Abstract: There is disclosed a method of forming a furnace cooling element involving casting a metal block or plate (10) around one or more elongated metal tubes (11) which have a protective coating applied thereto, and thereafter the, or each, metal tube (11) is explosively expanded by an explosive charge (12) closer to the surrounding material of the metal plate (10). Also disclosed is a method in which the protective coating on the metal tube is replaced by a full length external shroud tube (21) to increase the thermal efficiency of the cooling element, the inner metal cooling tube (20) is explosively expanded by an explosive charge (23) closer to the surrounding shroud tube (21) prior to casting a metal plate or block (28) around the tube combination (20, 21) to minimize the gap (22) between the outside wall of the metal cooling tube (20) and the inside wall of the metal protective shroud tube (21).