Iron rich concretions accelerate localized degradation of marine recovered bluish white glaze

Iron-rich concretions are common on marine-recovered ceramics, but their role in glaze degradation remains poorly constrained. We combine archaeological characterization of bluish-white porcelain from the Nanhai I shipwreck with controlled replica exposures to evaluate how iron-rich microenvironments affect glaze alteration. Archaeological samples contain heterogeneous Fe(III)-bearing deposits and reduced and oxidized sulfur species along glaze cracks. Fe-exposed replicas develop comparable deposits, localized pitting, and pit-associated cracking, whereas Fe-free controls show only mild alteration. Raman spectra indicate stronger modification of the surface silicate network under Fe exposure. Thermodynamic modelling suggests plausible post-recovery risk pathways involving Fe speciation, secondary precipitation of Fe(III) oxides/oxyhydroxides, and destabilization of sulfur-associated crack fills under oxygenated and humid conditions. Together, these findings support interpreting iron-rich concretions as reactive factors in localized glaze deterioration rather than merely passive surface deposits and highlight the need to assess them before prolonged aqueous treatment.