Tosefta for Pesachim 90:4

Tanners' bowls into which flour is placed [as part of the tanning process (see Pes. 45b:19)], [if the flour was placed there less than] three days before Passover, behold, he needs to remove [the flour]. Others say, once he placed hides inside [the bowl], there is no need to remove it [since the flour is no longer edible]. An eye salve (קִילּוּר not קלור, see Minchat Bikkurim) or a compress or a plaster into which flour is mixed, there is no need to remove it. A poultice [containing flour] that became putrid, there is no need to remove it. The chamasit (=חמסית, a mustard-like plant, see Minchat Bikkurim; alt., חרוסת (charoset), see GR"A) into which flour is mixed, behold, it is forbidden. Loaves [made from leavened] dough that became moldy or that dried out, behold, they are forbidden.