Reference for Shabbat 32:1
רב אשי אמר לעולם לכלי חרס דמו ודקא קשיא לך לא ליטמו מגבן הואיל ונראה תוכו כברו
R. Ashi said: After all, it is similar to earthen utensils, and as for your difficulty, 'let them not become unclean through their [flat or convex] backs', [the reply] is because its inside is as visible as its outside.<span class="x" onmousemove="('comment',' From without; hence it is all regarded as the inside. ');"><sup>1</sup></span> 'Simeon b. Shetah instituted a woman's marriage settlement and imposed uncleanness upon metal utensils.' But [the uncleanness of] metal utensils is Biblical, for it is written, howbeit the gold, and the silver [… etc.]?<span class="x" onmousemove="('comment',' Num. XXXI, 22. The text continues: everything that may abide the fire, ye shall make go through the fire, and it shall be clean; nevertheless it shall be purified with the water of separation. ');"><sup>2</sup></span> — This [the Rabbinical law] was necessary only in respect of former uncleanness.<span class="x" onmousemove="('comment',' V, supra a. ');"><sup>3</sup></span>