Spectroscopy Guides Long-Term Conservation of Renaissance Murals in Valencia

Discovered in 2004, mural paintings depicting the Angel Musicians in the vault of the Valencia Cathedral in Spain are considered one of the earliest examples of Renaissance painting in Europe. Although these murals were restored after discovery, salt efflorescence and polychrome lifting reappeared in 2014. Because of the large scale of the artwork, an innovative multi-analytical approach conducted by researchers from the University of the Basque Country (Leioa, Spain) and Català Restauradors S.L (Rocafort, Valencia, Spain) was used to diagnose the conservation state and identify the cause of salt emergence prior to a new conservation works with long-term results. The murals were analyzed with laboratory Raman and micro-energy-dispersive X-ray fluorescence (μ-EDXRF) spectroscopies combined with ion chromatography (IC). A paper based on their work was published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (1).

The loss of the pictorial layer of wall paintings through the formation of surface salts (efflorescence) can be the result of many factors, including humidity, temperature, pH, atmospheric pollutants or biological action can cause the loss of the pictorial layer of wall paintings through the formation of surface salts (efflorescence) (2,3). In addition, treatments used for previous restorations may also affect deterioration and should always be taken into consideration (4). Soluble salts such as nitrates, sulfates, chlorides and carbonates can have many different origins; nitrates, for example, can form naturally because of the metabolism of certain living organisms and the degradation of organic matter, as well due to anthropogenic activities, such as the deposition of NOx gases in areas with high environmental pollution (3, 5–7). Sulfate salts can arise in the presence of SOx gases (3,5,6); these and chloride salts have also been connected to groundwater influence or marine aerosols (8). Carbonate salts in buildings have been determined to primarily originate from materials such as cement (9).

Any of these events may have caused the damage observed in the Valencia Cathedral, where an extensive salt efflorescence's formation was evident (10). The cathedral is renowned for the Renaissance mural paintings which dominate the central vault, known as the Angel Musicians (11). Painted in the 15th century, these paintings disappeared in the 17th century after the construction of a new dome, extended about 80 cm below the previous one. The angels were discovered under the baroque dome in 2004, and the initial restoration and conservation work was carried out until 2007. Despite that recent restoration, problems began to emerge in 2014 with the appearance of salts and the lifting of the polychromy, or the practice of decorating architectural elements in a variety of colors (12). A new intervention was planned in 2019 to address the issues that had developed to that date, with a multidisciplinary project undertaken to study the conservation state of the paintings, quantify the effect of the salts, identify their origins, and design a restoration and conservation plan which avoided short-term conservation results like those of the previous works (1).

The research team used in situ portable Raman spectroscopy for point analysis and hyperspectral image analysis, for the coverage of the expanse of the artwork with ease, to create damage maps. Furthermore, the techniques allowed targeted sampling to avoid unnecessary sampling, collecting 28 samples of efflorescences and 15 depth-samples. Principal component analysis (PCA) and multivariate curve resolution (MCR) characterized areas in the artwork affected by efflorescence as well as differentiation of regions affected by syngenite and gypsum, likely the result of water infiltration carrying residues from generations of pigeons which have inhabited the roof of the cathedral for centuries (1).

The researchers wrote that the results of their analysis are crucial in the guidance of more effective restoration processes, as well as the prevention of inadequate interventions which might require additional treatment within a few years, ensuring the long-term conservation of the paintings. In addition, they stated that their analytical proposal is appropriate for the thorough investigation of other large-scale artworks (1).

Close up of Valencia Cathedral. © xiao zhang - stock.adobe.com

References

1. Etxebarria, I.; Vázquez-de la Fuente, I.; Huidobro, J. et al. Advanced Analytical Solutions for Large-Scale Works of Art: Revealing Damage in the Renaissance wall Paintings of Valencia Cathedral (Spain). Spectrochim. Acta A Mol. Biomol. Spectrosc. 2025, 346, 126846. DOI: 10.1016/j.saa.2025.126846
2. Coccato, A.; Moens, L.; Vandenabeele. P. On the Stability of Mediaeval Inorganic Pigments: A Literature Review of the Effect of Climate, Material Selection, Biological Activity, Analysis and Conservation Treatments. Herit. Sci.2017, 5 (1), 12. DOI: 10.1186/s40494-017-0125-6
3. Huidobro, J.; Arana, G.; Madariaga, J. M. Raman Spectroscopy Against Harmful Nitrogen-Based Compounds in Cultural Heritage Materials. J. Raman Spectrosc.2024, 55 (12), 1224-1235. DOI: 10.1002/jrs.6724
4. Costantini, I.; Veneranda, M.; Irazola, M. et al. The Green Grass Was Never Green: How Spectroscopic Techniques Should Have Assisted Restoration Works. Microchem. J.2018, 138, 154-161. DOI: 10.1016/j.microc.2018.01.010
5. Maguregui, M.; Sarmiento, A.;  Martínez-Arkarazo, I. et al. Analytical Diagnosis Methodology to Evaluate Nitrate Impact on Historical Building Materials. Anal. Bioanal. Chem.2008, 391 (4), 1361-1370. DOI: 10.1007/s00216-008-1844-z
6. García-Florentino, C.; Maguregui, M.; Morillas, H. et al. Portable and Raman Imaging Usefulness to Detect Decaying on Mortars from Punta Begoña Galleries (Getxo, North of Spain). J. Raman Spectrosc. 2016,47 (12), 1458-1466. DOI: 10.1002/jrs.4949
7. Siedel, H. Sefflorescence as Indicator for Sources of Damaging Salts on Historic Buildings and Monuments: A Statistical Approach. Environ. Earth Sci.2018, 77 (16). DOI: 10.1007/s12665-018-7752-4
8. Szendrei, G.; Tóth, T.; Kovács-Pálffy. P. Sulfate and Chloride Salt Efflorescences and Their Relationships with Groundwaters and Soils in Hungary. Agrokém. Talajt.2014, 63 (1), 49-58. DOI: 10.1556/agrokem.63.2014.1.6
9. Dow, C.; Glasser, F. P. Calcium Carbonate Efflorescence on Portland Cement and Building Materials. Cem. Concr. Res.2003, 33 (1), 147-154. DOI: 10.1016/s0008-8846(02)00937-7
10. Catedral de Valencia. La Iglesia Catedral. https://catedraldevalencia.es/la-catedral/la-iglesia-catedral/ (accessed 2022-02-04)
11. Catedral de Valencia. Historia de los Frescos de la Catedral. https://catedraldevalencia.es/los-frescos-renacentistas/historia-de-los-frescos-de-la-catedral/ (accessed 2022-02-04)
12. Polychrome. Wikipedia.https://en.wikipedia.org/wiki/Polychrome (accessed 2025-08-29)