Using novel natural gels for the cleaning of iron leachates on marble

Abstract

Marble is vulnerable to metallic leaching, resulting in structural damage and the alteration of its appearance. With an increase in the use of steel in heritage works, there is a high risk of iron mobilisation, which can result in the orange staining on nearby surfaces. This issue becomes especially relevant when dealing with marble structures of historical significance. To address this, conservators seek cleaning materials tailored to the specific structure without causing harm to the original surface. Agar, a substance that has been extensively studied, proves to be effective as a hydrogel by creating a controlled water release system, accommodating the needs of conservators for optimal cleanliness without compromising the material. In order to enhance cleaning efficiency, chelating agents like EDTA can be incorporated into the agar hydrogel composition. However, drawbacks such as microbiological growth and resultant damage on the surface of the original material must be considered. To overcome these challenges, alternative natural cleaning materials like gels made of kudzu and konjac are explored, combined with green chelating agents such as citrate, l-cysteine and oxalate. Different analytical techniques were used to characterize the mock-ups, before and after the staining, and to evaluate their cleaning potential, such us Raman spectroscopy, colorimetry, X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). Moreover, two reproducible and easily applicable methods to generate iron staining in marble mock-ups have been developed because of their absence in the literature. Raman spectroscopy confirmed the presence of goethite and lepidocrocite, the main minerals in real iron staining. Using XRF spectroscopy, it was found that the leachate method produced a staining with the highest amount of iron, while the Mohr method produced more homogeneous staining. Konjac gels were found to be the most effective for removing iron staining from marble surfaces when compared to other gels. In addition, konjac gels require less energy in their preparation because no heat is used in their preparation. Their efficiency is further enhanced by the addition of the chelators l-cysteine and citrate, being an ideal alternative to EDTA in cleaning iron staining from marble surfaces. The use of EDTA chelator is not recommended on marble surfaces due to its high affinity for calcium and the fact that, unlike the l-cysteine and citrate chelators, it is highly toxic to the environment.