Kelim, Chapter Nine, Mishnah Five
1) Potsherds that had been used for unclean liquids which fell into the air-space of an oven, if the oven was heated, it becomes unclean, for the liquid would eventually come out.
2) And the same with regard to fresh olive peat, but if it was old, the oven remains clean.
a) If it was known that liquid emerges, even after the lapse of three years, the oven becomes unclean.
Section one: The potsherd which was used in a vessel which contained unclean liquids would have absorbed some of that liquid. When it falls into a heated oven, the liquids will eventually come out and therefore the oven is unclean.
Section two: Olive peat is the olive waste that is left over after the olives have been pressed. If the olive press was unclean (or the pressing was done by unclean people), the oil is also unclean. Fresh olive peat will still retain some of this oil, so if fresh peat falls into a hot oven, the oven is unclean. The oil will emerge due to the heat. Old olive peat (after twelve months) no longer contains the oil and therefore impure oil will not seep out and defile the oven. This is true unless we know that liquid came out of the old olive peat. If we know that liquid came out, then it can defile the oven even if the peat is as old as three years.