Bechorot 23 - Dormant Impurity Wakes Up
Resh Lakish said, "One who buys fish brine from an ignorant person, not knowledgeable in the laws of purity, can purify this brine in the following way: make it slightly touch (kiss) the waters of a mikveh, and it will be pure. If this brine contains more than half water, then the water will be purified, and if it has more than half brine, then it's not proper food, does not accept food impurity, and the water in it is nullified."
Rabbi Yirmiyah added, "This advice is only good to dip your bread in it, but not to put it into a pot for cooking. Why not? Because the dormant impure water will combine with the water in the pot, its impurity will wake up and make the whole pot impure."
Abbaye was surprised, "Can nullified impurity re-awaken? Consider the blood cake of a miscarriage we learned about previously . It carries no impurity. But if, as you say, the impurity lies there dormant, one should become impure by carrying it, since in it are found all the pieces of the fetus, which are impure!" Rabbi Yirmiyah was silent and did not find a good answer.
Art: David The Younger Teniers - An interior scene with pots, barrels, baskets, onions and cabbages