Clinkers meaning

The lava is emitted from the volcanic vent at a high temperature, but on exposure to the air it rapidly consolidates superficially, forming a crust which in many cases is soon broken up by the continued flow of the subjacent liquid lava, so that the surface becomes rugged with clinkers.

Where coke or ordinary hard coal are used, the removal of clinkers should be done systematically, and the fires stirred.