text: Clinker often is reused as a cheap material for paving footpaths. It is laid and rolled, and forms a hard path with a rough surface that presents less risk of slipping than most loose materials. In sufficient thickness such a layer drains well and is valuable for controlling muddiness. However, if laid without sufficient adhesive, it needs frequent rolling and addition of more clinker to maintain the path in good condition if it is subject to heavy foot traffic.  
  In sewage treatment works, the foul water is first screened to remove floating debris. Then it is sedimented to remove insoluble particles.  After this, it is sprayed over a filter bed of clinker. Aerobic microbes soon grow in hollows in the clinker, where they kill harmful anaerobic bacteria in the water and remove much of the offensive organic waste.
  Historically, clinker from coal-burning steamships simply was discarded overboard, leaving detectable trails on the seabed of some prominent steamship routes. As such, the deposits have proven to be of biological, historical and archaeological interest.
  Naturally occurring clinkers exist.  For example, in the Powder River Basin is covered by clinkers from coal-seam fires, i.e., "baked, welded and molded rocks formed by the natural burning of coal beds."