Kelim, Chapter Four, Mishnah Two
1) If a jar was cracked and cannot be moved with half a kav of dried figs in it, it is clean.
2) If a damaged vessel (gistera) was cracked and it cannot hold any liquid, even though it can hold foodstuffs, it is clean, since remnants do not have remnants.
Section one: Half a kav of dried figs is an amount deemed worthy of a meal (see Peah 8:5). If a jar cannot hold this minimum measure of food, it is not susceptible to impurity.
Section two: A damaged broken piece of an earthenware vessel that is still used to hold liquids is still susceptible to impurity. However, if it then becomes cracked such that it cannot hold liquids, it is no longer susceptible to impurity. This is because there is a general rule that once a damaged piece of earthenware becomes further damaged, it is usually discarded and therefore no longer susceptible to impurity. Note that is true even though this broken piece of a utensil could be used to hold food. Since people generally throw such a thing away, it is no longer considered a vessel.