Firing Bizen Ware

The size and number of chambers may vary, but the principles remain the same. Only the wood of the Japanese red pine is used for firing. Due to the nature of the Bizen clay, temperatures must be raised very slowly. In comparison to the Echizen clay which Mr. Tatsumi could fire in five days, Bizen clay requires thirteen days to bring the temperature up to 1250 deg. centigrade. As the temperature rises in the kiln, there are particularly dangerous points around 400 deg and than again around 600 deg. Moving through these points too quickly as the crystalline structures change, can cause cracking and the possible loss of an entire firing. In the structure of the noborigama, there are two shelves for stacking in each chamber. The shelves or the chamber walls will absorb the heat as it moves through the kiln. A rule of thumb is that each shelf reduces the heat by 50 deg. and each wall by 100 deg. as the peak temperatures are reached.

Thus, the critical points pass at different times in each chamber and cause the firing process to lengthen. The master potter for each kiln supervises all of the firing. As the peak temperatures of 1250 deg. are reached at the mouth of the kiln, the temperatures in each chamber may be tested using the small curled pieces of clay shown here. They are extracted on the end of a steel rod.

Additional wood is added though openings in the side of each chamber. At this point, the rate at which wood is added controls the color and texture of the pieces. Adding the wood slowly produces red and purplish tones from the gasses released by the burning wood. Adding the wood faster results in gray and black tones due to the deposition of ash on the piece or the carbonization of the clay. The following photo shows the range of color effects that are possible from the same unglazed clay. The set of tea bowls shown are nearly identical in size and form. Each was obviously under slightly different conditions in the kiln and the result is a wide range of colors.