Commentary for Avodah Zarah 149:1

How does one scour earthenware vessels? While it might seem that Rav and Rabbah b. Hanah disagree, the Talmud believes that they do not. Both say that one needs ashes (a cleansing agent) and water. If the vat is wet, first he puts some ashes and then he cleans it off with water. If the vat is dry, he first puts some water and then the ashes clean it off.