Foundry burn on is a name given to a wide variety of surface defects that are produced at high heat during the metal sand casting process. Carbonaceous materials in the mold, such as high-anthracite coal dust can prevent such defects from forming.
Bentonite is a form of clay that has found numerous uses over its thousands of years of use - but perhaps most notably in foundries from the 1900’s onwards, alongside powdered coal.
Anthracite is one of the hardest forms of coal and is known for its high carbon content. Aside from as a fuel, anthracite finds use in high temperature applications - especially when calcined - due to its relatively high resistance to thermal shock, strength and chemical inertness.