Tosefta על נזיר 50:3

The fat of a corpse that was complete and they cut it up is impure. [If] it was scattered around and they melted it^*והתיכו (GR"A), it is pure, for that which is joined by human [agency] is not [considered] joined [for purposes of impurity]. An etrog that burst open, and they stuck it [back together] with a reed or a splinter of wood is not [considered] joined, for that which is joined by human [agency] is not [considered] joined. A marrow-bone of a corpse that was fashioned into the handle of a knife, behold, that is not [considered] joining by human [agency]. [If] one joined an olive's bulk of a corpse that had split into two, it is impure, and one need not be concerned lest it is deficient. An etrog that was cut into two is pure, and one need not be concerned lest it is deficient. A grain-offering that was mixed by five priests is fit [for the Altar], and one need not be concerned lest it is deficient. [If] it was transferred from vessel to vessel it [remains] fit. Rabban Shimon ben Gamaliel says, if he apportioned it [among the priests], behold, it is disqualified.
The equivalent of an olive's bulk from a corpse, and the equivalent of an olive's bulk of decayed matter, a ladleful of corpse-mold from two corpses join together with one another. A ladleful of corpse-mold: Whenever there is even a small amount of dirt in it, it is impure, and Rabbi Yehoshua renders it pure because it was neutralized.