Clinker is a general name given to waste from industrial processes — particularly those that involve smelting metals, burning fossil fuels and using a blacksmith's forge which will usually result in a large buildup of clinker around the tuyère. Clinker often forms a loose, black deposit that can consist of coke, coal, slag, charcoal, grit, and other waste materials.
Clinker may be reused for pavingfootpaths. It is laid and rolled, and forms a hard path with a rough surface.
Clinker often has a glassy look to it. It is much heavier than coke.
In sewage treatment works, the water is first of all screened to remove floating debris. Then it is sedimented to remove small insoluble impurities. After this, it is sprayed over a filter bed of clinker, where microbes in the clinker kill harmful anaerobic bacteria in the water.