The distinctive ironworking technology of northwestern Tanzania was researched by combining ethnographic and archaeological observations with laboratory analyses. Modern smelting was studied in an area where ironworking extends from 600-400 BCE to 600 CE. Smelting methods, different from early ones in Europe, are described. The iron produced has an unusually high phosphorus content of up to 10%. Possible sources of the phosphorus--from the ores and from the charcoal--are suggested, and the effect of phosphorus on smelting and forging is described. The chemistry and microstructure of ancient and modern samples of bloom are described and discussed.