Bottom ash is part of the non-
combustible residue of combustion in a
furnace or
incinerator. In an industrial context, it usually refers to
coal combustion and comprises traces of combustibles embedded in forming clinkers and sticking to hot side walls of a coal-burning furnace during its operation. The portion of the
ash that escapes up the
chimney or stack is, however, referred to as
fly ash. The
clinkers fall by themselves into the bottom hopper of a coal-burning furnace and are cooled. The above portion of the ash is referred to as
bottom ash too.