Reference for Chullin 49:1
ואפילו מלא חרדל
even though it is filled with mustard seed.<span class="x" onmousemove="('comment',' All the mustard seeds in the vessel are rendered unclean, even those which are in the center of the vessel. Now these latter become unclean only because they are in the air-space of an unclean earthenware vessel. It cannot be suggested that the seeds which touch the side of vessel convey uncleanness to those next to them, one seed conveying uncleanness to the other, and so on as far as the seeds in the center of the vessel, for the following reasons: (a) no foodstuff less than the size of an egg can convey uncleanness; (b) one foodstuff cannot convey uncleanness to another foodstuff; and (c) with unconsecrated food uncleanness can at the most be extended to the second degree, but no further. In our case, therefore, the vessel, being unclean in the first degree, would convey uncleanness by contact to the seeds next to it, and these would be unclean in the second degree and could not pass on the uncleanness to others even by contact.');"><sup>1</sup></span>