Analytical Investigation of Iron-Based Stains on Carbonate Stones: Rust Formation, Diffusion Mechanisms, and Speciation

Abstract

In cultural heritage, unaesthetic stains on carbonate stones due to their close contacts with metals are of concern for the preservation of sculptures, monumental facades and archeological finds of various origin and antiquities. Rust stains made up of various oxidized iron compounds are the most frequent forms of alteration. The presence of ferric iron on rust-stained marble surfaces was confirmed in previous studies and oriented the choice of the best cleaning method (based on complexing agents specific for ferric ions). However, the composition of rust stains may vary along their extension. As the corrosion of the metallic iron proceeds, if the oxygen levels in the surroundings are low and there are no conditions to favor the oxidation, ferrous ions can also diffuse within the carbonate structure and form a variety of intermediate compounds. In this study, the iron stains on archeological marbles were compared with those artificially produced on Carrara marbles and Travertine samples. The use of integrated techniques (optical and scanning electron microscopy as well as Mössbauer and XPS spectroscopy) with complementary analytical depths, has provided the overall information. Rust formation and diffusion mechanisms in carbonates were revealed together with the evolution of iron speciation and identification of phases such as ferrihydrite, goethite, maghemite, nanomagnetite, and hematite.

Efficacy of Chitosan-Carboxylic Acid Hydrogels in Reducing and Chelating Iron for the Removal of Rust from Stone Surface

“…Thus, the rust dispersion used for the first experiments should consist of large aggregates that can effectively adhere to the sample surface but are unable to penetrate into the material. Differently, the rusted iron grid could transfer nanometer-sized particles, which spread more easily and deeper into the stone [ 9 ].…”

“…Water plays an important role for iron oxyhydroxide nanoparticles, both as a dispersing medium and as a carrier through the stone. Then, the nanoparticles can interact with the substrate and undergo a series of transformations before being converted into more stable iron compounds [ 8 , 9 ]. There is a plethora of iron-based compounds that form rust, which can interconvert each other according to the environmental conditions, i.e., pH, relative humidity (RH), and the counterions involved in the oxidation processes.…”

Advances in Analytical Strategies to Study Cultural Heritage Samples

“…Among the various forms of degradation affecting stone artifacts, colored stains that form on surfaces in contact with metals or alloys are of particular concern. In the work of Reale et al [5], the mechanisms of rust formation and diffusion in carbonates are defined. The different oxidation states of Fe are studied through the combined use of various analytical techniques, such as optical microscopy (O.M.…”