Biological Patinas Research Articles

Prospection of the Red Biological Patinas Influencing the Urban Scenery Architecture in Portuguese Territory

Portugal’s architecture reflects a rich history influenced by Roman, Moorish, Neoclassical and Romantic styles, with the 20th century marked by Art Deco and colonial influences. Regional styles vary, with white-painted buildings commonly experiencing color changes due to biofilm formation. Visually striking are the red-colored biological patinascommon in the coastal areas. A survey of 120 historical buildings affected by bio-colonization helps to understand the reasons for the patinas’ growing, which beyond natural factors, is often linked to construction defects. A characterization of four samples utilizes Next-Generation Sequencing (NGS) to identify the microorganisms composing the red biofilm, while the SEM-EDS, FTIR-ATR and XRD techniques provide further insights into the biofilm and substrate features. The comprehensive data of biochemical characterization indicate a wide variety of microorganisms, including bacteria and fungi, some of which exhibit potential as producers of a UV-tolerant red/yellow pigment (carotenoid) responsible for the macroscopic coloration of bio-colonization.

Assessment of marine and urban-industrial environmental impact on stone acting as the base of a quaternary bronze sculpture

This work investigates the influence of a marine and urban-industrial environment on the deterioration of a funerary memorial in Getxo (Basque Country, Spain). The sculpture and artificial stone materials as well as their degradation products were characterised via a multi-analytical approach. An in situ characterisation of the bronze statue and its artificial limestone base was carried out with portable Energy Dispersive X-Ray Fluorescence (ED-XRF) and Raman spectrometers. In addition, several samples of white crusts as well as metal runoff crusts and biological patina were collected and some of them were prepared as cross-sections. High Resolution Micro-X-ray fluorescence and micro-Raman spectroscopy were used as non-destructive methods to detect original and degradation compounds. The test of soluble salts was also carried out on the samples and ion chromatography was employed in the laboratory. Exclusively calcite salt-efflorescences were found at the monument side with ED-XRF and Raman spectroscopy. While their morphology was compact at unsheltered sites, they showed a lamellar structure at sheltered locations, whose increased volume strongly influences the destruction of the memorial. The spectroscopic characterization of metal runoff showed that its excessive occurrence on the memorial surface was due to the dissolution of the copper-alloy caused by chemical reactivity with the surrounding acidic atmosphere. The formation of brochantite Cu4(SO4)(OH)6, posnjakite Cu4(SO4)(OH)6·(H2O), langite Cu4(SO4)(OH)6·2(H2O), or malachite Cu2(CO3)(OH)2 was favored exclusively in metal runoff crusts at unsheltered locations. However, much thinner crusts collected at sheltered sites showed a different composition, with copper species such as moolooite (Cu(C2O4)·nH2O) and atacamite Cu2Cl(OH)3. The soluble salt extraction showed a variable concentration of soluble ions depending on the area where the samples come from. The sample from north west sheltered location showed the highest content of sulfate, nitrate and chloride compounds. In addition, the concentration of fluoride and nitrate reflects the impact of contamination of surrounding industries.

NYMPHA, a natural product for the conservation of ancient wood

In recent years, there has been a significant interest on sustainability and health-safety across various domains. Notably, many traditional chemicals employed in the preservation of Cultural Heritage pose environmental and human health risks. The NYMPHA project aimed to develop an eco-friendly solution using microalgae-derived polysaccharides to remove biological patinas from cultural heritage wooden materials, addressing sustainability and health concerns. To validate its efficacy, the product underwent testing on diverse woods such as silver fir, beech, and sessile oak, selected for their distinct anatomical characteristics. The analytical methodology involves three key steps: 1) determining the optimal extraction and application method through spectro-colorimetric measures and UV imaging; 2) evaluating surface color stability; and 3) assessing the product's effectiveness before and after exposure to a biological attack using spectro-colorimetry. Results indicate that the NYMPHA product can induce a color variation on some wood surfaces. Moreover, although it is reported that algae can have biocidal effects, in this experiment, this action is not observed probably due to the absence of sulphates in the polysaccharide molecule extracted from this specific strain. This emphasizes the necessity for further research and to explore new solutions beyond controlled laboratory conditions, specifically on naturally degraded materials.

