Soak the Sichuan peppercorns in the oil for at least eight hours or overnight. In a three-quart saucepan cook the oil and Sichuan peppercorns on low heat. Add the ginger root, scallion, cloves, cassia bark, star anise, black cardamom, dried whole red chilies and purple gromwell. Simmer the oil for about 20 minutes.

Put the dried red chili flakes and powder in a three-quart mixing bowl. Strain the oil through a wire mesh strainer and pour it onto the chili flakes and powder. Discard all the other ingredients from infusing of the oil. Let the oil steep in the chili flakes and powder for two or three hours before using.