An in-depth study on the agar gel effectiveness for built heritage cleaning

•Different agar gel formulations were tested for copper stain removal from marble.•Hydrogel lyophilisation allowed to systematically compare gel formulations.•The relationship between gel morphology and effectiveness was assessed.•AgarArt 1% with TAC was the most effective gel for copper removal from marble.•EDTA in gels leads to a broader pore size distribution and to a lower gel strength.