Makhshirin, Chapter Four, Mishnah Ten
1) If [unclean] liquids fell upon wood and rain came down upon it and [the rain water] exceeded [the liquids] in quantity, they are clean.
2) If he took [the wood out] so that rain would come down upon it, they are unclean even though [the rain water] exceeded in quantity.
3) If [the wood] had absorbed unclean liquids, even if he took the wood outside in order that rain should come down upon it, it is clean.
a) But one may not light the wood in an oven except with clean hands.
b) Rabbi Shimon says: if the wood was moist and then he lit it, and the liquids that came out of it exceeded in quantity the liquids which it had absorbed, they become clean.
Section one: Two liquids fall on this woodrain and unclean liquids. Rain does not cause susceptibility to impurity, unless one wants something to get wet. Unclean liquids do cause susceptibility. If the rain water is greater in quantity than the unclean water, the wood remains pure.
Section two: If he wanted the wood to get wet, then the rainwater does cause it to be susceptible. Since unclean water fell on the wood, the wood is now unclean. We should note that others explain that the water wasn’t unclean, just that it caused susceptibility to impurity. Therefore, the wood wouldn’t be actually unclean. It would only be susceptible to impurity.
Section three: In this case, the wood absorbed the unclean liquid and this liquid can no longer be seen. The wood was therefore never susceptible to impurity. When the rain comes down upon it, it is not defiled, even if he took the wood outside so that it would get wet.
However, when he puts the wood into the oven to light it he should make sure his hands are pure. If his hands are impure, his hands will defile the liquids on the wood and then the liquid on the wood could defile the oven.
Rabbi Shimon says that if the wood was moist (meaning freshly cut) and then he lit it in the oven and more water came out then the quantity that it absorbed from the impure water, then the impure water that comes out is considered clean. Don’t even ask me how one would practically measure such a thing, I have no idea.