Celadon pottery, with its characteristic clear jade color and elegant curves, originated in China as long as 2,000 years ago. The glazing process was perfected through precisely mixed clay, wood ash glaze, and double firing in a specially sealed kiln. The valuable celadon pottery of ancient Korea, China, Japan, and Thailand are preserved in many museums and cultural centers.
All earthenware begins as wet, molded clay that can be fired with or without glaze. Yet celadon pottery bakes twice in a kiln to give it that unique color and sheen. Early potters found that special river clay kept its shape and also supported glaze. First they formed the shape of the vase, bowl or box. Then the plain object was fired in a large kiln at 1400° F (750° C). After the piece hardened, it was removed and coated with a glaze rich in calcium carbonate. Returning to the kiln for a second firing at 2400° F (1300° C), the muddy glaze fired to a gorgeous sage green.
Ceramicists had to be very careful about the temperature of the kiln. No thermometers existed and still they could keep the chamber at an even temperature for hours, only using wood for fuel. Also, the kiln had to be airtight to control the amount of oxygen present. The second firing needed low amounts of oxygen and a higher concentration of carbon monoxide and carbon dioxide. Otherwise, the clear glaze would oxidize into a murky brown. Tiny crackles, called "crazing," are original to the glaze and don't devalue the finish.
Celadon from Korea represented the finest work of artisans. Under King Injong and Uijong, during the 9th and 10th century, jade and celadon symbolized an afterlife full of wealth, peace, and honor. During that Koryo Dynasty, ceramicists developed intricate methods of clay inlay called saggam, probably inspired by lacquer and metal inlay. To achieve designs of flowers, cranes, clouds, and willow branches, the outlines were carved out of the pot. Then, clay of another color was smoothed into the crevices. White clay stayed bright white during firing while red clay turned an inky black.
Modern technology has never improved upon these flawless pieces. Although rare, celadon from the Koryo Dynasty exhibits some of the most practiced and perfected techniques of any culture. The bowls, vases, boxes, and jugs that were once displayed in Korean courts and temples have been moved to museums throughout Asia and Europe.