Jun wares were fired in a reduction kiln—a type of kiln which deprives the fire of oxygen by closing air vents, inducing smoke, or other means. This forces the fire to draw oxygen from the very clay of the vessels within the kiln, altering the naturally occurring iron molecules within the clay. Once this process was complete at the conclusion of the firing cycle, oxygen was readmitted to cool the kiln. At this moment, thinly glazed, unglazed, or unprotected parts of the vessels reabsorbed oxygen molecules, turning reddish or brown as the iron oxidized. In the case of this vase, the glaze is blue as intended on one side, but it misfired to brown on the other.