Raman spectroscopy assisted by other analytical techniques to identify the most deteriorated carbonate‐stones to be consolidated in two monuments of Vitoria‐Gasteiz (Spain)

AbstractThis work describes the diagnostic study on the building materials, mostly carbonated, belonging to Santa Maria Cathedral and the Medieval Wall of Vitoria‐Gasteiz (Spain) with the aim to design the best conservation procedure. Both the studies of the lithology and the secondary compounds originated by environmental impacts on the Cathedral and on the Medieval Wall were carried out using laboratory instruments (μ‐Raman and micro‐energy‐dispersive X‐ray spectroscopy, X‐ray diffraction and ion chromatography) on selected samples provided by the restorers. The systematic presence of black crusts in the stones of the Cathedral was related to the growth of microcrystalline structures of secondary compounds and biological patinas and the deposition of atmospheric particles from traffic and house heating systems. In fact, the main components identified were carbon, and iron compounds such as hematite, goethite, magnetite and lepidocrocite. In addition, the detection of lead compounds (lead‐rich hydroxyapatite) suggested in the same way the impact of the urban environment on the degradation and blackening of stone materials. The presence of sulfates, mainly gypsum, and, to a lesser extent, epsomite, anhydrite and bloedite could be caused by the sulfation of carbonated compounds as a result of an acid attack of atmospheric pollutants. The results on the secondary products of the Medieval Wall showed a greater presence of degradation by microorganisms compared to the Cathedral. This is probably related to the large garden surrounding the fortification, where the grass is in direct contact to the lower part of the structure. Markers of biological activity, such as carotenoid pigments and calcium oxalate weddellite, together with other soluble oxalates were identified. The presence of ammonium nitrate, characterised by means ion chromatography, causes a chemical degradation of carbonate stone materials over time, due to the acidic nature of the ammonium ion. In both cases considered in this study, the presence of nitrate compounds, nitratine and potassium nitrate, was attributed to both natural factors (ammonium nitrate is coming from the decomposition of plant and animal excretions), and anthropogenic contamination.

Recolonization dynamics of marble monuments after cleaning treatments: A nine-year follow-up study

The prevention and control of biological patinas on outdoor stone monuments represent a demanding challenge for the conservation of cultural heritage also due to some microorganisms, particularly resistant to treatments, such as black meristematic fungi, an eco-physiological group well known for its tolerance to extreme conditions. Even if several methods and eco-friendly products have been proposed as new alternatives, traditional biocides are still far from being completely replaced. Recolonization is a natural process that occurs sooner or later after cleaning. The time that elapses until its occurrence can vary considerably depending on environmental conditions and the used products; unfortunately, the papers describing the effect of treatments over time are rare. This work aims to shed light on the recolonization process of marble surfaces in the ancient monumental cemetery of Bonaria (Cagliari) after nine years from treatments, evaluating the long-term efficiency of two different cleaning methods, namely dimethyl sulfoxide-based gel (DMSO-based gel) and Biotin T (a didecyldimethylammonium chloride-based product-). In this context, the microflora present before treatments and in the following years was assessed by culture-based methods and identified by molecular techniques, with attention on black meristematic fungi, which were used as reference for the most resistant lithobiontic organisms. Different environmental parameters, such as temperature, exposition, dominant winds, and rainfall, were considered, and infrared thermography, portable light microscopy, and image analysis were used. This research evidenced the influence of water availability and lightning in recolonization processes, the transition from the pioneer fungal community versus more resistant black fungal species after Biotin T treatment, and the long-lasting efficiency of the DMSO-based gel. These findings prove that this low-impact method deserves more attention in the conservation of outdoor marble monuments, emphasizing the importance of long-term studies.

ASPECTS REGARDING THE ATTACK OF SOME BIODETERIOGENS ON SOME CULT OBJECTS FROM ARGES COUNTY

Cultural heritage has a cultural significance, which refers to the aesthetic value of the heritage asset, the historical or social value of monuments for past, present or future generations. In conservation areas where there are open environment conditions, there are ecological producers (autotrophic bacteria, algae, lichens and higher plants). In terms of trophic relationships and the main food chain, the presence in the conservation environment of collections from archives, libraries and museums of excessive food resources for biodeteriogenic pests determines the possibility of the absence of producers, which is why most populations settled in the environment conservation of movable cultural heritage goods are consumers (egs insects) and destroyers (bacteria and fungi). In the present study, various fragments of cultural heritage goods taken from cemeteries, churches, for the purpose of analyzing biological patinas and establishing the etiopathogenic complex were subjected to analysis. Fungi of the genera Alternaria, Penicilium, Aspergillus, etc., various bryophytes, lichens and blue-green algae have been identified.

