Since most coals have a tendency to oxidize on exposure
to air, the air-drying procedure should not be prolonged past
the time necessary to bring the sample to equilibrium with the
temperature and humidity of the air in the room in which
further reduction and division are to be made. Easily oxidized
coals must not be air dried at a temperature exceeding 10°C
above ambient temperature. In no case shall the air drying be
done at a temperature over 40°C. Air drying of low-rank coals
should not exceed 18 h because of oxidation. In the case of
lignite, the goal of reaching equilibrium should be weighed
against the possibility of oxidation.