The Secret Color Porcelain Lotus Bowl in the Eyes of a Ceramic Glaze Specialist: Glaze Secrets Locked in Millennium-old Celadon
As a ceramic glaze practitioner, I stood for a long time in front of the Five Dynasties Secret Color Porcelain Lotus Bowl during my visit to Suzhou Museum in China.
It is not merely a top treasure of the museum and a national cultural relic, but also a living textbook recording ancient craftsmanship wisdom. Today, I will talk about what makes this artefact extraordinary from a glaze technology perspective.
Secret color porcelain was once the top classified craft of the ancient Yue Kiln. The so-called "secret color" does not refer to a single fixed hue. Instead, it represents the supreme mastery of glaze formulation, firing atmosphere and temperature control. This lotus bowl serves as the standard specimen of secret color porcelain, achieved not by chance, but by a complete and mature craft system.
Let us start with the ceramic body. It adopts high-purity greyish-white porcelain body with extremely low impurities and fine, uniform texture. For glaze technicians, this is an ideal "clean canvas". The body will not interfere with the glaze tone, allowing the celadon glaze to display its purest texture.
Next comes the core
the glaze surface. Secret color porcelain adopts high-calcium lime glaze. It features good fluidity at high temperature, soft and mild luster, tight bonding with the body and uniform, stable glaze layer.
Its lake-green colour comes from precise control of iron oxide, whose content is kept within an extremely narrow range. Slightly higher iron content leads to dark brown tone, while less iron results in pale whitish glaze. More critically, it is fired under reducing flame. Strict oxygen-deficient conditions inside the kiln reduce ferric iron to ferrous iron, yielding this translucent, restrained, water-like visual effect. Even minor air leakage or parameter deviation will distort the final colour.
Then we look at firing and sagger technology. Craftsmen of the Five Dynasties already used individual firing inside ceramic saggers, which acted like an independent sealed "constant-temperature chamber". It prevents contamination and minimises temperature difference. The firing temperature was stably maintained at 1250–1280℃ with a slow cooling schedule for full glaze crystallisation. Without pyrometers or chemical analysers, ancient masters perfected the matching of "body – glaze – flame" purely through accumulated experience.
Most people admire secret colour porcelain for its beauty, history and legendary stories. For us glaze makers, what we see is minimalist formulation, precise process control and systematic thinking. Using simple raw materials, ancient craftsmen created premium texture that remains difficult to replicate even today.
After over a thousand years, this lotus bowl still shines with jade-like mildness. It tells us: truly top-tier craftsmanship never relies on stacking expensive materials, but on achieving perfection in every single process step.
This is the inspiration that Secret Color Porcelain passes down to modern ceramic professionals.
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