Reference for Numbers 19:15

A vessel of earthenware can protect everything [in it from contracting impurity], according to Beth Hillel. But Beth Shammai says: “It protects only food and liquids and [other] vessels of earthenware.” 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.