Rava explains that Rebbi Meir and Rebbi Yehuda are discussing (parts of) an oven which was initially heated when detached; therefore according to R”Y they are muktza. Although it will not attain a din of Tanur, why are they muktza – since the shards are suitable in their current form (to cover a barrel etc.)?
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If it was never a Tanur then it was never a Keli. Being broken off what wasn’t a Keli doesn’t give it a status of Keli — especially according to Rebbi Yehuda who holds that we need Me’en Melachto.