The soil comes with grit and lumps, and so must be washed, which in the old days was usually done in settling pools. The soil was dumped into the pools, which were then filled with water. The soil and water would then be stirred and left to settle overnight. The clear liquid was drained off the next day, while the fine-grained clay was transferred to another settling pool.
剛從大自然採來的原礦土裡，還摻有雜質和粗顆粒礦石，為了得到質地細緻的黏土，必須「洗土」──早期多在地上挖幾個池，把原礦土加水攪拌，任其靜置一夜；次日引流卸除上層清水，中間細泥則引入第二沉澱池。