Bottom ash. Coloured scanning electron micrograph (SEM) of coal bottom ash. Bottom ash is part of the non-combustible slag waste residue produced in the furnace of coal-fired power stations. Coal ash particles may contain large quantities of toxic metals, including mercury, arsenic, beryllium, cadmium, chromium, nickel, and selenium. It can also be recycled and used as an additive for concrete, cinder blocks, and asphalt. Magnification: x1654, when printed 10 centimetres wide.