# Biologically Derived Gels for the Cleaning of Historical and Artistic Metal Heritage
## 4 Organogels for Organic Coatings Removal
### Bio-Based Organogels for Cleaning
#### 4.3.2 Bio-Organogels in Art Conservation  
In the first case study here reported, PHB was employed together with alginic acid to create a functionalized organogel for the treatment of archaeological wooden findings [123]. The core idea was the addition of iron chelators to the gel, to examine its potentialities to develop innovative in situ applications. Regrettably, to enhance polyhydroxybutyrate solubility, chloroform was employed to obtain lower molar mass polymer chains. This intermediate step would represent a critical drawback for large-scale use in conservation, due to the severe health danger associated with chloroform [125]. Its use and the unclear outcomes highlighted the need of improving the preparation protocol and exploring different chelating agents.  
The first fully bio-based organogel in CH was made from polyhydroxybutyrate (PHB) as a thickening agent and \(\gamma\)-valerolactone (GVL) as an organic solvent, with or without the addition of triethyl citrate (TEC) as a plasticizer to test the different mechanical properties of the prepared gels [124]. The resulting PHB-GVL and PHB-GVL-TEC systems led to the removal of terpenic resin from oil paintings, leaving only little residues of GVL that did not appear concerning in terms of retention into underlying layers. Finally, no improvements were observed with the addition of the TEC plasticizer, making feasible and reliable the formulation with only PHB and GVL components [81].  
However, the unwanted deposition of solvent on the object is a weakness, especially when referring to rigid gels. It is a crucial point that attracted the attention of supplementary studies [126]. An additional implementation of the PHB-GVL formulation has recently been published and involves the use of cutting-edge electrospun material, interposed between gel and painting, able to further minimize the residues of \(\gamma\)-valerolactone (GVL) on the target [109].  
Another PHB-organogel was designed exploiting ethyl lactate (EL) as solvent. Indeed, the bio-based solvent was able to dissolve the polyhydroxybutyrate, thereby creating the so-called PHB-EL gelled system [107]. The obtained formulation performed satisfactorily to remove dammar from the surface of oil paintings [107]. Notably, its action was comparable both to traditionally applied free solvents and to the PHB-gel carrying dimethyl carbonate (DMC) as a green--but not bio-based--alternative solvent. The drop in mechanical stress is remarkable relating to ethyl lactate englobed in the gel or used neat for the same purpose of cleaning. Still impressive is the little amount of EL residues after treatment despite its inherent low vapor pressure, ascertained by solid-phase microextraction (SPME) analysis [107].  
Figure 2: Bio-organogels (i.e., bio-polymer and bio-solvents) employed in cultural heritage and the respective organic coatings targeted. The structural formula of polyhydroxybutyrate, \(\gamma\)-valerolactone, ethyl lactate, and biodiesel.  
Finally, the last fully bio-derived polyhydroxybutyrate-gel involves biodiesel (BD) as a key solvent [84]. As biodiesel itself is not able to create a gel out of PHB, the formulation included also dimethyl carbonate (DMC) to dissolve the PHB polymer. Interestingly, the PHB-DMC/BD system was the only one applied on bronze objects coated with waxes. Previous tests demonstrated that dimethyl carbonate alone does not affect non-polar materials (i.e., wax); therefore, the sole action of biodiesel for the removal of such coating was proved. However, the research points out also the well-known non-volatility of biodiesel, which appears as the main drawback of such a procedure. Indeed, even if retained in a gelled system, biodiesel tends towards leaving residues on the treated surface, implicating an additional cleaning with free DMC to remove these undesired remains.