Bottom ash is typically grey to black in color, angular, and has a porous surface texture.
Bottom ash is usually a small portion of the total ash produced by the boiler. It may be vitrified or clinkered, but is friable. It will often show metals in the leachate and may tend to be acid producing. Boiler Slag - Larger particles of coal ash that have settled to the bottom of the boiler and have completely melted. A vitreous material which is normally the major portion of residues produced at the bottom of cyclone boilers.