Application and Monitoring of Oxidative Alginate–Biocide Hydrogels for Two Case Studies in “The Sassi and the Park of the Rupestrian Churches of Matera”

The removal of biological colonization on building materials of cultural heritage is a difficult challenge, as the treatment must completely eliminate the biological patina without altering the treated substrate and possibly delaying new colonization. With the aim of searching for systems to minimize the biocide impact on the substrate, the environment and the operators, different alginate–oxidizing biocide hydrogels were previously tested and optimized in the laboratory and here selected for application in situ. The churches “San Pietro Barisano” and “Madonna dei Derelitti”, located in the Sassi of Matera (UNESCO World Heritage Site in Basilicata region, Italy), were chosen as case studies. They differ in terms of both the environmental conditions and the microorganisms responsible for colonization. Colorimetric measurements and microscopic investigation proved the efficacy of biocide hydrogels in removing biopatinas and in restoring the original chromaticity of the selected treated surfaces of both sites. After the biocidal treatments, new protective acrylic coatings were applied to prevent recolonization and minimize the loss of material grains. Samples collected, immediately after and two years later, established the absence of biological colonization, demonstrating the long-term efficacy of the proposed restoration protocol.

AUTOMATIC POINT CLOUD SEGMENTATION FOR THE DETECTION OF ALTERATIONS ON HISTORICAL BUILDINGS THROUGH AN UNSUPERVISED AND CLUSTERING-BASED MACHINE LEARNING APPROACH

Abstract. The article describes an innovative procedure for the three-dimensional analysis of decay morphologies of ancient buildings, through the application of machine learning methods for the automatic segmentation of point clouds. In the field of Cultural Heritage conservation, photogrammetric data can be exploited, for diagnostic and monitoring support, to recognize different typologies of alterations visible on the masonry surface, starting from colour information. Actually, certain stone and plaster surface pathologies (biological patina, biological colonization, chromatic alterations, spots,…) are typically characterized by chromatic variations. To this purpose, colour-based segmentation with hierarchical clustering has been implemented on colour data of point clouds, considered in the HSV colour-space. In addition, geometry-based segmentation of 3D reconstructions has been performed, in order to identify the main architectural elements (walls, vaults), and to associate them to the detected defects. The proposed workflow has been applied to some ancient buildings’ environments, chosen because of their irregularity both in geometrical and colorimetric characteristics.

New oxidative alginate-biocide hydrogels against stone biodeterioration

To address the problem of the biodeterioration of stone monuments by developing innovative methodologies, two new alginate hydrogels were obtained by using titanium dioxide as photoactivatable biocide and sodium dichloroisocyanurate as reservoir of hypochlorous acid. The effectiveness of both hydrogels in removing the biological patina on artificially colonized Lecce stones without altering the properties of the treated substrates was demonstrated by measurements of light microscopy, colorimetry, SEM/EDS analyses and 1H-NMR relaxometry. The biocidal activity of the new developed hydrogels has been also tested on natural cyanobacterial biofilms collected in Roman Catacombs and grown on agar medium and on calcareous stones. The results are very promising with the complete inhibition of cyanobacterial photosynthesis and without any side effects on the substrates tested.

Alginate-biocide hydrogel for the removal of biofilms from calcareous stone artworks

One aspect of heritage science is to examine and, if possible, reduce the chemical and physical interactions between the restoration products and artworks’ constituent materials. This research aims to identify an alternative process to minimize the impact on materials and health risk for users when hypochlorite is used as biocide on stone materials. In this study, we tested a hydrogel comprised of sodium alginate as an inert matrix and hypochlorite ions as biocide agents for the removal of biological patinas from calcarenite Lecce samples. These biofilms resulted formed mainly by filamentous cyanobacteria and green microalgae. We carried out 1H-NMR T2 relaxation measurements to check the hydrogel’s cleaning efficacy on artificially bio-deteriorated samples through a portable NMR equipment. Simultaneously, optical microscopy and colorimetric measurements allowed the establishment of color differences induced by treatments. Our biocide hydrogel proved to be effective in removing microbial colonization from the surface of Lecce stone specimens and from a section of an altar rail in the rupestrian Church of San Pietro Barisano, located in the “Sassi of Matera”.

