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The objectives of this study were to examine the role of the addition of kaolin on the firing of white clay for Korean porcelain by characterizing the fired composites at 1000–1300°C, and finally to estimate the production conditions of ancient Korean white porcelain in the Joseon Dynasty. Yanggu white clay disks composed of 57.1 wt% quartz and 38.8 wt% muscovite showed bloating phenomenon at temperatures over 1175°C due to the evolution of gas incorporated in the disk. Potassiumfrom muscovite (KAl2(AlSi3O10)(F,OH)2) in Yanggu white clay played a significant role as melting agent to form a liquid phase. To produce Korean white porcelain at temperatures over 1175 oC without bloating, the addition of 20–40 wt% of Korean kaolin to Yanggu white clay was necessary. Increasing the amount of kaolin added and firing temperature enhanced the formation of mullite. We estimated the manufacture of a portion of the old Korean white porcelains in the Joseon Dynasty from mixtures of Yanggu white clay and kaolin to improve the refractoriness and whiteness of the body.