What is the main composition of the gray glaze on porcelain?

Some people divide it into lime glaze and lime alkali glaze according to the content of calcium oxide, and mainly use lime as the flux of silicon dioxide.

Without ash, there is no glaze. In the Yuan Dynasty, Man Jiangqi recorded in his "Ji Tao" that "the method of making stone and chalk was destroyed by rubbing persimmon leaves"; Zhu Yan, a Qing Dynasty man, recorded in Tao Shuo that "it is impossible to have glaze without ash. The ash from Leping County, 70km southeast of Jingdezhen, is made of bluestone and Pteris multifida and washed with water. The chalk and bluestone mentioned in the above two places are limestone. Ancient Jingdezhen porcelain makers introduced calcium oxide into porcelain glaze through the processes of calcining limestone, digesting, simmering and elutriation, mainly in the form of calcium carbonate, and a small amount of calcium phosphate was used as silica flux. This is why we call the traditional glaze lime glaze.