Multidisciplinary Approach to Characterize Archaeological Materials and Status of Conservation of the Roman Thermae of Reggio Calabria Site (Calabria, South Italy)

This multidisciplinary research focuses on diagnostic investigations to characterize the archaeological materials, as well as the alteration and degradation forms detected at the Roman Thermae of Reggio Calabria (Calabria, South Italy) site. The thermal complex, (dating I–II century B.C.), was built around three main rooms such as the caldarium (hot bath), the tepidarium (warm bath) and the frigidarium (cold bath), all connected to a central room through several entrances. The central hall still preserves a suggestive mosaic floor dating between the II and III century A.D., characterized by geometric motifs and black and white tesserae. Fragments of various archaeological stone materials, such as bricks, mortars, sedimentary, volcanic and metamorphic rocks have been studied with different and complementary techniques. Particularly, polarized optical microscopy (POM) and X-ray diffractometry (XRD) were performed to characterize the materials employed to construct the site and evaluate their state of preservation. Finally, laboratory microbiological culture analysis was conducted to identify the main microorganisms composing the biological patinas detected on the sampled materials. Results allowed us to evaluate the most suitable restoration procedures to conduct at the archaeological site, considering the different stone materials present in the studied area and their state of conservation.

A review of the nature, role and control of lithobionts on stone cultural heritage: weighing-up and managing biodeterioration and bioprotection.

Lithobionts (rock-dwelling organisms) have been recognized as agents of aesthetic and physico-chemical deterioration of stonework. In consequence, their removal from cultural heritage stone surfaces (CHSS) is widely considered a necessary step in conservation interventions. On the other hand, lithobiontic communities, including microbial biofilms ('biological patinas'), can help integrate CHSS with their environmental setting and enhance biodiversity. Moreover, in some cases bioprotective effects have been reported and even interpreted as potential biotechnological solutions for conservation. This paper reviews the plethora of traditional and innovative methodologies to characterize lithobionts on CHSS in terms of biodiversity, interaction with the stone substrate and impacts on durability. In order to develop the best management and conservation strategies for CHSS, such diagnosis should be acquired on a case-by-case basis, as generalized approaches are unlikely to be suitable for all lithobionts, lithologies, environmental and cultural contexts or types of stonework. Strategies to control biodeteriogenic lithobionts on CHSS should similarly be based on experimental evaluation of their efficacy, including long-term monitoring of the effects on bioreceptivity, and of their environmental safety. This review examines what is known about the efficacy of control methods based on traditional-commercial biocides, as well as those based on innovative application of substances of plant and microbial origin, and physical techniques. A framework for providing a balanced scientific assessment of the role of lithobionts on CHSS and integrating this knowledge into management and conservation decision-making is presented.

Encapsulation of environmentally-friendly biocides in silica nanosystems for multifunctional coatings

In cultural heritage field, significant research efforts have been recently made to improve the efficacy of anti-vegetative treatments and to reduce the environmental impact caused by biocides high concentration. According to the pro-ecological approach, this work reports a novel approach based on the encapsulation/incorporation of environmentally-friendly biocides in different silica nanosystems in order to control the development of biological patinas on outdoor cultural heritage. Two different green biocides have been selected and tested in silica nanosystems: the zosteric acid sodium salt (ZS), a natural antifoulant compound produced by Zostera marina (eelgrass), and the usnic acid (UA), a secondary metabolite produced by some saxicolous lichens. ZS was previously successfully encapsulated but never entrapped in mesoporous silica; instead, UA is, for the first time, encapsulated and in situ entrapped into the silica nanosystems in order to control the release over time. Both silica nanosystems have been characterized as far as their dimensions and superficial properties and loading capability. The antifouling activity was assessed against microorganisms from biopatinas colonising the Aurelian Walls in Rome. Our results have shown that the two nanosystems have complementary properties, thus it is possible to tune the antifouling efficiency by combining the two in different proportions.

The Essential Oil of Thymbra capitata and its Application as A Biocide on Stone and Derived Surfaces.

Many chemicals used nowadays for the preservation of cultural heritage pose a risk to both human health and the environment. Thus, it is desirable to find new and eco-friendly biocides that can replace the synthetic ones. In this regard, plant essential oils represent effective alternatives to synthetic substances for the preservation of historical monuments. Thymbra capitata (syn. Thymus capitatus) is a medicinal and aromatic plant growing in the Mediterranean area and endowed with important pharmacological properties related to its essential oil. Among them, the antimicrobial ones make the T. capitata essential oil an ideal candidate for industrial applications; for instance, as biocide for the inhibition and elimination of biological patinas of cyanobacteria and green algae on historical monuments. In the present work, we studied the chemical composition of the essential oil from T. capitata growing in Malta by gas chromatography-mass spectrometry (GC/MS). The major volatile component is the phenolic monoterpene carvacrol (73.2%), which is capable of damaging the cytoplasmic membrane and to interfere both in the growth curve and in the invasive capacity, though the contribution of minor components γ-terpinene and p-cymene cannot be disregarded. For the oil application on the stone surface, Pickering emulsions systems were prepared with an essential oil/water 1:3 mass ratio stabilized with kaolinite at 4 mass% in the presence of Laponite®; this allowed to limit the fast volatility of the oil and guaranteed a better application and an easier removal from the artefacts attacked by biodeteriogens both indoor and outdoor. This formulation caused the elimination of biodeteriogens from treated surfaces without residuals or films on artworks surface, and the effect was retained up to four months.

