Celadon Clay Collection — Gangjin Iron-Rich Clay Harvesting and Aging

The jade-green color (翡色) of Goryeo celadon originates from the iron oxide content in the clay body. Celadon clay is iron-bearing clay containing 1~3% iron oxide (Fe2O3). During reduction firing, iron oxide transforms into ferrous oxide (FeO) and creates a distinctive jade-green color. If there is too much iron, it becomes brown; if too little, it becomes white.

Gangjin-gun, Jeollanam-do and Buan-gun, Jeollabuk-do are representative clay sources for Goryeo celadon. In the Daegu-myeon area of Gangjin, clay layers formed by the weathering of granite are widely distributed. Grayish-blue clay layers are found on hillsides or valley slopes. The clay layers are usually located 50cm to 2m below the ground surface.

Prepare the tools necessary for clay mining. Use a shovel and a pickaxe to remove the topsoil and access the clay layer. A bucket and a tarpaulin are used to transport the excavated clay. A magnifying glass and a magnet help roughly confirm the iron content of the clay at the site.

At the selected mining point, remove the topsoil (organic layer) with a shovel to a depth of 30~50cm. Soil mixed with leaves, humus, and sand is unsuitable for celadon. Begin mining when a uniform clay layer of gray-blue or gray-brown color appears. The more uniform the color of the clay layer, the better the quality.

Remove the clay layer with a pickaxe and place it in a bucket with a shovel. Be careful not to mix in stones, tree roots, and sand. Mine approximately 20~30kg from one location. If the clay is too dry, it breaks easily, so it is best to mine the day after it rains.

Mined clay is spread on a waterproof sheet and naturally weathered outdoors for 2-4 weeks. Exposure to rain, wind, and sunlight loosens the bonds between clay particles and decomposes impurities. This process is called air drying (風乾), which increases the plasticity of the clay. The clay is occasionally turned over to ensure even weathering.

Place weathered clay lumps in a large container and pour water about 3 times the weight of the clay. After completely soaking for 24~48 hours, stir with a wooden rod to create a suspension. This process is called sobi (水飛) and evenly disperses clay particles in water. Large particles and sand settle to the bottom.

Pour the clay suspension into a 60~80 mesh sieve and filter. Coarse particles such as stones, sand, and organic matter are filtered out. The filtered suspension is then passed through a 120 mesh fine sieve for secondary filtration. After this double filtration process, a fine clay suspension with uniform particle size is obtained.

Place the filtered clay liquid in a wide container and let it sit undisturbed for 3-5 days. Once the clay particles settle to the bottom and the water above becomes clear, carefully pour off the water. Place the remaining clay slurry on a plaster board or cloth bag to remove additional moisture. Dehydrate until it reaches a dough-like consistency.

Wrap dehydrated clay with vinyl and mature it in a cool place for a minimum of 3-6 months. Traditionally, it was aged for more than 1 year. During aging, microorganisms decompose organic matter and the charge on the clay particle surface changes, greatly improving plasticity. The longer it ages, the easier it becomes to shape.

Remove the matured clay and repeatedly knead it on the work table. This process is called wedging (練土) or rope-making. Repeat pushing, folding, and pushing the clay 100-200 times. The purpose is to completely remove air bubbles and make the moisture and particles uniform.

Cut the finished clay thinly and check the cross-section. It passes if there are no air bubbles and the color is uniform. Take a small amount to make a test piece and perform reduction firing at 1250~1300 degrees to check if the color develops. Store the qualified clay sealed in vinyl in a cool place.