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Aims This study reports the results of the application of a new agar‐gauze biogel system activated with viable bacterial cells to altered wall paintings. Methods and Results Biocleaning using agar biogel and agar‐gauze biogel systems was performed onsite by direct application to altered wall painting surfaces (25–1000 cm2). The treatments were performed for the restoration of two original Italian sites: (i) at the Vatican Museums, Cristo che salva Pietro dalle acque—La Navicella, a wall painting by Giovanni Lanfranco (1627–1628) and (ii) at Pisa Cathedral Cupola, Incarnato, a wall painting by Orazio Riminaldi (1593–1630) and his brother Girolamo Riminaldi. The novelty of this study is the use of viable Pseudomonas stutzeri A29 cells in an advanced agar‐gauze biogel system and the short bio‐application contact times of between 3 and 12 h. The historical artworks were altered by lipid and protein residues from past restoration, as confirmed by Py‐gas chromatography–mass spectrometry and FT‐IR data. The effectiveness of the biological treatment was assessed, and general considerations were discussed. Conclusions The short bio‐application contact time of advanced agar‐gauze gel activated with viable P. stutzeri cells makes this biotechnology promising as an alternative method to the traditional onsite cleaning techniques currently in use for altered historical wall paintings. Significance and Impact of the Study In this study, we report for the first time the biocleaning of altered materials located in vertical and vaulted areas using agar‐gauze biogel with short application times. These findings are of great significance for future restoration activities and are crucial for determining the best preservation strategies in this field.

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The first academic studies on the use of microorganisms in cleaning procedures appeared in the late 1980s/early 1990s. In the past thirty years, most of such studies have addressed the removal of nitrate and sulphate salts and organic matter from surfaces by using non-pathogenic anaerobic microorganisms, mainly sulphate-reducing bacteria. The successful use of microbes in the removal of graffiti paint remains, however, a work in progress. Biocleaning surfaces to remove graffiti is not a simple task, because of the complex chemical composition of graffiti paints. This study looks at ways of improving the bioremoval of graffiti and presents the latest findings regarding different methodological aspects of cleaning natural and man-made stone. Granite and concrete substrates were coated with silver and black graffiti spray paints for comparison of the efficacy of the biocleaning method on these different materials. Visual and microscopic examination along with colour and infrared measurements made after application of the bacterial strains tested (previously shown to be suitable candidates for bioremoval of graffiti) revealed remarkably successful results. The findings presented thus represent progress in the development of a biocleaning protocol applicable to the in-situ removal of graffiti. Important improvements have been made regarding the time of treatment, which has been reduced by up to 20 days, and the use of a culture medium enriched with powdered graffiti, which facilitates and accelerates the adaptation of the microorganisms to the target surface.

High levels of nitrate contamination of granite stone are a major problem, affecting large surfaces of many historical monuments, particularly in the north-west of Spain. This study showed a comparison between different traditional and biotechnological desalination methods in order to evaluate the most appropriate cleaning treatment for nitrate desalination of granite. Three types of traditional desalination methods (with cellulose and/or sepiolite) were compared with two types of bacterial denitrifying treatments that used Pseudomonas stutzeri (with cotton wool or with agar 2% as delivery systems). The in-situ tests were carried in the Cristo Chapel of Stª Mª de Conxo in Santiago de Compostela (Spain), which has a high nitrate salt content in its granite pavement. Conductivity and nitrate content measurements, biological monitoring and digital image analysis were performed to determinate the efficacy of each method. The findings showed that both techniques succeeded in reducing salt content, but bio-desalination was the more effective method tested. This work contributes to the practical implementation of BTCH (Biocleaning Technologies for Cultural Heritage) for the bio-desalination of granite surfaces, and to the evaluation of the use of non-destructive cleaning techniques based on digital imaging.