# Biologically Derived Gels for the Cleaning of Historical and Artistic Metal Heritage
## 1 Metal Care in Cultural Heritage: An Overview  
In the motley world of cultural heritage, metals are among the main actors since the emergence of metalworking of gold, silver, and copper [1]. Due to their inherent chemo-physical properties and aesthetic potential, metals have had a broad spectrum of applications in art through the ages: sculptures, musical instruments, jewelry, utensils, scientific instruments, decorative arts, and so forth [2; 3; 4; 5; 6; 7]. Such a massive presence in cultural heritage has inevitably yielded a plethora of studies on metal preservation to safeguard this inheritance through generations. The core problem for the preservation of metals is the spontaneous process of corrosion towards which they naturally tend [8]. Indeed, corrosion is a thermodynamically favored phenomenon, which alters metals' original features and represents a tremendous economic loss in whichever sector, as well as a conservation issue in the heritage field [9]. Different aspects can influence corrosion processes: along with the basic dual presence of water and oxygen, the characteristics of the surrounding environment rule and meddle the phenomenon. Factors such as burial soil, marine location, microbiological activity, atmospheric pollutants, and, in general, alloys' complexity and exposure conditions play an important role in this detrimental process [10; 11; 12]. From a preventive perspective, despite the utopic willingness to artificially control the micro-environment, not all the active agents in corrosion can be avoided or adjusted,especially if we consider outdoor artifacts. Therefore, in metal care, the first phase consists of tackling the corrosion layer present. This usually means either stabilizing or removing the corrosion compounds, depending on the substrate nature and the type of alteration exhibited, in addition to an evaluation according to restoration principles (e.g., noble or unstable patina, corrosion as proof of authenticity or anesthetic degradation) [13]. Both issues can be addressed from different angles. For stabilization, the current techniques go from electrolytic reduction, dichloromethane, and immersion in an alkaline sulfide solution for archaeological iron artifacts, to the use of chemical chelating agents or corrosion inhibitors such as benzotriazole (BTA) for copper-based relics [14; 15; 16; 17]. On the other side, for corrosion removal, we can cite potentially invasive mechanical methods such as wet or dry abrasion (sanding paper or cloth, sandblasting, scalpel, laser, or cryogenics), as well as chemical approaches, exploiting electrochemical reduction or synthetic complexing agents such as ethylenediaminetetraacetic acid disodium salt dihydrate (Na\({}_{2}\)EDTA), tetraethylenepentamine (TPEA) and triammonium citrate (TAC) [18]. In the following step, a solid solution is to prevent the interaction between the metal surface and the external detrimental factors. The use of organic coatings is thus a favored method, being able to achieve a film with the required physical, mechanical, and aesthetical characteristics [19]. The typical products employed are both from natural or synthetic origin--e.g., beeswax, epoxy resin, acrylic-based varnish [20]--however, the latter has acquired more relevance since they are designed with the desired features and fine-tuned to be the most environmental-proof as possible [21]. These compounds are also sensitive to detrimental factors through time, even if to a lesser degree or merely not like metals. Therefore, there might be the necessity of removing--and replacing--them once their preservative purpose is compromised (e.g., film cracking and detachment, harmful reaction with the underlying substrate itself, visual appearance no more acceptable according to conservation standards) [22; 23]. To achieve this purpose of removal, the most employed approach is the dissolution of the no more-desired coating present [24; 25; 26; 27].