coke-bed

from The Century Dictionary.

• noun A layer of coke which is put into a cupola before charging any of the iron. The amount of coke in this layer is proportionate to the amount of iron to be melted.
• noun A porous stratum of coke and clinker in pieces of various sizes, laid to a depth of several inches below a mold for casting a piece of work having large surfaces and which is bedded in.