High, is usually of brick, red brick on the outside, firebrick on the inside; sometimes it is made of wrought iron waterjackets.
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.
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.
This method has received considerable extension, notably in furnace-smelting of iron ores containing manganese, where the entire hearth is often completely water-cased, and in some lead furnaces where no firebrick lining is used, the lower part of the furnace stack being a mere double iron box cooled by water sufficiently to keep a coating of slag adhering to the inner shell which prevents the metal from being acted upon.
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.