10.1.1. Physical Properties
The color of ash varies from tan to different shades of gray, depending on the presence of carbon particles because of incomplete combustion. The lighter shades of gray generally indicate a higher quality of ash and lower carbon content. A tan color is normally a result of lignite or subbituminous coal use.
Ash in its powdery form (i.e., fly ash or crushed/ground bottom ash) normally has a spherical shape and glassy appearance. Particle sizes vary, however, in accordance with the type, grade, or rank of coal used. The specific gravity of ash may vary in the range of ∼2.0 to 3.0.