Avodah Zarah 76 - Purifying a Knife
To purify a non-kosher knife, one wipes it, and it is clean. Rav Ukva bar Chama explained this to mean that he sticks the knife into the ground ten times. In addition, the ground has to be hard and not ploughed, and the knife - smooth, without dents. This only suffices to purify the knife to be used with cold foods.
Mar Yehudah and Bati bar Tuvi were sitting at a meal with the Persian king Shapoor. They brought an etrog to the king. The king sliced a piece and ate it. Then he sliced a piece and handed it to Bati. Then he stood up and stuck the knife into the ground ten times, cut a piece, and gave it to Mar Yehuda. Bati took offence and said, "And this person (meaning himself), is he not a Jew?" The king answered, "I know about Mar Yehudah that he is observant, but I don't know this about you."
In another version the king said, "Remember what you did last night?" It was a custom to send the guest a woman, and Mar Yehudah did not accept his gift, but Bati did. In truth, Bati was a half-slave, and he was allowed to have relations with that woman who was non-Jewish, but the king did not know that.
End of Mesechet Avodah Zarah
Art: Kamal-Al-Molk - Persian Musicians