Bujing grass.
What was burned was a small tree called acacia. “Grey water” is a clear liquid in which the ashes of the Bujing plant are dissolved and precipitated. It is a natural alkaline water. In addition to using horsetail, soybean plants (pods) can also be used. For soybeans grown in rural areas, after harvesting and removing the beans, the dried plants can be burned to ashes to produce gray water. Additionally, "grey water" can be produced by burning ashes, soaking in water and filtering the stems of grass, pulsatilla, gall heads, mulberry roots, etc.