Furnace residue (7)

I believe the answer is:
clinker
(Other definitions for clinker that I've seen before include "Fragments left after a fire" , "Furnace slag" , "Stony residue from burnt coal" , "Stony residue from a furnace" .)
I believe the answer is:
clinker
(Other definitions for clinker that I've seen before include "Fragments left after a fire" , "Furnace slag" , "Stony residue from burnt coal" , "Stony residue from a furnace" .)