Introduction:
Yixing clay (simplified Chinese: 宜兴; traditional Chinese: 宜興; pinyin: Yíxīng; Wade–Giles: I-Hsing) is a type of clay from the region near the city of Yixing in Jiangsu province, China. Its use dates back to the Song Dynasty (960 - 1279) when purple clay was first mined around Lake Taihu in China[1]. 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 known as Purple Sand ware, and are 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; yíxīng zǐshā hú - Yixing Purple Sand Pot).
Chemically, Zisha is a mixture of kaolin, quartz and mica, with a high content of iron oxide. It is mined principally at Huanglongshan and Zhaozhuangshan and has a somewhat sandy texture. The process of preparing the clay is lengthy and was traditionally regarded as a trade secret. Typical firing temperature is between 1100C - 1200C, higher than regular clay, in an oxidizing atmosphere.
Manufacturing:
The raw materials for yixing clay are buried deep underground, 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 consist 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 underlying 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.
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.
