Cleaning is a fundamental phase of the conservation and maintenance activities of the Cultural Heritage. According to the current guidelines [1–3], its primary objective is the removal of all deposits, undesired layers and potentially harmful substances which can cause irreversible damage as a result of the prolonged interaction with the stone substrate . It is therefore to be considered a very delicate and irreversible procedure, because under no circumstances the removed material can be retrieved and even the most respectful methodology may cause slight damage of the surface . A wide range of methodologies has been developed and are currently available for stone cleaning, based on several mechanical, chemical or physical mechanisms [6–8]. Each one of them has specific features, advantages, and drawbacks which have been extensively reported in the literature . The selection of the most appropriate cleaning methodology should follow the precise knowledge of the substrate characteristics (i.e. constituent materials, stratigraphy, possible presence of previous conservative treatments, etc.) as well as of the composition of the materials to be removed (i.e. chemical composition, thickness of overlapped layers, penetration depth of deposits, adhesion to the stone substrate, etc.). As a general rule, the cleaning methodology is required to be highly effective and specifically selective in the removal of undesired deposit and harmful compounds alone, being at the same time extremely respectful of the substrate. The fulfilment of such conditions can be assessed through the evaluation of basic criteria which include: physical and chemical harmfulness; homogeneity of the removal of the deposits; efficiency of cleaning; absence of aesthetic alteration; durability . A wide range of non-destructive or micro-destructive techniques can be used on selected pilot areas to preliminary asses all the previously mentioned criteria, prior to the global intervention, even though a shared protocol for the in situ evaluation of the cleaning results has not been defined so far. The durability criteria, in particular, obviously require at least a medium to long-term monitoring of the overall treated surface to define the actual effectiveness of the procedure over time with respect to the exposure conditions (microclimate, quality of air, particulate matter composition, etc.), as well as to detect every possible alterations of the substrate.
In the present work, a sustainable approach for the cleaning of indoor surfaces of the cultural heritage is presented. A cleaning methodology based on agar gel applied on stone sculpted surface of the Duomo of Milan was defined, preliminary tested and then applied. The Duomo is one of the most iconic and well-known Italian monuments, a document of historic and material culture of paramount importance, and a primary touristic resource. Its location in the very centre of a particularly polluted city , as well as the great number of people constantly visiting the interior of the church, determines the need for a continuous maintenance of the marble surfaces and of the sculpted decoration. In particular, the progressive accumulation of particulate matter is a primary cause of blackening. This phenomenon results in the alteration of perception of the interiors and potentially enhances the decay rate of the stone material [12, 13]. The availability of a sustainable, cost-effective and low time-consuming cleaning methodology therefore represents an urgent need for the experts in charge of the monument maintenance. Moreover, the maintenance operations are usually performed during the opening hours of the Duomo, thus in presence of visitors. The demand for totally safe conditions for both the operators and the visitors is a further fundamental criterion for the selection of the optimal cleaning procedure.
The evaluation of the agar gel cleaning procedure was conducted on the “Fuga in Egitto” (“Flight into Egypt”) high-relief and on the two lateral caryatids, a sculpted group located in the deambulatory of the Duomo (in the so-called “Tornacoro”), behind the main altar. The relief is part of the Baroque sculptural cycle dedicated to the Stories of the Virgin Mary, made in Candoglia and Carrara marble during the XVII century by some of the most important sculptors of Milan: Gian Andrea Biffi, Marcantonio Prestinari, Giovanni Pietro Lasagna, Giovanni Bellanda, Gaspare Vismara . The entire cycle is composed of seventeen panels, each one sided by a couple of caryatids in forms of sculpted angels. The “Fuga in Egitto” was realized by Gian Andrea Biffi around 1612, whereas the two lateral angels were designed by Francesco Brambilla and made by unknown authors. The overall state of conservation of the sculpted group was not particularly worrying with respect to the substrate cohesion. The marble did not suffer any evident loss of material neither due to disaggregation, nor to surface erosion, thanks to the interior location. On the other hand, the presence of a diffused dark deposit completely altered the colour and aesthetic of the surface, so that the sculpted details could no longer be fully appreciated. Moreover, it is well known that deposits in urban environments can be particularly rich in soluble salts, metallic ions and other potentially harmful compounds which can enhance the degradation rate of the stone [15, 16]. The cleaning of the sculptures was then considered as mandatory in order to prevent future decay.
