Corrosion mechanisms of bluish-white porcelains of Hutian kiln of the Southern Song Dynasty excavated from the Nanhai No. 1 shipwreck

Understanding the influence of marine-organism attachment on the corrosion of ancient porcelains is crucial for the conservation of underwater cultural heritage. In this study, the mutually beneficial relationship between siliceous sponges and ancient silicate ceramics was elucidated for the first time. The complex interactions between the sponges’ secondary metabolites (nitrogen-containing alkaloids and carbon-containing organic mucus) and corrosion products were studied in detail, along with their filtration and purification effects. We comprehensively analysed the morphological structures and chemical compositions of the surface concretions and glaze surface of bluish-white porcelains excavated from the Nanhai No. 1 shipwreck. The results indicated that the concretions consisted of sponge remains, with the skeleton primarily composed of silicon carbonate. The main corrosion products were iron(III) oxide-hydroxide and iron(III) oxide. The sponge-secreted alkaloids formed secondary corrosion products which bonded with the primary corrosion products in the surface alteration layer, resulting in significant differences in corrosion morphology across the glaze surface depending on concretion detachment. This study demonstrates the presence of an inorganic/organic silicon cycle and elucidates the complex interactions therein. Our findings serve as an important reference for research on the conservation of excavated underwater ancient ceramics, marine palaeontology, and geochemistry.