When the raw base material is sufficiently dried after molding, it is first unglazed. The purpose of unglazing is to prepare for drawing and glazing, or simply to harden the clay weakly. Vessels that are fired in this manner and not glazed are called unglazed. Earthenware that does not require glazing is fired only unglazed, or it is hardened by increasing the firing rate of unglazed ware and then shimeyaki is performed to complete the process. Soyaki refers to the firing of the base only, while in the case of ceramics, it is called shimeyaki. Unglazed pottery is usually fired in the range of 800 to 110 degrees Celsius, so the base is not yet fully baked and tightened, but pottery is baked to a much higher temperature of 110 to 125 degrees Celsius. Porcelain, on the other hand, is fired at low temperatures in the beginning, and after glazing, both the base and glaze are fired at the same time with high temperatures. In other words, the base becomes semi-molten and the glaze is also melted. Since the unglazed firing method is the first time to give high heat to the dried base, it must be heated very gradually and avoid a rapid increase in temperature.
Otherwise, uneven shrinkage will cause cracks or distortion. Therefore, the heat must be applied gradually to dissipate the evaporated moisture without allowing it to stagnate, and sometimes to saturate the moisture to a certain extent. In this way, the parts of the piece that are too dry are softened by this moisture, while the wet parts are dried out, and each part is gradually dried and heated on average to complete a satisfactory firing. Kilns do not need to be built specifically for the purpose of unglazed firing, but they are generally suitable for unglazed firing because it is convenient for continuous unglazed firing and because kilns for unglazed firing do not require particularly high-quality materials due to their low firing temperatures. In large kilns in large factories, the residual heat from the main firing process is used, or an unglazed chamber is built in the middle of the flue to use the residual heat. Depending on the nature of the base material, it is important to be very careful about the correct temperature of unglazing, as failure to obtain the proper temperature may cause various problems in the subsequent processes. For example, if the firing temperature is too low, the glaze will not adhere properly. Also, some substrates shrink considerably with a slight difference in temperature. In such a case, a slight difference in the firing temperature at the end of the firing process will prevent the required dimensions from being obtained, and if the firing temperature in the kiln is uneven, the pieces will be extremely uneven. Some pieces do not require unglazed firing depending on the nature of the raw material. Examples include porcelain from Jingdezhen, China, and parts of Aizu and Mino ware in Japan.
