• by vinux

There are many kinds of Yixing pottery, which are widely distributed in the hilly area of southern Yixing. Dingshan, Zhangzhu and Zhudong are the main production areas. Generally, the local clay is divided into three categories: white clay, jia ni and soft mud. White clay is a grey-white sand-like aluminum-based clay; jia ni is a multicolored sandy clay (commonly known as shale), which is called stone bone before weathering, with hard, crisp and fine material; Soft mud is a multi-colored clay with yellowish brown or grayish white as its main color and soft, tender and fine material. All kinds of natural clays produced by Yixing Pottery Town, whether Jiani or Nen Mud, contain a large amount of iron oxide. The content varies from more than 8% to less than 2%. Because the iron content of various Jiani and Nen Mud differs, the slurry can be blended in an appropriate ratio and fired with different types of flame to present various colors such as black, brown, red, purple, yellow and green. This is why purple sand kettle presents various gorgeous colors.

Yixing clay is the main raw material of Yixing Ceramics, with a wide variety of types, but never classified clearly. Some are named after the place of origin, such as Benshan Jia Ni (the clay mined in Huangling Mountain area is commonly known as Benshan Ni), Dongshen Jia Ni, Xiangshan Nen Ni and Nanshan Bai Ni; some are distinguished by the combination of place of origin and appearance and color, such as Benshan Zi Ni, Benshan Lu Ni, Benshan Hong Zong Ni and so on. The most common way is to distinguish by the hardness and plasticity of the clay. It is usually divided into three categories: Jia Ni, Bai Ni, Nen Ni. Among these three categories, there are fine clay materials for making purple sand teapots.

Jia Ni is a type of clay used for construction and decoration in Chinese architecture. It is made from a mixture of different minerals, such as kaolin and quartz. The texture of Jia Ni can vary from very fine to coarse, depending on the region it comes from. Its color ranges from white to yellowish grey. Jia Ni is valued for its high strength, durability, and ability to withstand extreme temperatures. It is often used to make roofs, walls, floors, sculptures, and other decorative elements in traditional Chinese architecture.

Jia Ni, formerly written as “Jia Ni”, was named because of its deep embedment in the interlayer of quartz sandstone in the mountain belly; it was also said that Jia Ni was the hardest in Yixing pottery, similar to the ancient “iron armor”, hence its name. The former explanation is more reasonable, as Jia Ni is not all hard like iron, and it does indeed exist deeply within the interlayer of quartz sandstone. From a geological perspective, Jia Ni ore bodies are found in the upper part of Wu Tong Group in the Devonian System and in the middle of Gao Li Mountain Group in Carboniferous System; from regional distribution, Jia Ni is mainly found in Huang Long Mountain, Qing Long Mountain and Nan Shan of Ding Shu Town, Yixing City; Bai Ni Mountain, Da Chao Mountain and Lan Shan of Yuan Dong Town; Yang Dian of Tai Hua Town and Ping Yuan, Xiang Shan of Ming Ling Town; Jun Shan Wu , Jian Wu, Cang Wu Border Paper Workshop and Hong Ling of Hu Mo Town as well as Yixing Forest Farm etc. According to the origin place, they are named respectively as Ben Shan Jia Ni, Ping Yuan Jia Ni、Yang Dian Jia Ni、Lin Chang Jia Ni、Jian Wu Jia Ni etc.

Jia Ni is the most abundant pottery with 83% total amount amongst Yixing pottery. Before 1990s, 6738.61 million tons have been preliminarily surveyed after 471.21 million tons have been mined so that 6267.40 million tons were reserved. There are over 30 mining areas or points which are mostly now closed for restoration purposes. Huang Long Mountain (Ben Shan) of Ding Shu Town is main production area for Yixing JiaNi while Jun Shan Wu at Hu Mo Town follows suit.

Looking at the distribution of the ore deposits, kaolin is deeply embedded in the interlayer of quartz sandstone in mountain waist. The ore body is abundant and the thickness of the main ore body reaches 30 meters. The thickness is relatively stable and mostly layers are produced. There are four ore layers in Huanglongshan Kaolin Mine from bottom to top: I, II1, III and IV. Kaolin is a kind of purplish sandy clayey (commonly known as shale), mainly purple in appearance, with light purple, dark purple, grayish purple, purple red, blue purple, green, gray green, bluish white and light gray colors. It has a spotty structure of sandy clay shale and clayey shale structure. Kaolin ore bodies are multi-layered and multi-colored distributed. The ore quality is not pure and there are gravels and pebbles mixed in it which can be usually referred to as “mixing three with four” characteristics. Pure kaolin can be used to make Zisha pottery teapots while most Zisha pottery teapots produced in Yixing use kaolinite ore materials from kaolin mines.

The characteristics of kaolin are as follows: first, it has good plasticity and bonding strength, which can be used to make batches of any size. Some have high plasticity and most of them are medium plasticity. For example, the liquid limit of Ben Shan kaolin is 22.88%, the plastic limit is 13.68%, and the plastic index is 9.20; second, its mud performance is good, the dry shrinkage rate of blank pieces and burning rate is relatively small, about 10%; third, it has good stability and wide firing temperature range, usually 1150℃~1250℃. Kaolin is the raw material for producing large ceramics products and grouting products.

