The critical factor is the intended material being processed: this heading applies specifically to furnaces designed for the heat treatment of ores, pyrites, or metals—not for ceramics, glass, or food products. Technical documentation confirming the furnace's design parameters (temperature range, atmosphere control, feed material) and the manufacturer's stated application are the primary classification evidence. A furnace capable of processing both metals and other materials may require a principal-use determination.

Due to their enormous size, blast furnaces are almost always imported in prefabricated sections or as components and assembled on-site. This raises classification questions about whether individual sections are classified as parts of the furnace or as structural steel or refractory products. The classification of each section depends on its degree of manufacture and whether it is identifiable as a furnace component at the time of importation; a binding ruling from CBP is advisable for large modular furnace projects.