text: Clinker is a generic name given to waste from industrial processes, particularly those that involve smelting metals, welding, burning fossil fuels and use of a blacksmith's forge, which commonly causes a large buildup of clinker around the tuyere. Clinker often forms a loose, dark deposit consisting of waste materials such as coke, coal, slag, charcoal, and grit.  Clinker often has a glassy look to it, usually because of the formation of molten silica compounds during processing. Clinker generally is much denser than coke, and, unlike coke, generally contains too little carbon to be of any value as fuel.  It is also applied to the byproduct of combustion and heating by those who use anthracite or lignite coal-fired boilers.  
  Clinkers can occur naturally, for example in underground deposits of coal that has been altered by heat from nearby molten magma; volcanic clinkers are jagged pieces of lava that look similar to industrial clinker.