Eduyos, Chapter 7, Mishnah 8. This mishnah brings the testimony of Menachem ben Signai, who is not among the better known Tannaim. He was an expert dyer, and he had a tradition regarding the cauldrons used for dyeing.
The halachic background:
A vessel is susceptible to tumah. When something is added to a vessel, we have to examine the nature of that addition: if it is an essential part of the vessel, enabling it to perform its function better, then it is part of the vessel; and if not, it is not part of it. For example, a copper cauldron used for boiling water into which a ring of clay or earthenware was inserted. On its own, earthenware of this sort is not susceptible to tumah, but if it is considered part of the cauldron, it becomes susceptible to tumah just as the cauldron is. The question, then, is whether that clay ring placed inside the cauldron is subsumed into it and becomes part of the vessel, and that depends on the use of the vessel and on the role the ring plays within it.
For this reason the mishnah distinguishes between two types of use:
The cauldron of olive pressers: After the olives are pressed and ground to extract the oil, the pulp remains. It is boiled in a cauldron, and in the course of the boiling the oil is drawn out of the pulp and rises to the top, so that additional olive oil can be extracted from it. While it boils, the contents are liable to spill over the walls of the cauldron, so a rim of earthenware clay is added inside it, which allows the cauldron to be filled to the very top and prevents the oil and water from overflowing and being lost. It turns out that the rim improves the functioning of the vessel as an instrument for producing oil, and therefore it is an essential part of it, is subsumed into it, and is susceptible to tumah.
The dyer's cauldron: Into this cauldron one places linen or white wool in boiling water in which the dye has been dissolved, so that the dye will be absorbed into the fabric and color it. Here too one does not want the water and dye spilling over the walls of the cauldron, but preventing the overflow does not improve the dyeing process at all; it merely prevents a mess. For that reason the rim is not considered an essential part of the dyeing vessel, is not subsumed into it, and is not susceptible to tumah.
The words of the mishnah:
"He'id Menachem ben Signai" - a testimony that he testified.
"Al mosaf hayorah" - the addition that was added to the cauldron, that inner ring of clay or earthenware.
"Shel shukei zeisim - shehu tamei" - in the cauldrons of the olive pressers, the inner rim is part of the cauldron, since it helps it function better, and therefore it is susceptible to tumah just like the cauldron itself.
"Veshel tzeva - shehu tahor" - in the dyer's cauldron, where the rim serves only to prevent a mess and not to improve the dyeing, it is not considered an essential part of the vessel, and it is not susceptible to tumah.
Why was this said?
At first glance there is nothing novel here. But as we have seen consistently throughout Maseches Eduyos, something is stated because there was someone who said the opposite: "shehayu omrim chiluf hadevarim" - for they used to say the reverse. According to that opposing tradition, the rim of the dyer's cauldron is susceptible to tumah, while the ring in the cauldron used for producing olive oil is tahor. The reasoning behind that view is the value of the material: dye is expensive, and one does not want to lose any of it, so a rim is added; whereas the pulp and the oil that comes out of it are not valuable, so one is not particular about the little that spills. They therefore held that the rim of the dyer's cauldron is the more significant one, and hence that it is susceptible to tumah.
But Menachem ben Signai's point is that the law is precisely the reverse, since susceptibility to tumah depends on the functioning of the vessel and not on the value of the material. Preventing the overflow in the dyer's cauldron serves only to save on expense, and there is no need whatsoever to fill the cauldron to the top in order to dye. By contrast, the loss of oil that spills out of the olive pressers' cauldron is a direct inefficiency in the vessel itself, and therefore the rim that prevents it is part of the vessel and is susceptible to tumah.
The halachah: The ruling follows the view of Menachem ben Signai, and not the tradition that said the reverse.