Medieval Korean potters achieved jade-green celadon glazes by controlling iron oxide reduction in wood-fired kilns. The exact technique was lost and took modern chemists decades to reverse-engineer.
Iwanafish at English Wikipedia, Public domain, via Wikimedia Commons
The chemistry of a king's color
The coveted blue-green color of Goryeo Dynasty (918–1392) celadon is not the result of a pigment; it is a chemical transformation in a low-oxygen fire. The secret lies in the glaze, a mixture of refined clay and wood ash containing a small amount of iron oxide (Fe2O3), typically between 1% and 3%. Potters in Gangjin, the center of royal celadon production, used local iron-rich clays for the vessels and a glaze composed of iron oxide, manganese oxide, and quartz particles.
When fired in a traditional mud kiln at temperatures reaching 1,150 to 1,250°C, the atmosphere is deliberately starved of oxygen. This is called a reducing atmosphere. The flame, seeking oxygen to burn, chemically strips oxygen atoms from the ferric oxide (Fe2O3) in the glaze, converting it to ferrous oxide (FeO). This change in the iron's oxidation state alters its color. Ferric oxide, if fired in an oxygen-rich environment, produces a simple yellow or brown. Ferrous oxide, however, produces the prized translucent blue-green hue known as bisaek, or "kingfisher color." This chemical process, perfected over centuries, distinguished Goryeo celadons from the warmer olive-green wares produced in Chinese kilns.
Reverse-engineering a lost art
After the Goryeo Dynasty collapsed in 1392, followed by invasions, the precise techniques for creating perfect bisaek celadon were lost. For centuries, Korean potters could not replicate the color and translucency of these earlier masterpieces. The quest to rediscover the method in the 20th century was a project of intense historical and scientific inquiry. Modern scientists armed with advanced analytical tools were finally able to decode the chemistry.
Using techniques like scanning electron microscopy (SEM) and X-ray fluorescence (XRF), researchers analyzed shards excavated from the hundreds of kiln sites in Gangjin. This analysis revealed the exact elemental composition of the clay and glaze. Scientists discovered that the glaze layer and the clay body fused in a deep interaction zone, a microstructure that created the glaze's optical depth. They also found that tiny, well-distributed bubbles within the glaze scattered light, creating a soft, jade-like luster. The control over kiln temperature, the duration of the firing (often lasting three to four days), and the precise moment of oxygen reduction were all variables that Goryeo potters mastered empirically. It took modern science to translate their art back into chemistry.
💡Fun Facts
Goryeo potters used a two-step firing process: a first firing at around 800°C to harden the vessel, followed by a second, hotter glaze firing.
Of the roughly 400 Goryeo-era kiln sites found in Korea, about half (188) are located in the Gangjin area.
The fine, web-like cracking visible on many celadons, known as crazing, was a controlled effect, achieved by manipulating the cooling rate of the finished piece.
The preferred fuel for the kilns was local pine wood, as its resin content and burning properties were essential for achieving the correct reducing atmosphere.
