Metalworkers use furnaces to smelt ores-to separate valuable metals from impurities, or slag. A furnace may be a simple bowl dug in the ground. Or it may be a complicated structure that forces air across a hearth and up through a chimney-like shaft.
African peoples have smelted iron for centuries. This kind
of furnace was used in the 1920s by the Dimi smelters in Ethiopia. Though
it looks simple, this kind of forced-draft furnace uses a complex process
to make metals out of ore (iron-bearing rock).
To make the walls of these furnaces strong, smelters mix clay with other materials, including animal dung, termite mounds and rotting millet straw.
A fast-burning furnace smelts ores in just a few hours, but many workers must pump the bellows to keep it going. Communities with fewer workers build furnaces powered by natural air flow. Although these furnaces take much longer to smelt ores, they work with less manpower-one person checks the furnace until smelting is done.
the process of smelting carbonized iron or steel, the smelter has to pack
the furnace with dry grass. After setting the grass on fire, he adds charcoal.
When the charcoal is red hot, the smelter inserts clay pipes, or tuyeres,
into the furnace's sides. The tuyeres connected the furnace to clay bowls.
Bellows are made by attaching goat skins to the bowls. When pumped, the bellows are able to force air and oxygen into the furnace.
Layers of charcoal and ore are poured into the furnace until the shaft is full. Then smelters must pump the bellows to force air into the furnace.
Inside the furnace, airflow makes the charcoal hotter causing the ore to begin breaking down. Red-hot ore and charcoal fall through the white hot area between the tuyeres. As the ore melts, it gives off oxygen and absorbs carbon from the hot coals. Puddles of iron slag form beneath the tuyeres. As the slag absorbs more carbon from the bed of charred grass and as oxygen escapes from each slag puddle, pieces of carbonized iron, or steel, form in the center. The master smelter knows the iron is ready when the color of the flame changes, when the furnace's contents burn down, and when the sound of dropping slag stops. The furnace is allowed to cool overnight.
The next morning, smelters enter the furnace to collect pieces
of carbonized iron, called bloom. The iron is ready to be forged into
To watch a video on this subject, select one of the following options:
4.1 MB 6 frames/sec presentation. (high speed connection recommended)
1.2 MB 1 frames/sec presentation. (low speed connection okay)
|Next page - A Smith's Workshop|