Shinza is the Japanese term for a type of red glaze or underglaze decoration achieved using copper oxide as a colorant and firing in a reduction atmosphere. It includes both a red glaze made entirely with copper and underglaze red decoration where copper oxide is used to paint patterns on the body, which are then covered with a transparent glaze and fired to produce a red design. In Chinese, these are referred to as hongyou (red glaze) and youlihong (underglaze red).
Because copper red only develops its vivid color under ideal firing conditions, it is common for parts of the decoration to blur or turn blackish. This technique originated at the Jingdezhen kilns during the Yuan dynasty and was especially popular during the Hongwu reign of the early Ming dynasty. In the Qing dynasty, the technology for producing a stable and brilliant red color was finally perfected.