Celadon: Reduction, Iron Structure and Colour Formation

Celadon glaze is admired for its subtle green and blue shades and their deep historical and cultural associations. What makes these colours so distinctive? The answer involves the chemistry of iron in the glaze and its relationship with composition and firing conditions.

Colour formation in iron-containing glazes involves the oxidation state of iron and its structural environment. Hidaka and colleagues’ study, published in Cerâmica in 2012, examines structural and electronic properties of specific celadon samples. Its results should not be generalised to all celadon glazes.

Reduction and Iron in Celadon Glaze

The firing atmosphere influences the chemical conditions within a glaze. Fe³⁺ and Fe²⁺ represent oxidation states of iron; Fe₂O₃ and FeO are oxide formulae. This distinction matters: describing the local environment of iron by comparison with an oxide structure does not, by itself, demonstrate that the oxide forms a separate crystalline phase in the glaze.

In the cited study, the authors investigate the local arrangement around iron, including FeO₆ coordination units, and electronic interactions between iron and oxygen. This approach allows discussion of short-range order in a glassy material without describing the entire glaze as a crystalline structure.

Why Does This Matter for Colour?

Light absorption depends on the electronic properties of iron-containing centres. The paper discusses how local structure and interactions between iron and oxygen contribute to the optical properties of the samples examined. Explaining the colour requires considering these aspects alongside composition and firing conditions.

These results broaden the discussion of celadon colour, but do not demonstrate that reduction never changes the oxidation state of iron. Nor do they justify replacing explanations involving Fe³⁺ and Fe²⁺ with a single structural explanation for all celadons.

Conclusion

The beauty of celadon is related to a complex chemistry in which composition, firing atmosphere, oxidation state and the local environment of iron must be considered together. The study contributes to understanding specific samples rather than establishing a universal rule for every celadon glaze.

Source

Hidaka, M., et al. Structural and Electronic Properties of Iron Oxides in the Celadon Glazes (II), Cerâmica 58 (2012), 534–541.