Iron smelting and forging technology was present in southern Africa well before the 18th century, along with other parts of the "Bantu" package (agriculture, cattle herding, language, types of social organization, etc). This was well established by Chris Ehret of UCLA and many scholars who have followed. See Norman Etherington's excellent 2010 article, "Historians, archaeologists, and the legacy of the discredited short iron-age chronology" in African Studies, 2010 (69).
Now if your question is about exact forge technology and techniques, I don't really know much, but T. Maggs did write an interesting if speculative [article] (http://www.sahumanities.org/ojs/index.php/SAH/article/viewFile/118/100) about this. I cannot judge its accuracy.
I would also like to point to this documentary "The Tree of Iron" by Peter O'Neill and Frank Muhly, Jr. with Peter Schmidt. It deals primarily with smelting, but also smithing, techniques in East Africa that date as far back as 2,000 years ago.