PRODUCTION PROCESS
After preparing the mixture of Shigarki clay, together with a special blend of other raw materials and water, the first step is to shape the tub. On a circular or oval modelled base plate, the material is applied by hand, in the form of strands, layer by layer, all around - until the wall height is reached. A distinction is made between two methods: rokuro seikei, where a potter's wheel is used to form rotationally symmetrical hollow bodies (exclusively for round tubs up to 130 cm in diameter), and teneri seikei for larger or oval components, which are formed entirely by hand. The resulting blank with a solid, thick tub wall requires 2 to 3 weeks of air drying, so that the initial humidity of 20% is reduced to a value of 5%. As a further preparation for firing, a 2-day stay in the drying chamber follows. The drying process causes shrinkage, which has to be taken into account in the shaping process beforehand. Firing takes about three days, whereby the temperature in the kiln is slowly increased to 1250 °C; cooling in the kiln is equally slow and takes about two days. As a surface finish, for smoothing and sealing against liquids, glazes in different colours are applied, for example kogeyohen (burnt brown) or kuromatto (matt black).
