Probably because they had been carefully selected for Zhuang Qingning, these sweet potatoes were large, uniformly shaped, and unblemished. They were impeccably clean after a simple rinse with water.
Next, they were grated.
A metal board with numerous holes drilled into it was used to grate the sweet potatoes. Then, the sweet potato was strained through a piece of cloth used for filtering tofu. This process was repeated several times until all of the starch in the sweet potato chunks had dissolved in the water. Left to settle for a day or two, the water above the settled starch was drained off, fresh water added, and the settling process repeated.
After doing this several times, all the water was cleared away, leaving the leftover starch that had settled at the bottom. This soft white starch was then dried under the sun until completely dry, then ground into a fine powder. This is what is now known as sweet potato starch.