As previously discussed, the cleaning operation conducted in this specific context must not interfere with the regular touristic and religious use of the site. Excessive noise, water percolation, production of dust as a result of the deposit removal, or use of aggressive chemical compounds had to be absolutely avoided. It was therefore decided to test the application of agar gels which granted high efficacy, no liquid water release and limited cost. Moreover, agar is completely non-toxic since it derives from natural organic raw materials and can therefore be considered as an eco-friendly and “green” technique.
Agar is a water-soluble, neutral, polysaccharide, extracted from several genera of red seaweeds of Rhodophyceae class (Gelidium, Gracilaria, Gelidiella, Pterocladia, Sphaerococcus genera). It is made of two polysaccharides, agaropectin and agarose. The latter derives from disaccharide agarobiose units sided by 3-linked β-D-galactopyranosyl and 4-linked 3,6- anhydro-α-L-galactopyranosyl units . Thanks to its gelling properties, agar is applied in several different fields: in biology for the preparation of culture media , in chemistry as electrode binder for electrolyte cells , in pharmacy for drug delivery , in the food industry as thickening agent. Once the agar powder is mixed in water suspension and heated up to 80°C, it develops a random coil structure which is able to progressively rearrange and gel during the cooling stage, below 40°C. It then forms thermo-reversible physical gels, characterized by ordered double helix chains linked by hydrogen bonding . The terminations of the chains remain in the coil form, available to join with other terminations and creating a 3-dimensional reticulum . The resulting microstructure is well-ordered, with a high number of pores, having rather homogeneous pore size distribution, which enhance water retention and allow liquid water migration within the gel . When agar is applied as a water-based poultice, the solvent effect of water is therefore promoted: in presence of soluble deposits the dissolution is favoured and the dissolved material is drained into the gel structure, which acts as a “sponge”. Agar gels can also be prepared by adding surfactant or chelating agents into the water solutions, to create gels characterized by a variable pH for the treatment of specific substrates .
All these characteristics make the use of agar gel particularly valuable in the cleaning of cultural heritage surfaces. Its high water retention promotes the soluble salt extraction, whereas its transparency allows a constant visual control of the underlying material. As for the possible application techniques, agar can be used as a dense solution applied by brush on a surface, as well as a preformed rigid gel overlapped to the substrate. In both cases, its final removal at the end of the procedure is facilitated by the low adhesion to the substrate, and it leaves almost no residues on the surface . During the cleaning, agar gel can limit the contact of liquid water with the treated surface respect to more traditional poultices based on cellulose pulp, japanese paper or sepiolite, thus preventing solvent migration throughout the surrounding areas . This aspect becomes very important in the case of particularly absorbent or delicate materials to be cleaned, such as paintings on canvas and on panels, mural paintings [26, 27], wood , stones [29, 30], plaster , paper and textiles . Moreover, differently from agar, some other gels such as cellulose ethers (Klucel G, Tylose, HPMC) or polyacrylic acid-based gels (Carbopol or Carbometer) are reported to have compatibility issues if applied on specific substrates and in presence of solvents . It is also worth noting that agar is capable to retain the deposit and materials removed, thus compounds allowing a more efficient control of the cleaning procedure. Agar has been already used in the cultural heritage field as a successful cleaning methodology, but only a limited number of applied case studies are reported in the scientific literature [25–34], whereas a thorough evaluation of its efficacy and harmfulness is still lacking so far.
The diagnostic activity to support the conservative intervention of the sculpted group of the Duomo started in 2012 and lasted for almost two years. It supported the entire cleaning procedure: starting from the preliminary characterization of the substrate and deposits, to the subsequent set-up of the cleaning operations on pilot areas, followed by the constant monitoring of the cleaning results and to the final assessment of the cleaned surfaces. In order to set-up the final method, a multi-analytical approach was carried out both in situ and in the laboratory to compare the efficacy of different cleaning conditions in pilot areas by using the following techniques: spectrophotometric measurements (colorimetry), optical observations, ESEM-EDX analyses, Fourier Transform Infrared Spectroscopy, and X-Ray Diffraction. At the end of the intervention, an investigation of the marble surfaces was performed to evaluate the cleaning result and to confirm the absence of any damage to the stone substrate.