White mud

Kaolin, in geology, is found in the Lower Member of the Maoshan Group and Wutong Formation of the Silurian System, and in quartz sandstone of the Lower Member of the Gaoli Mountain Formation of the Carboniferous System (Huangshi interlayer). It is mostly a primary sedimentary deposit. The ore body is distributed on the wing part (waist part) of the mountain, showing layered, thin layered or lens-like shape, and there are often multiple ore layers in general ore bodies. The thickness of ore layer varies from 10 meters at its thickest to only 0.14 meter at its thinnest, usually with a sharp termination along strike or dip direction. Experienced miners with long-term mining experience metaphorically describe the shape of kaolin distribution in mountains as “basket shape”, “mung bean shape” and “chicken nest shape”.

In the region, kaolin deposits are mainly distributed around Nanshan, Baidi Mountain, Daichao Mountain, Dongshanzui and Lanhou in Dingshu Town; Cangfengjie in Hulou Town; Yangdian in Zhangzhu Town; Shendong, Mingling Plain and Xiangshan. Since ancient times, Daichao Mountain has been an excellent production area for kaolin. Zhou Gaoqi of the Ming Dynasty wrote in Yangxian Meihong Series: “Kaolin is produced from Daichao Mountain. It is used for making pottery and amphorae jars. This mountain has not yet been developed.” After surveying, the proven reserves of kaolin are 5.3874 million tons, the mined reserves are 1.6279 million tons and the remaining reserves are 3.7495 million tons, which should be richer than the proven reserves. At present, there are nearly 40 mining areas or points that have been mined for kaolin, most of which have also been sealed off and restored to greenness.

White clay is a grey-white colored, single-component, sandy clay. People may generally assume that white clay ore is white in color, but this is not the case; white clay ore usually has a mainly grey-white color with blue-white, carbon black, ash yellow and light red when fired into pottery. This is collectively referred to as white clay, mainly because all of these have similar properties. White clay ore has a platy or blocky structure with a sandy clayey texture and is dense but not hard and has a slippery feel. Its main mineral components are muscovite, sericite, kaolinite, quartz and muscovite fragments, free iron and so on. Compared to ordinary pottery clay, white clay has finer soil structure, finer particles with better plasticity and adhesion performance but heavier sandiness which belongs to medium-high plastic clays. For example the mountain white clay has a liquid limit of 28.81%, plastic limit of 14.20% and plastic index of 14.79; secondly the drying shrinkage rate of white clay pieces between 16~21% when dry or fired is higher than ordinary pottery; thirdly its firing range is slightly narrower than ordinary pottery with firing temperature range from 1150℃~1230℃C (50℃C~80℃). White Clay is widely used for making sand pots, daily wares and art ceramics etc., while it was rarely used to make pots directly in ancient times but now it is being used in small amounts.

Mud that is soft and tender.

Nen Mud, located in the upper section of the Longtan Formation in the Permian system, is overlain by the Quaternary aeolian deposits. Nen Mud is a sedimentary weathering-induced deposit with ore bodies close to the surface, usually below the red clay layer on hillsides suitable for open-pit mining. The ore body is lens-shaped and 3-15 meters thick. The main distribution areas or points of Nen Mud are Xianshan, Hongmiao, Baishan, Shashan, Zhao Zhuang and Shizhutou around Yingxin Town; Guanzhaung and Changgangling at Taihua Yangdian in Zhangzhu Town; Bridge North, Liulipeng and Jiulishan in Hufu. In the initial survey, the proven reserves were 4.422 million tons and 1.16 million tons have been mined, leaving 3.282 million tons available. Areas or points that have been mined include Xianshan Mining Area, Chuanbu Shashan Point, Hongmiao Point, Jiulishan Point, etc. Nen Mud is a yellowish-greenish sticky clay mainly composed of muscovite, kaolin claystone, hydrated kaolinite and quartz with iron components and mica debris as impurities.

The main characteristics of the lean mud are: firstly, it has a fine particle size and good viscosity, and has good plasticity and bonding capacity, which belongs to a high plasticity clay. For example, Xianshan lean clay has a liquid limit of 43.28%, plastic limit of 21.50%, and plasticity index of 21.70. Secondly, it has good forming performance and dry strength. Thirdly, the shrinkage is large, the firing temperature is relatively low, and the firing range is narrow. The linear shrinkage rate of lean mud blanks during drying and firing sometimes reaches as high as 25%, the general firing temperature is around 1150℃C, close to 1200℃ when sintered, and the sintering temperature range is less than 50℃C. Lean mud is often used in Yixing ceramic industry as an adhesive clay. In Ming Dynasty’s Zhou Gaoqi’s “Yangxianming Pot System”, it said: “Lean mud comes from Zhaozhuang Mountain, which can be stuck with all colors of soil to build pots.” Zhu Ni type clays mixed in purple sand clay mostly contained among lean mud layers.

Pearls of the human world are enough, how can it compare to a lump of earth from Yangxi’s stream? This poem illustrates the preciousness of purple-clay mud. In the broad and profound tea ceremony, purple-clay teapot has become an essential complement for tea. For thousands of years, purple clay has been highly acclaimed and praised by people. Nowadays, purple clay has become the name card of Yixing city and indeed China.