A multi-analytical approach for the study of copper stain removal by agar gels

• a systematic study was performed on the copper stain removal from stone materials by agar gels.
• several agar gel formulations were tested, both pure and with additives.
• pure agar gel almost behaves as a “free water” reservoir.
• the coordination of copper centers by additives and by agar gels, the latter never previously observed, has been highlighted.
• the effectiveness in stain removal for model samples suggests the applicability of the protocol in heritage cleaning.

The effectiveness and the mechanism of copper stain removal from stones by agar gels was systematically studied using marble laboratory specimens, stained and cleaned in well-controlled and reproducible conditions. The same cleaning procedure was also applied on the marble base of Napoleon's statue by A. Canova. The water release from agar gels to stones was investigated by capillarity absorption and unilateral Nuclear Magnetic Resonance. The cleaning by different agar gel formulations (pure and added with chemicals) was studied both on the stone substrate (optical microscopy and colour measurements) and in the gels (inductively coupled plasma-atomic emission spectrometry, electron paramagnetic resonance spectroscopy). Among the considered cleaning systems, the most effective ones for copper removal were agar gels 3% containing additives, with no significant difference among the used additives. However, agar gels with additives host copper in different ways: in gels added with ethylenediamine tetraacetic acid (EDTA), all copper centers are coordinated, while copper centers are also dispersed in water within gels added with ammonium citrate tribasic (TAC). The stain cleaning process of stones probably starts with the diffusion of water at the gel-stone interface, but it finds the driving force in the copper coordination.