Special Issue on Interdisciplinary Researches for Cultural Heritage Conservation

1. Introduction

UNESCO defines cultural heritage as “the legacy of physical artefacts and intangible attributes of a group or society that are inherited from past generations, maintained in the present and bestowed for the benefit of future generations”.

The roots of each country and region are anchored in its own cultural heritage. Cultural heritage is an invaluable legacy and is integral to our future, but due to its fragile and finite nature we sought to identify the best, most sustainable means of preserving cultural heritage. This implies the conservation of movable (paintings, sculptures, artifacts) and immovable (monuments, archaeological sites, industrial archaeology) properties. Today, cultural heritage is exposed to air pollution, flooding, earthquakes, wrong management activities, etc., which threaten its integrity. To mitigate damages, research should focus on analyzing and alleviating deterioration and provide technological solutions for enhancing the conservation of cultural heritage. These goals can be achieved through interdisciplinary and transdisciplinary studies.

This Special Issue on Interdisciplinary Researches for Cultural Heritage Conservation in Applied Sciences aims to bring together some of the latest researches in this field. A total of 10 papers focusing on different aspects of cultural heritage are categorized into the three topics detailed below and summarized.

2. Rock Art, Mural Paintings and Stone Monuments

Some of the earliest forms of rock art are cave paintings and petroglyphs, carved or scratched into the rock surface. The most famous examples of rock art include Altamira and Lascaux caves, well-known examples of flawed management that produced substantial biodeterioration phenomena, widely discussed in the scientific literature. The pioneer studies on these two caves promoted the extension to other caves affected by similar problems.

Altamira, Lascaux and many other caves presented different types of colonization by phototrophic biofilms, a severe alteration produced by artificial lighting. The most noticeable effect of the lighting was the growth of a dense phototrophic community of cyanobacteria, algae and bryophytes on the speleothems, walls and ground of Tesoro Cave, Rincon de la Victoria, Spain. Jurado et al. [1] tested the application of different mechanical and chemical cleaning procedures and showed the effectiveness of hydrogen peroxide and sodium hypochlorite for the successful cleaning of green biofilms from the entire cave.

Cañaveras et al. [2] studied the deterioration processes affecting the sandstones supporting the paintings and carvings of the southern Spanish shelters of Tajo de las Figuras (Cádiz) and Peñas Cabrera (Málaga). The data showed that the most common deterioration mechanisms corresponded to natural physical and chemical phenomena, including mechanical (frost) and chemical (carbo-hydrolysis) weathering, in addition to biological colonization, for which corrective measures are difficult to apply without producing unforeseen consequences, mainly when the state of conservation of the paintings is acceptable.

Rabbachin et al. [3] investigated the factors involved in the degradation of petroglyphs in the Negev desert of Israel, with a focus on biodeterioration processes. The multi-analytical approach used in this study allowed the characterization of the microbiomes of limestones covered with black varnish, in close proximity to the petroglyphs, as well as the weathering of the limestones, thus unveiling mineral–microbial interactions.

Earliest known examples of wall paintings are found in Crete and in Egyptian tombs that were continued in Etruscan necropolis, Roman, medieval and Renaissance monuments. The Salón de Reinos is a remnant of the 17th century Palacio del Buen Retiro (Madrid, Spain) built between 1632 and 1640. The Salón housed famous paintings, now deposited in the Museo Nacional del Prado, and mural paintings located in the ceiling and window vaults with a poor conservation status, evidenced by fissures, water filtration and fungal colonizations. Jurado et al. [4] stated that most of the isolated bacteria and fungi from the paintings originate from plants, saprophytes or pathogens, data consistent with the abundant vegetation in parks and gardens near the Salón de Reinos (Retiro Park and Botanical Garden). Likewise, the presence of a few bacteria and fungi with biodeteriorative activities, previously detected in other murals and frescoes, was also noteworthy.

The monuments have not only suffered the ravages of times, but are also damaged by vandalism in modern times. An example is the Cross of the Inquisition located in the Seville City Hall, Spain. The Cross was vandalized in September 2019 and the restoration concluded in September 2021. Along with the restoration, a mineralogical and microbiological study was carried out in a few small fragments. The exposure of the Cross of the Inquisition to an urban environment for more than 100 years promoted the colonization of the limestone by a lichenic community, associated with bacteria, fungi and bryophytes, which were detrimental for the stone. Appropriate biocides and consolidants were used in the restoration [5].

3. Stained-Glass Windows

Two papers deal with materials used in stained-glass windows. Palomar et al. [6] determined the composition of flashed glasses that have been used since medieval times in stained-glass windows. They used optical microscopy, field emission scanning electron microscopy, and linear energy dispersive spectroscopy to determine the thickness of the colored layers, and laser-induced breakdown spectroscopy for the analysis of major and minor components, which provided valuable information on chronology, provenance, and manufacturing processes of these types of glasses.

Machado et al. [7] studied the production of grisaille, the first glass-based paint to be used in stained-glass windows spread throughout Europe from the 12th century onwards and still used today. The authors determined that raw materials directly affect the properties and appearance of grisailles and the proportions between the different grisaille components (base glasses, coloring agents, firing processes, etc.) were key factors in their stability.

4. Miscellaneous Papers

Morales-Martín et al. [8] explored the use of optical sol–gel environmental pH sensors for air evaluation in exhibition halls of the Museo Nacional de Ciencias Naturales (Madrid, Spain). The effects of Madrid air pollution affected the exhibition halls near the entrance with slightly low pH values, while halls located far from the entrance showed neutral pH values. Environmental pH sensors allowed the museum to make decisions on preventive conservation strategies.

Bernabéu-Larena et al. [9] analysed the case of Canal de Isabel II, a water supply public work and its historic infrastructure built in the second half of the 19th century and the early 20th century. The study showed that the water supply that maintained its original use has also taken on new functions through the conversion of some of its parts and added new functions to the existing ones, such as the enhancement of its heritage value.

Wang et al. [10] proposed Lunyu ontology to formally represent concepts concerning morality in The Analects of Confucius. In this research, the authors explored ways to develop a class hierarchy from discrete and abstract ideas and also adopted a term-and-characteristic-guided methodology derived from taking into account the ISO principles on Terminology, whereby “a term is a verbal designation of a concept” and “a concept is a unique combination of (essential) characteristics”.

Overall, the main themes of the papers published in this Special Issue cover important achievements on diverse Cultural Heritage aspects that were also complemented by other Applied Sciences Special Issues devoted to Cultural Heritage.