Multidisciplinary Approach Applied to the Diagnosis of the Facade of the Arciprestal Church of Santa María de Morella (Castellón, Spain).

This paper deals with the development of a multidisciplinary study on the current state of conservation of the facade of the Arciprestal Church of Santa María de Morella (Castellón, Spain), a work of the Gothic period of great historical and artistic value. The aim of this diagnosis was to undertake the preventive conservation actions required and increase the knowledge about the conservation of paintings on stones. During the diagnosis scanning, electron microscopy was demonstrated to be a valuable analytical method for wall paintings on stone. The facade, which since its construction has not undergone major architectural changes, has reached our days as it was configured in its creation, adding the traces of the passage of time and interventions that have suffered polychromies. Because of the conservation situation, it was decided to have an interdisciplinary project for the structural study of the work, an exhaustive study of the materials and their state of conservation. The study of the materials includes the identification of stone supports, mortars, the pictorial technique of the original and added polychromies, and the superficial patinas. On-site studies were carried out by ground penetration radar (GPR) and X-ray fluorescence (XRF). Among the techniques used in laboratory were optical polarized light microscopy (MO-LP), X-ray diffraction (XRD), scanning electron microscopy with microanalysis (SEM-EDX) and Fourier-transform infrared spectroscopy (FTIR). The study allowed to determine the different pathologies of alteration and degradation of stone substrate and polychromies, chromatic alterations, biological patinas, etc. During this study, it was demonstrated that the diagnosis of wall paintings is a complex issue that needs to be addressed in a multidisciplinary approach, where scanning electron microscopy with microanalysis is the key methodology to get a deeper understanding of subsurface characterization of wall paintings and highlight the weathering processes. In a second phase of previous studies, this technique (SEM) has been used in assessing the viability of consolidation systems and cleaning both the stone and the polychrome.

Evaluation of the role of biocolonizations in the conservation state of Machu Picchu (Peru): The Sacred Rock

Machu Picchu Inca sanctuary (Cusco Region, Peru) was constructed on a granitic plateau, better known as Vilcabamba batholith. One of the most important carved granitic rocks from this archaeological site is the Sacred Rock, used by Inca citizens for religious rituals. Due to the location and climatic conditions, different rocks from this archaeological site are affected by biocolonizations. Concretely, the Sacred Rock shows flaking and delamination problems. In this work, a non-destructive multi analytical methodology has been applied to determine the possible role of the biodeteriogens, forming the biological patina on the Sacred Rock, in the previously mentioned conservation problems. Before characterizing the biological patina, a mineralogical characterization of the granitic substrate was conducted using X-ray Diffraction, Raman microscopy (RM) and micro energy dispersive X-ray fluorescence spectrometry. For the identification of the main biodeteriogens in the biofilm, Phase Contrast Microscopy was used. RM also allowed to determine the distribution (imaging) and the penetration (depth profiling) of the biogenic pigments present in the biopatina. Thanks to this study, it was possible to asses that some colonizers are growing on inner areas of the rock, reinforcing their possible assistance in the delamination. Moreover, the in-depth distribution of a wide variety of carotenoids in the patinas allowed to approach the penetration ability of the main biodeteriogens and the diffusion of these biogenic pigments to the inner areas of the rocky substrate.

