The furnace consisted of a flat, rectangular, firebrick box, packed with a layer of finely-powdered charcoal 2 in.
The central space of the furnace was filled with a mixture of corundum, coarsely-powdered charcoal and copper; and an iron lid lined with firebrick was luted in its place to exclude air.
Coke dust or graphite is used for the same purpose in crucible making (see Firebrick).
For the latter purpose, however, lime is generally used; but as this substance has only small stability, it is usually bedded in a casing of firebrick.
Puddling furnaces are usually entirely cased with iron plates, and blast furnaces with hoops round each course of the stack, or in those of thinner constructions the firebrick work is entirely enclosed in a wrought iron casing or jacket.