Clay for making celadon should be viscous, possess high refractoriness, have homogenous particles, have adequate iron composition and contractility, and appear grayish-blue after 2nd firing. If it lacks refractoriness or exceeds iron content, white soil is added.
The soils are not immediately used after collected. They are dissolved in water (nowadays we use agitator to make muddy water) and then filtered using 12 slits to remove iron contents. Then it is dried at a drying room for dozens of days