What is Chinese celadon?

Celadon is a term for pottery denoting both wares glazed in the jade green celadon color, also known as greenware (the term specialists now tend to use), and a type of transparent glaze, often with small cracks, that was first used on greenware, but later used on other porcelains. Celadon originated in China, though the term is purely European, and notable kilns such as the Longquan kiln in Zhejiang province are renowned for their celadon glazes.
Celadon production later spread to other parts of East Asia, such as Japan and Korea as well as Southeast Asian countries such as Thailand. Eventually, European potteries produced some pieces, but it was never a major element there. Finer pieces are in porcelain, but both the color and the glaze can be produced in stoneware and earthenware. Most of the earlier Longquan celadon is on the border of stoneware and porcelain, meeting the Chinese but not the European definitions of porcelain.
For many centuries, celadon wares were highly regarded by the Chinese Imperial court, before being replaced in fashion by painted wares, especially the new blue and white porcelain under the Yuan dynasty. The similarity of the color to jade, traditionally the most highly valued material in China, was a large part of its attraction. Celadon continued to be produced in China at a lower level, often with a conscious sense of reviving older styles. In Korea the celadons produced under the Goryeo Dynasty (918–1392) are regarded as the classic wares of Korean porcelain.
The celadon colour is classically produced by firing a glaze containing a little iron oxide at a high temperature in a reducing kiln. The materials must be refined, as other chemicals can alter the color completely. Too little iron oxide causes a blue colour (sometimes a desired effect), and too much gives olive and finally black; the right amount is between 0.75% and 2.5%. The presence of other chemicals may have effects; titanium dioxide gives a yellowish tinge. Pieces made with a celadon glaze are themselves often referred to as "celadons".

Chinese celadons
Greenwares are found in earthenware from the Shang dynasty onwards. Archaeologist Wang Zhongshu states that shards with a celadon ceramic glaze have been recovered from Eastern Han Dynasty (25–220 AD) tomb excavations in Zhejiang, and that this type of ceramic became well known during the Three Kingdoms (220–265). These are now often called proto-celadons, and tend to browns and yellows, without much green.
The earliest major type of celadon was Yue ware which was succeeded by a number of kilns in north China producing wares known as Northern Celadons, sometimes used by the imperial court. The best known of these is Yaozhou ware. All these types were already widely exported to the rest of East Asia and the Islamic world.
Longquan celadon wares were first made during the Northern Song, but flourished under the Southern Song, as the capital moved to the south and the northern kilns declined. This had bluish, blue-green, and olive green glazes and the bodies increasingly had high silica and alkali contents which resembled later porcelain wares made at Jingdezhen and Dehua rather than stonewares.
All the wares mentioned above were mostly in, or aiming to be in, some shade of green. Other wares which can be classified as celadons, were more often in shades of pale blue, very highly valued by the Chinese, or various browns and off-whites. These were often the most highly regarded at the time and by later Chinese connoisseurs, and sometimes made more or less exclusively for the court. These include Ru ware, Guan ware and Ge ware, as well as earlier types such as the "secret colour" (mi se) wares, finally identified when the crypt at the Famen Temple was opened.
Large quantities of Longquan celadon were exported throughout East Asia, Southeast Asia and the Middle East in the 13th–15th century. Large celadon dishes were especially welcomed in Islamic nations. Since about 1420 the Counts of Katzenelnbogen have owned the oldest European import of celadon, reaching Europe indirectly via the Islamic world. This is a cup mounted in metal in Europe, and exhibited in Kassel in the Landesmuseum. After the development of blue and white porcelain in Jingdezhen ware in the early 14th century, celadon gradually went out of fashion in both Chinese and export markets, and after about 1500 both the quality and quantity of production was much reduced, though there were some antiquarian revivals of celadon glazes on Jingdezhen porcelain in later centuries.
Decoration in Chinese celadons is normally only by shaping the body or creating shallow designs on the flat surface which allow the glaze to pool in depressions, giving a much deeper colour to accentuate the design. In both methods carving, moulding and a range of other techniques may be used. There is very rarely any contrast with a completely different colour, except where parts of a piece are sometimes left as unglazed biscuit in Longquan celadon.