Efficacy of laser cleaning in the removal of biological patina on the volcanic scoria of the rock-hewn churches of Lalibela, Ethiopia

The Lalibela rock-hewn churches are one of the most important religious pilgrimage sites in Ethiopia. These churches are carved from the scoriaceous basalt rock substrate, which has been exposed to attack by biological agents with significant loss of surface material. Particularly, the widespread growth of lichens and other microorganisms on the carved surfaces of the churches has proven to represent a substantial threat for the preservation of the site. In this study, laboratory tests have been conducted to assess the feasibility of using laser technology as an efficient cleaning method of biological patina from polymineralic stone substrates. Multi-analytical techniques were applied for the characterization of the stone samples collected from two of the Lalibela churches: Bete Giyorgis and Bete Amanuel. Stone samples artificially inoculated with bacteria, yeast, fungi isolates, and lichen-encrusted samples were laser cleaned using UV and IR laser wavelengths. The high content of Fe and Ti oxides and the high porosity have made the stone surfaces easily susceptible to low-energy laser treatment. Results indicate that laser cleaning can be applied to polymineralic lithotypes and UV irradiation can successfully remove lichen colonies. Further studies need to be conducted to optimize the laser procedure in polymineralic, high porosity stones.

Biological colonization on stone monuments: A new low impact cleaning method

Abstract In restoration and conservation practices, biocide treatments are considered one of the most practical approaches to remove biological colonization on artworks, including stone. Numerous studies have focused on the short- and long-term effects of these treatments and recently many alternative methods to reduce their potential hazards to human health and the environment have been proposed. In this study, a solvent gel containing dimethyl sulfoxide (DMSO), already used to clean paintings, was applied on colonized marble artifacts at the monumental cemetery of Bonaria (Cagliari – Italy) to remove biological patinas. The protocol efficiency was evaluated by scanning electronic microscopy, rugosimetric and colorimetric measurements and growth tests. A comparative study also was performed to validate the method using biocides currently used in conservation. The results demonstrate that DMSO solvent gel is efficient at removing patinas on stone, of low impact, easy to use, inexpensive and can be considered a more practical alternative to biocide treatments.

Biodeterioration of Pompeian mural paintings: fungal colonization favoured by the presence of volcanic material residues

This work was focused on the study of the biodegradation processes jeopardizing a mural painting conserved in the basement of Ariadne House (archaeological site of Pompeii, Italy). The fresco stood out for its peculiar state of preservation: the upper part, recovered in 1988, was just barely colonized by microorganisms. On the contrary, the lower part (excavated in 2005) was almost completely covered by extensive biological patinas. The genomic characterization carried out by polymerase chain reaction (PCR) highlighted the presence of seven different fungi strains on the mural surface. Beside, in situ and laboratory analyses were performed with the purpose of identifying the causes of the heterogeneous spatial distribution of the biopatinas. The in situ Raman spectroscopy and energy dispersive X-ray fluorescence (ED-XRF) spectroscopy measurements excluded any link between the heterogeneous colonization and the original materials present in the wall. On the other side, X-ray diffraction (XRD) and scanning electron microscopy (SEM) on microsamples proved the presence of a thin volcanic material layer overlying the lower part of the fresco. Considering that most of the biofilms of the studied mural painting only growth over these residues, it was confirmed the role of volcanic material as a suitable support for biological colonization. Thanks to the obtained results, this research helped to understand more in depth an important degradation pathway threatening the artworks from one of the most important archaeological sites in the world.

A multianalytical approach to investigate stone biodeterioration at a UNESCO world heritage site: the volcanic rock-hewn churches of Lalibela, Northern Ethiopia

A multianalytical approach combining Optical Microscopy (OM), Backscattered Variable Pressure Scanning Electron Microscopy + Energy Dispersive X-ray Spectroscopy (VP-BSEM + EDS), Powder X-ray Diffractometry (PXRD), Raman Spectroscopy, and Microbiological techniques has been applied to characterize decay products and processes occurring at the surface of two rock-hewn churches (Bete Gyorgis and Bete Amanuel) at the UNESCO’s World Heritage site of Lalibela, Northern Ethiopia. The two churches were carved into volcanic scoria deposits of basaltic composition. In their geological history, the Lalibela volcanic rocks underwent late to post-magmatic hydrothermal alteration together with partial laterization and are therefore characterized by a decay-prone highly vesicular microtexture with late stage to post-magmatic precipitation of secondary mineral phases (calcite–zeolite–smectite). The main objective of the study was to gain a better insight into the weathering products and mechanisms affecting the surface of the stone monuments and to assess the relative contribution of natural “geological” weathering processes versus biological/salt attack in stone decay at this unique heritage site. Results indicate that while the main cause of bulk rock deterioration and structural failure could be related to the stone inherited “geological” features, biological attack by micro- (bacteria) and/or macro- (lichens) organisms is currently responsible for severe stone surface physical and chemical weathering leading to significant weakening of the stone texture and to material loss at the surface of the churches walls. A prompt and careful removal of the biological patinas with the correct biocidal treatment is therefore recommended.