Slide 13: Post-Firing Decoration - Blackening Process II

The black broth (see previous page) is splashed on the pot that was just pulled from the firing with a bunch of grass or leafy twigs. On contact with the hot clay, great clouds of steam erupt and the broth sizzles and bubbles wildly as it evaporates. Some liquid is splashed into the interior of each pot where it collects in a pool. As the pot is turned to permit the potter to splash its exterior, the interior is blackened. Some potters, most notably the Ashanti, thoroughly blacken the entire pot by repeating the process two or three times. Other groups prefer the patterns produced when some areas are blackened while others are left the natural red color of oxidized pottery.

Besides adding to the visual attractiveness of the pottery, blackening decreases its porosity by filling tiny open areas between clay molecules with carbon. As a result, water storage jars are rarely blackened, as they would no longer be able to cool their contents by evaporation, while cooking pots are almost always blackened.

Supplementary Information on the Science of the Blackening Process

The blackening process produces a very heavy reducing atmosphere (excess fuel--volatile vegetation in this case--and too little oxygen) around the pottery, an atmosphere which cannot be produced during the firing (except, perhaps, when fairly volatile fuel like dry dung is used) because the potter cannot control the oxygen supply to her open kiln. The reducing atmosphere turns the iron molecules in the clay into ferrous oxide, which is black, while the presence of extra oxygen in an oxidation firing produces ferric oxide, which is red. In addition, a resinous film may be left on the surface of the pot by some fuels, producing a very shiny finish.

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