It’s salt.

According to Baidu’s experience query, the salt-baking and simmering soup stove uses salt instead of stones. Salt has good thermal conductivity and can quickly transfer the heat generated by the fire to the inside of the soup can, thereby heating the soup to simmering temperature.

Salt simmering soup stove, also known as earthen pot simmering soup stove, uses the good thermal conductivity of sea salt to simmer and heat earthen pots.