Halakhah zu Parah 2:13

And work [with it] disqualifies it, as it is written (Numbers 19:12), "that has borne no yoke upon it." And any labor is [considered equivalent] to a yoke; therefore, they, may their memory be blessed, said that even if he placed a cloak on it, it is disqualified. If, however, it needed to be guarded and it was tied by a rope, it is acceptable (Mishnah Parah 2:3, Mishneh Torah, Laws of Red Heifer 1:7), but if it did not need guarding, it is disqualified, as any protection that is unnecessary is a burden. And it was purchased with money from the collection of the cell (Mishnah Shekalim 4:2). And a heifer that became blemished can be redeemed and it goes out to being non-sacred. And [also] that which they, may their memory be blessed, said (Yoma 88b) about the matter that it is the one who burns the heifer that is impure, that this is the one who assists in the burning - such as the one who turns over the meat, or throws in firewood, or [moves] the fire or stokes the coals to have the fire burn [better], and similar to these - but the one who lights the fire in the furnace, or arranges the wood, is pure, as is anyone who deals with it after it has become ashes.