Within the range of Korean ceramic production, the term porcelain is applied to a broad range of ceramics. As commonly understood, porcelain is made from a clay body that becomes white and hard when fired to a sufficiently high temperature. In Korea during the Joseon period, this type of porcelain was made at the Bunwon kilns, which served the royal court.
Meanwhile, provincial kilns making more economical products for regional markets used porcelain clay of lesser quality and fired to lower temperatures. The resulting wares are not fully vitrified and appear off-white or buff. Korean archaeologists classify these provincial wares as porcelain, although they may appear to fall within the Western category of stoneware. Such vessels in the Freer collection are termed unvitrified porcelain, following Korean archaeological terminology, to distinguish them from darker brown stoneware clay typical of Buncheong ware. Many provincial unvitrified porcelain vessels were exported to Japan, where they became widely appreciated for use as tea bowls.
The white porcelain made earlier during the Goryeo period is also soft and cream-colored as the result of lower firing temperatures.