I heat with Coal..Very Hard Anthracite. "Clinkers" are made by a process of Melting of the coal ash. as air passes by from below, they can move and grow to be bigger than Stove size coal.
Clinkers can occur on long burns and seem to happen more with less than 88% carbon coal.
I screen out the clinkers and use in the driveway as well.
Some can be sharp as stated.
