(1) Fuel and substance in contact.
(a) Height of furnace greater than diameter = shaft furnaces.
(α) No blast = kilns.
(β) With blast = blast furnaces.
(b) Height not much greater than diameter = hearth furnaces.
(2) Substance heated by products of combustion = reverberatory furnaces.
(a) Charge not melted = roasting or calcining furnaces.
(b) Charge melted = melting furnaces.
(3) Substance is not directly heated by the fuel or by the products of combustion.
(a) Heating chamber fixed and forming part of furnace = muffle furnaces.