Hon-yaki is the glaze firing process in the production of ceramics, and the kiln used for this process is called a hon-yaki kiln (as opposed to an unglazed kiln). There are various types of honyaki kilns, such as round, old, round, and square kilns, depending on the style, but in any case, this firing process is the most difficult compared to other firing processes. In addition, because of the high firing temperature, if the firing method is not correct, the heat may not be applied properly, or the temperature distribution in the kiln may be uneven, resulting in irregularities in the ware in the kiln, and the entire process may be laborious and economically damaging. The porcelain firing process is divided into the following three phases. The first stage is called aburi (roasting), in which the porcelain is fired with oxidizing agents. The second stage is called seri, or firing in a reducing fire. The third stage is called kageage (firing), in which the pieces are fired with neutral flame. The first stage is the same as the unglazed and glazed stages, but the heat is gradually increased to dissipate the moisture sufficiently, and the kiln is fired keeping in mind the average temperature inside the kiln chamber.
When peering through a peephole in the kiln wall, the inside of the kiln gradually becomes visible in red. This is commonly referred to as soot exhaustion, and the kiln must be fired so that no soot remains from the initial fuel due to oxidization, and the temperature inside the kiln must be well balanced before moving on to the next stage. The second stage is also called the “blame” stage. During this period, the glaze is fired with a reducing fire, which is so named because more fuel is thrown into the firing chamber. The reduction furnace reduces the iron contained in the base material, which turns reddish when oxidized and bluish when reduced. The reddish tint is undesirable for white substrates. For this reason, the iron is fired in a reduction furnace to give it a bluish tinge. However, if reduction firing is used from the beginning, the carbon that has penetrated into the clay will remain and turn black, so oxidation firing is used to cut off soot at first, and reduction firing is used when the glaze finally begins to melt, so that the iron is fully returned to the Honnoji Bunrin Mai by the time the glaze melts. If the glaze is oxidized even for a moment, the iron is immediately oxidized and takes on a yellowish hue, which causes dirty red spots to appear on the surface, commonly referred to as staining, or, as in the old days, drunkenness. The fire cannot be restored after the glaze has melted. Because of this extremely difficult firing method, it is impossible to adjust the amount of charcoal thrown into the kiln at each firing point in order to achieve an average level in the kiln during this period, so in order to achieve a uniform level of fire in the kiln, it is necessary to achieve a sufficient average during the Aburi period. In the third stage, the glaze is melted and the substrate is close to the melting point, so there is no longer any need for reduction. And since the heat inside the kiln has risen considerably, a neutral flame is naturally produced even if a reducing flame is used. In this period, the kiln is heated sufficiently with neutral flame for the final firing.
