Sentence:  The furnace was again entered, and the crust to the chests broken down; it had partially vitrified and had absorbed iron oxide and was removed in black, craggy pieces, often referred to as <WSD>'crozzle</WSD>' - still sometimes to be found as a capping to walls in the vicinity of the old furnaces. 
Meaning of the word marked with <WSD> tags:  Hardened slag from a cementation furnace.