What is Jianzhan?

Jianzhan or Jian ware (Chinese: 建窯; pinyin: Jiàn yáo; Wade–Giles: Chien-yao) is a type of Chinese pottery originally made in Jianyang, Fujian province. It, and local imitations of it, is known in Japan as Tenmoku (天目). The ware are simple shapes in stoneware, with a strong emphasis on subtle effects in the glazes.
In the Song dynasty they achieved a high prestige, especially among Buddhist monks and in relation to tea-drinking. They were also highly valued in Japan, where many of the best examples were collected. Though the ceramic body is light-coloured, the wares, generally small cups for tea, bowls and vases, normally are glazed in dark colours, with special effects such as the "hare's fur" "oil-spot" and "partridge feather" patterns caused randomly as excess iron in the glaze is forced out during firing.