Tractate Tahorot - Chapter 1 - Mishnah 8

A piece of dough that had first degree uncleanness, and then others became stuck to it, they all become unclean in the first degree. If they were separated, it still remains unclean in the first degree but all the others are have only second degree uncleanness. If [the original piece] had second degree uncleanness and then others became stuck to it, they all become unclean in the second degree; If they were separated, it still remains unclean in the second degree but all the others are only unclean in the third degree. If [the original piece] had third degree uncleanness, and then other became stuck to it, it remains unclean in the third degree but all the others remain clean, whether they were subsequently separated from it or whether they were not separated.