) FTIR spectra of samples 1RS, 7RS, 9RS, and 10RS corresponding to the unaltered Vindhyan stone and (B) zoom of the spectrum of sample 10RS of the region 4000–2000 cm−1.
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Figure 9. Photo of sheltered sampling point (A) with the micrographs of the collected sample 2BC (B,C). Photo of unsheltered sampling point (D) with the micrographs of the collected sample 8BC (E,F).
Figure 9. Photo of sheltered sampling point (A) with the micrographs of the collected sample 2BC (B,C). Photo of unsheltered sampling point (D) with the micrographs of the collected sample 8BC (E,F).
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Figure 10. SEM images of 2BC sample not embedded in resin at 250X (A) and 600X (B), where acicular gypsum crystals are visible and formed within the BC.
Figure 10. SEM images of 2BC sample not embedded in resin at 250X (A) and 600X (B), where acicular gypsum crystals are visible and formed within the BC.
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Figure 11. SEM image of sample 2BC (200X), in which the spots for the point analyses are marked (A). Maps of elements obtained with SEM-EDX at points 1, 2, 3, and 4 of sample 2BC. Elements mapped are: Ca (B), S (C), Si (D), Al (E), and K (F).
Figure 11. SEM image of sample 2BC (200X), in which the spots for the point analyses are marked (A). Maps of elements obtained with SEM-EDX at points 1, 2, 3, and 4 of sample 2BC. Elements mapped are: Ca (B), S (C), Si (D), Al (E), and K (F).
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Figure 12. FTIR spectra of samples 2BC, 3BC, 4BC, 5BC, and 8BC.
Figure 12. FTIR spectra of samples 2BC, 3BC, 4BC, 5BC, and 8BC.
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Table 1. Samples collected at the Red Fort Complex with the indication of their collocation and short description.
Table 1. Samples collected at the Red Fort Complex with the indication of their collocation and short description.
Sample IDSampling PointDescription
1RSUpper window of the fortification wallRed sandstone, no black crust
2BCUpper window of the fortification wallRed sandstone with black crust
3BCUpper window of the fortification wallRed sandstone with black crust
4BCUpper window of the fortification wallRed sandstone with black crust
5BCLower window of the fortification wallRed sandstone with thin black crust
6BCLower window of the fortification wallRed sandstone with black crust
7RSVault of the kioskRed sandstone; no black crust
8BCColumn of the kioskRed sandstone thin black crust
9RSZafar MahalRed sandstone; no black crust
10RS“New” stone used in restoration worksRed sandstone; no black crust
Table 2. Annual average air pollutant concentrations in the Red Fort proximity (Anand Vihar, Ashok Vihar, Vivek Vihar, Pusa, Patpadganj, ITO, and Dilshad Garden monitoring stations) along with National Ambient Air Quality Standards.
Table 2. Annual average air pollutant concentrations in the Red Fort proximity (Anand Vihar, Ashok Vihar, Vivek Vihar, Pusa, Patpadganj, ITO, and Dilshad Garden monitoring stations) along with National Ambient Air Quality Standards.
Location→Red FortNAAQS Standards
Year→202120222023
Pollutants (µg/m3) ↓AverageσAverageσAverageσ
PM2.511291103711028040
PM1021412622611121812160
NO244255029452440
NH3462652284624100
SO214913810650
Table 3. Elemental analyses using EDX of sample 2BC.
Table 3. Elemental analyses using EDX of sample 2BC.
PointElementsComment
1Si, O, Ca, S, Al, Mg, K, Fecrust
2Si, O, Al, Mg, K, Ca, Feinterface crust/stone
3Si, Ostone
4Si, Ca, O, Alcement
Table 4. Trace elements present in 1RS and 2BC samples of the Red Fort.
Table 4. Trace elements present in 1RS and 2BC samples of the Red Fort.
Trace Elements (mg/kg)
SampleTiVCrMnNiCuZnBaPb
1RS34.211.0315.4616.431.801.3021.663.400.36
2BC865.1846.657.56154.2417.2652.70187.7081.7841.98
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Kumar, G.; Rusin, L.; Nagar, P.K.; Manjul, S.K.; Back, M.; Benedetti, A.; Gurjar, B.R.; Ojha, C.S.P.; Sharma, M.; Balliana, E. Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India. Heritage 2025, 8, 236. https://doi.org/10.3390/heritage8060236

AMA Style

Kumar G, Rusin L, Nagar PK, Manjul SK, Back M, Benedetti A, Gurjar BR, Ojha CSP, Sharma M, Balliana E. Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India. Heritage. 2025; 8(6):236. https://doi.org/10.3390/heritage8060236

Chicago/Turabian Style

Kumar, Gaurav, Lucia Rusin, Pavan Kumar Nagar, Sanjay Kumar Manjul, Michele Back, Alvise Benedetti, Bhola Ram Gurjar, Chandra Shekhar Prasad Ojha, Mukesh Sharma, and Eleonora Balliana. 2025. "Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India" Heritage 8, no. 6: 236. https://doi.org/10.3390/heritage8060236

APA Style

Kumar, G., Rusin, L., Nagar, P. K., Manjul, S. K., Back, M., Benedetti, A., Gurjar, B. R., Ojha, C. S. P., Sharma, M., & Balliana, E. (2025). Characterization of Red Sandstone and Black Crust to Analyze Air Pollution Impacts on a Cultural Heritage Building: Red Fort, Delhi, India. Heritage, 8(6), 236. https://doi.org/10.3390/heritage8060236

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