When a ceramic vessel sinks and settles into the seafloor, it becomes part of a complex and dynamic chemical system. Seawater, rich in dissolved salts, percolates through sediments that are typically depleted in oxygen and populated by communities of sulfate-reducing and other anaerobic bacteria. These microorganisms metabolize sulfate ions and generate hydrogen sulfide, which reacts with dissolved iron released from corroding iron fastenings, cannonballs, ballast and hull fittings scattered across the wreck site. The result is the precipitation of iron sulfides and, after subsequent oxidation when artifacts are disturbed or exposed to oxygenated water, a spectrum of iron corrosion products including oxyhydroxides such as goethite and lepidocrocite. Over decades and centuries, these products accrete into dense, cemented masses known as concretions, which frequently fuse ceramics, metals and sediment into a single hard conglomerate that must be mechanically separated during excavation or conservation.

The research team examined fragments of bluish white glaze ceramics recovered from marine contexts, characterizing both the glaze itself and the iron-rich concretion material in intimate contact with it. Using a combination of analytical techniques, including optical microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and micro-Raman spectroscopy, the investigators mapped the chemistry and microstructure of the glaze-concretion interface at high spatial resolution. Their goal was to determine whether the concretions were simply juxtaposed against the ceramic surface or whether a genuine chemical interaction had occurred between the two materials during burial, and if so, what consequences that interaction held for the preservation of the glaze.

The answer was unambiguous: the contact zones between iron-rich concretions and bluish white glaze displayed distinctive localized damage that was absent, or far less developed, on adjacent glaze surfaces that had not been in contact with iron-bearing material. The degradation took several characteristic forms. The researchers documented pitting and etching of the glaze surface, accompanied by enrichment of iron in the outermost layers of the glaze where concretion material had been in direct contact. Cracks and micro-fissures propagated from the interface into the glaze body, and in severely affected areas the glassy surface had lost its integrity entirely, exposing the underlying ceramic body to further chemical attack. The distribution of this damage, tightly correlated with the extent and thickness of the iron-rich deposits, provided compelling evidence of a causal relationship rather than a coincidental association.

The underlying chemistry, as the authors explain, is rooted in the redox-active nature of iron and the sensitivity of glaze to sustained chemical stress. Iron corrosion products, particularly under the alternating reducing and oxidizing conditions that can develop within sediments and after recovery, undergo cyclic transformations. Reduced iron species such as magnetite and iron sulfides can oxidize to form ferric oxyhydroxides, a process that generates acidity and produces expansive corrosion layers capable of exerting mechanical stress on adjacent surfaces. Glazes are essentially thin glass coatings, typically based on a silicate network modified with fluxes such as calcium, potassium and sodium, and reinforced optically by the presence of colorants and opacifiers. The bluish white appearance prized in many porcelains arises from the interaction of cobalt pigments and opacifying phases with the glaze matrix. This glassy layer, while chemically more durable than the porous ceramic body beneath it, is nonetheless vulnerable to attack by acidic solutions, by complexing agents and by the mechanical wedging action of growing corrosion products.

In the concretion environment, the researchers found evidence that soluble iron species and acidic micro-environments generated at the concretion-glaze boundary had penetrated the glaze surface through microscopic defects, producing localized dissolution of the silicate network. Iron ions migrated into the degraded glaze, where they were detected as elevated iron concentrations in the affected zones. The hydronatrous and altered glaze layers that resulted were softer, more porous and more soluble than the original glass, creating a feedback loop in which degradation facilitated further chemical ingress. In some samples, the investigators also identified iron phosphate and iron sulfide phases within the damaged areas, testifying to the complex interplay of biological sulfate reduction, iron corrosion and post-recovery oxidation that characterizes the burial and storage histories of marine artifacts.

One of the most consequential implications of the study concerns the moment of recovery. For centuries, an artifact buried in an anoxic sediment may reach a rough chemical equilibrium with its surroundings; the iron in its concretion may remain largely in reduced, relatively stable forms. When the object is raised, drained and exposed to air, that equilibrium collapses. Reduced sulfur and iron compounds oxidize rapidly, acidity is generated, and the reactions that had been slow or dormant during burial can accelerate dramatically. The study demonstrates that this post-recovery oxidation is precisely when iron-rich concretions become most dangerous to adjacent glaze surfaces. If concretion material is left in contact with ceramic during storage, even for relatively short periods, ongoing corrosion of the iron can continue to attack the glaze long after the artifact has left the sea. This reframes a familiar conservation dilemma: concretions are often left in place, at least temporarily, because their removal is labor-intensive and risks mechanical damage to the object they encase. The new findings suggest that delayed or incomplete removal carries a chemical price that may outweigh the mechanical risks of earlier intervention.

The research also carries lessons for the display and long-term curation of marine ceramics. Many museum collections include blue-and-white wares recovered from shipwrecks, some of which retain patches of concretion or visible iron staining as evidence of their underwater history. The study’s authors emphasize that such objects should be assessed for active corrosion risk, not simply for aesthetic or structural condition. Environmental control, including low relative humidity to suppress iron corrosion and appropriate desalination treatments to remove soluble salts, forms part of the standard toolkit for marine ceramic conservation. But the findings add urgency to the selective and careful removal of iron-rich deposits from glaze surfaces, and to monitoring programs that can detect early signs of localized glaze degradation before irreversible loss occurs. Analytical imaging of the kind employed in the study, capable of resolving the chemistry at the micrometer scale, offers a template for such condition assessments.

Beyond its practical consequences, the research illuminates the broader science of how materials interact within underwater archaeological sites. A shipwreck is not a static archive but a chemically coupled system in which the decay of one material accelerates the decay of another. Iron fastenings corrode and damage wood; copper alloys leach biocidal ions that alter microbial communities; and, as this study shows, iron corrosion products attack the glassy surfaces of ceramics that might otherwise have survived for millennia. Understanding these cross-material interactions is essential both for interpreting the archaeological record and for designing interventions that preserve what remains. The degradation of bluish white glaze at concretion interfaces is a vivid example of how the story of an artifact’s deterioration is written in the chemistry of its burial environment, and how attentive analysis of that chemistry can turn conservation from a reactive discipline into a predictive one. For the countless ceramic collections still held in marine sediments around the world, and for those already on museum shelves, the message is clear: the iron that clings to a sunken porcelain is not inert residue but a continuing threat, and managing it is as important to the artifact’s future as any treatment applied on the day of recovery.

Subject of Research: The role of iron-rich concretions in accelerating localized degradation of marine-recovered bluish white ceramic glaze.

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

Article References: Wu, J., Zhang, R., & Luo, W. (2026). Iron rich concretions accelerate localized degradation of marine recovered bluish white glaze. npj Heritage Science. https://doi.org/10.1038/s40494-026-02978-7

Image Credits: AI Generated

DOI: 10.1038/s40494-026-02978-7

Keywords: marine archaeology, ceramic conservation, iron concretions, glaze degradation, underwater cultural heritage, corrosion science, blue-and-white porcelain, materials characterization, desalination, heritage preservation, Iron, rich