Celadon is a grey-green glaze. Potters in China made it first; potters in Goryeo-dynasty Korea took it somewhere else. By the twelfth century, kilns in the south-west, around Gangjin and Buan, were firing bowls, ewers and tall maebyeong vases in a glaze that Korean writers compared to the colour of the sea after rain.
Inlay, not paint
The Korean step was sanggam, inlay. The potter carved a crane or a cloud into the leather-hard clay, filled the cut with white or black slip, scraped the surface clean, and only then glazed and fired. The pattern sits under the glaze rather than on it, like a drawing seen through water.
Cranes and clouds are the classic pair: cranes for long life, clouds for the sky they fly through. On the best pieces, cranes fly up inside circular medallions and down outside them, so the vase seems to turn even when it is still.
Where to see it
The National Museum of Korea in Seoul has the great collection; give it a morning, not an hour. The Leeum in Hannam shows celadon next to contemporary work. Then go south: Gangjin has a celadon museum and working kilns, and the drive takes you through tea country.
If you want to hold some, Icheon, an hour south of Seoul, is full of working potteries. A potter there will put you at a wheel for a morning. What you make will not be celadon, but you will understand why the old pieces cost what they do.