Hidasuki is a characteristic red-colored pattern on the surface of Bizen-yaki. Before green weares are heated, straw ropes are used to prevent their mutual contact and it is only where the ropes are used that Hidasuki appears. Potassium and chlorine, detected in the straw, must be considered to play a significant role in Hidasuki coloration through their reaction with Bizen-clay. During the heating process of a mixture of Bizen-clay and 8wt% of KCI, hematite was formed at between 600°C and 1100°C. KCI reacted with water at the dehydroxylation temperature of halloysite to form HCI. Glassy phase containing potassium was formed at above 1100°C and hematite dissolved in the glassy phase. At 1300°C, only glassy phase containing iron ions and corundum were found. When the mixture of the glassy phase and corundum were cooled down slowly in air, hematite was found at below 1050°C. The degree of red coloring of the sample was closely correlated with the shape and the amount of hematite. Hematite with glassy phase was responsible for the red coloring of Hidasuki. However, hematite was not formed when the mixture was quenched. A similar mineral composition to that of Hidasuki was obtained when the mixtures 8 or l0wt% KCI added were cooled down at the rate of 0.2°C/min in air.