text: Historically, the reduction of iron ore without smelting is the oldest process for obtaining steel. Low-temperature furnaces, unable to reach the melting temperatures of iron alloys, produce a burr, a heterogeneous agglomerate of metallic iron more or less impregnated with carbon, gangue, and charcoal. This process was gradually succeeded, from the 1st century in China and the 13th century in Europe, by the blast furnace, which simultaneously reduces and melts iron.
  Elaborate low furnaces, such as the tatara or the Catalan forge, survived until the early 19th century. Compared with the indirect process (reduction-melting in the blast furnace, followed by cast-iron refining), these processes only survived when they enjoyed at least one of the following two advantages:
  * ability to process ores that are incompatible with blast furnaces (such as iron sands that clog blast furnaces, or ores that generate slag that is too pasty to be drained);
  * a more "reasonable" size than that of giant plants and their constraints (ore and capital requirements, production to sell off, etc.).