Therefore, GB211 "Industrial Analysis Methods for Coal" specifies the following slow ashing procedure; The coal sample is placed in a cold muffle furnace and gradually heated to 500 ℃ at room temperature within 30 minutes (to gradually ash the coal sample and prevent deflagration). It is then left at 500 for 30 minutes (to fully oxidize and remove organic sulfur and iron sulfide). Then, the furnace temperature is raised to 815 ± 10 ℃ and kept for 1 hour (to fully decompose calcium carbonate).