# Biologically Derived Gels for the Cleaning of Historical and Artistic Metal Heritage
## 2 Gels: A Reliable Delivery System
### Cleaning Gels in Cultural Heritage  
At the beginning of the nineties, [PERSON] introduced gels to the world of art conservation [45]. His work focused on the treatment of paintings [46], which are water- and solvent-sensitive targets; thus, gel formulations seemed optimal for higher-controlled and less invasive interventions. Considering the promising evidences, this technique became rapidly captivating in several fields, including wall paintings and stone care [47; 48; 49]. Moreover, the employment of gels turned to be essential when dealing with paper or wood substrates, for which the conventional procedure by immersion into water solution was not ideal [50; 51].  
Nowadays gel-based cleaning protocols are recognized as well-grounded and effective methodologies, thanks to their high retention and interesting rheological properties that allow a precise and selective treatment confined to the upmost layer [52; 53]. In addition, such systems can adsorb the undesired substances to be removed (i.e., corrosion products and degraded protective materials) in the polymeric matrix. Finally, they drastically limit solvent fumes released in the air, thereby creating a less hazardous work environment for operators.  
Nevertheless, the use of gels is still anedotic on metal artworks, even though there is the need for localized treatments, considering how heterogenous they can be: composite artifacts, gilded artworks, painted metals, etc.  
In a comparative study on gel and swab cleaning on brass, AFM (Atomic force microscopy) pictures proved the existence of scratches after the swab action due to particles originally present on the metal. On the contrary, the gel performance, although slightly less efficient, was smoother and non-invasive, leaving no physical changes on the surface [54]. These observations led to the setting-up of a project named \"Gels-Metaux\", initiated in France and expanding to a larger-scale, calling forth an international appeal. The project tackles various tricky topics: gel preparation protocols, compatibility between formulations and active agents (i.e., chelators), and introducing gels into new branches of metal conservation like archaeological findings [55].  
Notably, some studies regarding the use of synthetic gels have been published for metal conservation. In 2014, a set of indoor bronze sculptures were successfully treated via Carbopol-gel applications to remove layer-by-layer dirt, protective waxes, and corrosion phases [56]. While water-based formulations made of the synthetic polymer poly (vinyl alcohol) (PVA) were designed by [PERSON] et al. as part of the NANORESTART project and assessed on copper alloys [16; 57]. Despite the captivating potentialities, high attention must be addressed to their waste management and the correlated ecological impact. For instance, PVA is readily water-soluble; however, inadequate disposal can cause pollution of groundwaters. Its biodegradation is performed by very peculiar bacteria, barely represented in nature [58; 59]. Another critical point is the sustainability of gelling agents' production. Al though bio-derived from cellulose, often the synthesis of cellulose-based polymers cannot be considered green, since it may require reagents hazardous for the aquatic environment (e.g., chloroacetic acid used for the synthesis of carboxymethyl cellulose) [60].  
In contrast, scarcer is the literature on green or bio-based alternatives. Therefore, an overview of both hydro- and solvent-gels is proposed here, highlighting their suitability for metal heritage.