Reference for Eduyot 7:8
הֵעִיד מְנַחֵם בֶּן סִגְנַאי עַל מוּסַף הַיּוֹרָה שֶׁל שׁוֹלְקֵי זֵיתִים שֶׁהוּא טָמֵא, וְשֶׁל צַבָּעִים שֶׁהוּא טָהוֹר. שֶׁהָיוּ אוֹמְרִים חִלּוּף הַדְּבָרִים:
Menachem b. Signai testified about the rim of the vat of the boilers of olives that it is tamei; and of dyers, that it is tahor. For there were those that said the opposite. [The olive boilers and the dyers used to have large vats on whose rims they placed a border of clay to contain the water when it boiled over. That (i.e., the rim) of the olive boilers was tamei because that addition was needed for the vessel and was used; and the Torah stated in respect to an oven or a stove (Leviticus 11:31): "They shall be unclean to you" — "to you," to all that you need; that is, the thing in the vessel that you need and use — it is that which is susceptible of tumah. ("and of dyers, that it is tahor":) The dyers do not make use of that addition, for they are afraid that it will spoil their dye.]