From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): (n.) Mineral coal charred, or depriver of its bitumen, sulphur, or other volatile matter by roasting in a kiln or oven, or by distillation, as in gas works. It is lagerly used where / smokeless fire is required.

(2): (v. t.) To convert into coke.

The Nuttall Encyclopedia [2]

Coal with a residue of carbon and earthy matter after the volatile constituents are driven off by heat in closed spaces.