Mishná sobre Pará 4:7

If an unclean person rests upon part of an object fit to lie upon, or a clean person rests upon part of an object fit to lie upon, it becomes unclean. If part of an unclean person rests on an object fit to lie upon, or part of a clean person rests upon an object fit to lie upon, it remains clean. Thus we find that impurity enters it and goes out of it by its lesser part. Similarly, if a loaf of terumah was placed upon an object fit to lie upon [that was unclean] and there paper in between, whether it was above or below, it remains clean. Similarly, in the case of a stone with scale disease it remains clean. Rabbi Shimon pronounced such a case unclean.