Slide 10: Final Firing on Bare Ground

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In Africa, the final firing of pottery is usually done in the open, on flat ground. The potters pile a layer of fuel, from one inch to one foot thick, on the bare ground. The dry or pre-fired pots are placed on top of this layer of fuel, either rim downward or with the rim leaning against the bottom of the pot next to it. The pottery is arranged in a single layer and more fuel is piled on top. A second or third layer of pottery may be added to the charge and the entire mass is topped off with a final layer of fuel. Finally, the entire charge is lit at the bottom. Flames quickly engulf the entire mass, producing a large bonfire. The charge is reduced to a mixture of hot ash, glowing coals, and red hot pottery in a period of one-half to two-and-a-half hours, depending on the size of the charge and the amount and type of fuel used. The goal is to drive off the water molecules that permit the clay to return to a plastic state, but not to fire to a temperature high enough to cause vitrification, which creates a rigid molecular structure that would make the pottery shatter if used over and open flame. The pottery is fired to a low temperature on purpose, because this produces the durable ware the client needs.

Supplementary Information of Types of Fuel

The type of fuel used is naturally dictated by the crops and vegetation in the area. In the Sahel, dried grass, millet chaff and stalks, animal dung, and wood are used. Fine chaff produces a thick blanket of ash over the firing which protects the pottery from breakage if the wind should suddenly increase. The farther north the firing takes place the less wood and the more dung and grass are used. Farther south, in the more heavily forested grasslands, wood is the most common fuel. In the rain forest areas, palm mid-ribs, bamboo, and grass are used.

Supplementary Information on Firing Temperatures

The duration of all African firings is very short when compared to the technically complex and lengthy firings of European, American, and Oriental potters. As a result of the nature of the kilns used and the length of the firings, firing temperatures usually do not exceed the minimum required to dehydrate the clay completely--about 600° C. At this temperature water, which is chemically bonded in the clay molecules and which even the longest and most thorough drying cannot remove, is driven off as steam. Pots that are not fired to this temperature will re-absorb water and rapidly disintegrate, reverting to shapeless lumps of mud. But once pottery has been fired to this temperature, the clay from which it is formed can no longer re-absorb water and the pots can be used to store liquids. (see Science page for in-depth explanation)

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