Purple sand clay

Understanding this material#

It is a family, not one mineral: sources group it into purple clay (zini), red clay (hongni, including zhuni), green clay (lüni) and blended buff clays (duanni or tuanni). Lab work on purple-clay samples finds a quartz, kaolinite and mica system with a few per cent of iron oxide, and a grainy structure that leaves two kinds of pores after firing. Ore, prepared body and fired pot are three different things, and a name or colour alone says little about any of them.

Fired colour#

Range

Purple-brown, red, buff to yellow, depending on the clay group¹

Depends on

Clay group and its iron content, blend, firing temperature and atmosphere

Names in the trade#

Firing and shrinkage#

Composition#

Structure#

• Grainy (aggregate) structure with small pores inside the grains and broken, chain-like pores between them; the authors see this double-pore structure as the basis of the ware's air permeability. Samples fired at 1080 and 1100 °C were compared. (Abstract read; full text not available.)³
• Clay aggregates of 300 to 900 µm and evenly spread quartz of 50 to 100 µm were seen only in the Huanglongshan sample, not in the Zhangzhu or Guangde samples.²

Body components#

The analytical results describe the named raw-clay samples, not the composition of every fired pot. Firing changes mineral phases; a clay name does not identify a laboratory sample. Hou et al. 2016

Xu Hantang recalled two ways he worked with grains on small flowerpots. In the 1960s he applied grains to the surface (鋪沙), varying their size and density. In the 1980s he mixed grains into the clay body (調砂) for a decorative effect. These dates describe his own practice. Both techniques can produce visible speckles.^{p. 37 · PDF 46}

Where the ore is said to come from#

• Dingshu, Yixing, Jiangsu: Huanglongshan (黄龙山), mined by underground tunnels. The museum text says the deposit is now under government control and private digging is banned, and that clay already stockpiled is said to last for decades.¹
• Yixing and neighbouring areas: Huanglongshan, Zhangzhu (Yixing) and Guangde (Anhui). Origins of three purple-clay samples bought as potting clay on the market, one per place; a lab comparison, not a map of all sources.²

A named mine is the source's statement, not a guarantee for any pot.

History#

1. Ming, 1640sZhou Gaoqi names the clays of his day by source hill and fired colour: 嫩泥 from Zhaozhuang hill, sticky and used to bind the other colours; 石黃泥, also from Zhaozhuang, which fires to a cinnabar colour; 天青泥 from Lishu, which fires dark liver-coloured, with offshoots 梨皮泥 (pear-skin) and 淡紅泥 (pale red); 老泥 from Tuanshan, which fires with white sand specks; and 白泥 from Dachao hill, used for jars and basins. He does not link these names to today's trade names, and none should be read as the same material.⁷
2. Ming, 1640sZhou says the pots from Gongchun to the early Shi Dabin were fine clay of a pale ink colour with silvery specks, and that mixing in coarse sand later gave a faintly granular surface.⁷

Known problems#

Documented practices, described with sources. None of this is said about any named seller or any single pot.

Standards#

Related#