Porcelain is the clay. Celadon is the glaze. Ju ware is a combination of the two. As someone who has worked with clay and glazes on and off for fifty years, I believe the story of Ju ware started with a river bank, that shifted perhaps as much as the rise and fall of different emperors in China, in a culture different than ours, in a time different than ours. Chinese emperors were what we might call patrons of the arts today, in addition to whatever military and social role they played. The emperors funded and sometimes supervised the whole process of ceramic production, which started with workers digging clay from the river banks, removing impurities, and preparing the clay for the potters. These potters were employed to make whatever the emperor wanted; this was considered an honor.
Some Ju ware kilns were quite small and built near the river, lost to time. Larger ones have been found, but were ransacked after a documentary disclosed the location. Since getting the desired result of celadon often took up to ten firings, think days and days: the firing of porcelain is done in gradual heating and cooling stages, each 12 hours or longer depending on the number of pieces being fired, the kiln, the weather, and other factors. The Henan Provincial Administration of Cultural Heritage has possession of temperature tester insert biscuits from the Ch' ing-liang Temple excavation site. Today these are called firing cones, which are put in the kiln as it is heating up to almost 2000 degrees Farenheit. The cones melt at various temperatures that give an indication of the heat reached inside the kiln.
Ancient kilns used wood or coal, whereas modern kilns use wood, gas, or electricity. During the stacking (placing the wares inside the kiln) and before the firing process, the air-hardened, raw clay objects were protected inside saggers (large fired clay plates placed under the objects) from touching each other and/or from being scorched by kiln flames. During the second firing, after the celadon glaze was applied, other means ranging from nails imbedded in clay to piles of sand were used, again for separation and support. The beginning student and many longtime ceramic artists do not glaze the bottom of vessels, as it will weld to the kiln shelf during the secong firing. Many Ju wares were covered completely with glaze, even on the bottom, with merely 3-6 rice-sized spurs to lift them up. That seemingly simple act required knowledge and devotion to correct placement, not only of the support, but of the placement in the kiln.
Sung dynasty texts record the glaze as having powdered agate, however that is not what caused the mysterious result entirely, as slight variations of silica in the clay body itself can play a factor as well. Ju porcelains are graded on surface luster and appearance (presence or absence of crackle/crazing), as well as form, edges, and the underside of the piece are all important. It was considered sacrilege to mark the emperor's name in something which could easily break, such as porcelain (as opposed to bronze); this makes it more difficult to determine the origin of some pieces. Another fact which helped their exporting trade at the time was the rumor that a celadon dish would change color if poisoned food was placed on it.
Celadon glaze was not exclusive to that 20 year period nor that particular geographical place. Due to the proximity of ceramics in neighboring countries, pottery techniques overlapped. Koryo celadons from Korea are just one example of many. Trends in color, pattern, availability of raw materials, the shifting of the river beds where the clay was collected, as well as the rise and fall of emperors all factored into Chinese ceramics. Since there are still celadon ceramic pieces being made today, it is highly likely that generations of long-dead artisans passed along their knowledge. In the current ceramic community, clay and glaze recipes are not kept secret. For certain people there is always a quest for shared experience, experiments, and striving for perfection.
Special thanks to mauler, who reminded me how much I love clay, and how hard it is to write about something which is essentially a wordless process.