Yixing clay
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Yixing clay (宜興) is a type of stoneware clay 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, which is used for 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 (宜興紫砂壺; yixing hú).
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[edit] 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 (紫砂 or 紫泥 ; literally, "purple clay sand"): dark brownish stoneware that gives its name to the type of stoneware usually related to yixing.
- Zhuni (朱泥; literally, "cinnabar clay"): reddish brown stoneware that is made from clay with a very high iron content. The name only refers to the sometimes bright red hue of cinnabar (朱砂; pinyin: zhūshā). There are currently 10 mines still producing zhuni[citation needed]. However, due to the increasing demand for Yixing stoneware, zhuni is now in very limited quantities. Zhuni clay is not to be confused with hongni (红泥, literally, "red clay", another red clay.
- Duanni (鍛泥; literally, "fortifed clay"): stoneware that was formulated using various stones and minerals in addition to zini or zhuni clay. This results in various textures and colours, ranging from beige, blue, and green (绿泥), to black.
[edit] Minerals Compositions
See Clay minerals
[edit] 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[citation needed], and produces a fired product that exhibits good strength[citation needed]. The appearance of yixing products, such as its colour or texture, can be enriched and altered 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).
