Yixing clay
Yixing clay is a type of stoneware produced in the region near the city of Yixing in Jiangsu province of China. Its use dates back to the Song Dynasty. From the 17th century on, the ware was commonly exported to Europe. The finished stoneware, teaware, and other small items are usually red or brown in colour. They are also typically unglazed. The clays used for the yixing-wares are very cohesive and can be formed by slip molding, coil forming, or most commonly, slab forming. The clays can also be formed by throwing. The most famous wares made for yixing clay are Yixing clay teapots ( yixing hú).
Types
The term “yixing clay”, and also “zisha clay”, is often used as a umbrella term to describe three distinct types of stoneware:
* Zisha or zini ( literally, “purple clay sand”): dark brownish stoneware that gives it’s name to the type of stoneware usually related to yixing.
* Zhuni ( literally, “cinnabar clay”): reddish brown stoneware that are made of clay very high in iron content. The name only refers to the sometimes bright red hue of cinnabar ( pinyin: zhusha) and does not contain the mineral itself. Also called hongni ( literally, “red clay”).
* Duanni ( literally, “fortifed clay”): stonewares that contains pulverized minerals and stone as well as zini or zhuni clay thus resulting in various textures and colours; from beige, blue, and green, to black.
Manufacturing
The raw materials for yixing clay are buried deep undergrounds, sometimes under heavy sedimentary rock formations. When excavated, it is usually located within stratified layers of other clays. The seam of yixing zisha can be as thick as a several decimeters, up to a meter. Yixing clays consistes of fine iron containing silt, with mica, kaolinite and varying quantities of quartz and iron ores as its main mineral constituents.
Processing of raw zisha yixing clay involves removing the clay from the underlaying strata, drying it under the sun in open stalls, and then pulverizing the dried clay pieces into fine particles. The clay powder then undergoes air screening to isolate clay particles of the finest grit size. The screened clay is then mixed with water in a cement mixer to a thick paste, piled into heaps, and vacuum processed to remove air bubbles, in addition to some moisture from the clay mixture. The quality and quantity of water in yixing clay is critical in that it determines the quality of the stoneware products produced. After this processing, the resulting clay is then ready to be used.
Yixing clay experiences little shrinkage in the firing process, and produces a fired product that exhibits good plasticity and crush strength. The appearance of yixing products, such as it’s colour or texture, can be enriched and altered by the artisans through the addition of various metal oxides into the yixing clay, through the manipulation of firing temperatures, and also from regulating the kiln atmosphere (oxidative versus reductive).
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