The gray white substance on the lid of the crucible is generated by the oxidation of the coal sample, which can cause the measured volatile content to be higher. This oxidation phenomenon is mainly caused by the loose lid of the crucible and the intrusion of air into the crucible. For bituminous coal, especially high volatile coal, it is not serious because a large amount of volatile matter escaping outside will prevent air intrusion. Especially for low volatile substances such as high volatile coal, the oxidation effect may be more severe. At this time, a few drops of volatile liquid (such as benzene) can be added to the coal sample. When heated, the volatile liquid evaporates and escapes outside the crucible, causing air intrusion.