Tractate Oholot - Chapter 5 - Mishnah 3 אהלות פרק ה׳ משנה ג׳

If [the pot] was whole:Bet Hillel says: it protects all [from uncleanness].A vessel of earthenware can protect everything [in it from contracting impurity], according to Beth Hillel. Bet Shammai says: it protects only food, drink and earthenware vessels.But Beth Shammai says: “It protects only food and liquids and [other] vessels of earthenware.” Bet Hillel changed their opinion and taught as Bet Shammai.Beth Hillel said to them: “Why?” Beth Shammai said to them: “Because it is [itself] impure with respect to an ignoramus, and no impure vessel can screen [against impurity].” Beth Hillel said to them: “And did you not pronounce pure the food and liquids inside it?” Beth Shammai said to them: “When we pronounced pure the food and liquids inside it, we pronounced them pure for him [the ignoramus] only, but when you pronounced the vessel pure you pronounced it pure for yourself and for him.” Then Beth Hillel changed their mind and taught according to the opinion of Beth Shammai.