# Biologically Derived Gels for the Cleaning of Historical and Artistic Metal Heritage
## 3 Bio-Based Hydrogels for Metal Preservation
### Physical Hydrogels  
#### 3.1.1 Agar  
Rigid gels are valid delivery systems due to their capability to retain solvents that allow to focus the cleaning on the uppermost layer, thereby limiting negative actions on eventual sensitive underlying materials. Moreover, their compact consistency makes them appreciated for the easy removability or even peelability [63]. However, their main advantage is also their main weakness. Being rigid, it is difficult to achieve good performances on rough substrates where the contact gel-surface is difficult to maintain. In art conservation, the leading thickening agent in this category is uncontestably agar. Agar is a gelling powder, extracted from red seaweed membranes, in particular _Gelidium_ sp., _Peteroladia_ sp. and _Gracilaria_ sp. [64], which forms a rigid hydrogel after being heated above 90 \({}^{\circ}\)C and cooled down [63]. Agar is usually prepared in concentration from 2 to 6% \(w/v\)  
Figure 1: Bio-polymers used for hydrogel formulations in cultural heritage: structural formula of xanthan gum, gellan gum, agarose and chitosan.  
Agar is broadly available, with various purity and composition according to different assigned purposes in the food industry, biomedicine, or art conservation [65; 66]. The primary compound is agarose, which is more often used imprinted together with agaropectin, being less expensive [63].  
The heating process can be a turn-off for conservators, especially when they are required to work out of their lab, with no heating devices. In addition, an agar gel cannot be formed with acidic or basic solutions, which, however, can be absorbed by immersion once the gel is created. Finally, it gives particularly satisfying results when directly applied hot on the target, as in a liquid state, it still has the chance to conform to the artifact surface [55].  
A gel called Nevek(r) has been recently developed and is mentioned as particularly suitable for the cleaning of metallic surfaces [55]. It is a ready-to-use 5% double gemified agar with the addition of 5% ethanol to avoid microbial growth during storage.  
In previous literature, several methodologies based on agar were tested on metals. In the case of archaeological silver horns, an agar gel was prepared and impregnated with the active compound before application [67]. Meanwhile, on Islamic inlays copper-alloys, agar powder was mixed to a complexing solution, buffered at pH 7, and then warmed up to gel [68]. In 2013, an agarose rigid gel was successfully exploited on Japanese lacquer objects, to face the complex combination of metals present, namely tin, lead, and iron. The use of a hydrogel system addressed the electrolytic reduction treatment exclusively on the corroded areas, without affecting and altering the surrounding water-sensitive lacquers [69]. Recently agar formulations resulted to be efficient carriers for the electrochemical cleaning of silver threads and silver leaves [70; 71]. However, they were not performing at the best on non-flat objects due to their rigidity and thus their tendency to crack [71].  
Still, attention should be drawn to the recent shortage of agar supplies, because of its increasing exploitation in numerous fields and the unpredictability of its production due to the dependence on marine conditions (i.e., warmth, infesters). Although the usage of agar and agarose is moderate in art conservation compared to other sectors, it is important to be aware of these issues and try to act accordingly, by choosing other gels whenever possible [64].