The characters are 「石泉槐火」, which refers to the passing of winter and coming of spring. Specifically,
- 「石泉」 means flowing of springwater (i.e. the melting of frozen rivers and streams)
- 「槐火」 refers to a fire ritual involving the burning of the Chinese Scholar Tree (Styphnolobium japonicum)
In Ancient China, it is said that the monarchs conducted fire rituals involving the burning of different types of trees during the year, to protect against seasonal diseases.
Officers of the fire rituals are in charge of the royal court's decrees on conduct involving pyromancy, changing the Tree-burning Ritual throughout the year to mitigate the spread of seasonal illnesses.
Later commentaries of the Rites of Zhou specified the seasonal trees to be burned:
春取榆柳之火， 夏取棗杏之火， 季夏取桑柘之火， 秋取柞楢之火， 冬取槐檀之火
During springtime, elms and willows are burned; in the summertime, jujube and apricot trees are burned; towards the end of summer, mulberry trees are burned; during autumn, oaks are burned; in the winter, scholar trees and rosewoods* are burned.
*The specific type of tree referred to is uncertain.