Clinkers, or slag, are made up of elements and minerals found in coal which are non-combustible.  When burnt, they melt and fuse together, forming lumpy ashes.  For as long as we have operated coal boilers, clinkers have been a recurring problem, occurring in small stoves used for heating a home right up to giant commercial/industrial boilers.  Boiler operators consider clinkers to be miserable stuff.

Resembling volcanic ash and lava, clinkers occur in the high-temperature furnace area of the boiler.  Coal boilers draw their combustion air from the bottom, up through the combustion grate. Lots of large clinkers on the combustion grate can hinder air passage, increase operation and maintenance costs and even force shut-downs of the boiler for cleaning out the clinkers.