Moisture is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analysed quantitatively. Moisture may occur in four possible forms within coal:
* Surface moisture: water held on the surface of coal particles or macerals
* Hygroscopic moisture: water held by capillary action within the microfractures of the coal
* Decomposition moisture: water held within the coal's decomposed organic compounds
* Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays
Total moisture is analysed by loss of mass between an untreated sample and the sample once analysed. This is achieved by any of the following methods;
# Heating the coal with toluene
# Drying in a minimum free-space oven at  within a nitrogen atmosphere
# Drying in air at  and relative loss of mass determined
Methods 1 and 2 are suitable with low-rank coals, but method 3 is only suitable for high-rank coals as free air drying low-rank coals may promote oxidation.
Inherent moisture is analysed similarly, though it may be done in a vacuum.