Machshirin, Chapter 4, Mishnah 10. This chapter has been working through cases in which liquid arrives on an object or on produce, and the question is always the same: did the owner want the liquid there or not? Water that comes with a person's consent, beratzon, is capable of giving hechsher, of preparing food to receive tumah, while water that comes against his wishes is halachically inert. Our mishnah applies that principle to a pile of logs, and it closes the chapter with a chain of cases that grow progressively more surprising.
The first case reads: "Eitzim shenaflu aleihen mashkin", wood upon which liquids fell, "veyardu aleihen geshamim, im rabu, tehorim", and then rain descended upon that wood; should the rain be the larger amount, the logs remain tahor. The Rav explains that the liquids spoken of at the start are tamei water. So visualize a stack of firewood with tamei water resting on it, water still sitting on the surface and not yet drawn into the wood. Rainfall now joins it and the two waters blend together. Because a person has no interest whatsoever in soggy firewood, that rain descends entirely against the owner's will, which means it neither carries tumah nor can it acquire any. When the quantity of rain surpasses the tamei water, the tamei water is swallowed up and voided within it, leaving the woodpile tahor.
The mishnah immediately shows the reverse: "Hotziam sheyerdu aleihen geshamim, af al pi sherabu, temei'in." If he carried the logs outside precisely so that the rain would fall on them, then even if the rain was the greater quantity, everything is tamei. Here the owner wanted the water. His intention turns the rainfall into something he values, and once it is beratzon the rain is no longer neutral. The moment it makes contact with the tamei water lying on the logs, it takes on that tumah, and the whole pile together with its liquid is tamei.
The next clause presents a situation in which no tamei water remains exposed at all: "Bal'u mashkin temei'in", the wood soaked up the tamei liquids, so that its surface is now dry, "af al pi shehotziam", and even supposing he brought the logs outdoors, "sheyerdu aleihen geshamim, tehorin", with the express purpose that rain should come down on them, they stay tahor. That the rain remains tahor requires little explanation, because there is nothing tamei left on the surface for it to meet and merge with. The novelty lies elsewhere: the wood too remains tahor. Wood is not a utensil, it does not have the standing of a kli, and as it stands now it simply lacks any receptivity to tumah. Therefore, although the entire act was performed willingly and with full intent, once the tamei water has been drawn inside and nothing but rain reaches the surface, no tumah takes hold.
Still, the mishnah appends a warning: "Lo yasikem", he may not use these logs for a fire, "ela biyadayim tehoros bilvad", unless his hands are tahor. Keep in mind the rabbinic institution of tumas yadayim, whereby hands themselves are treated as bearing tumah. Having deliberately set the logs out in the rain, their wetness is something he desired, and damp wood grasped by such hands will absorb tumah from them. From there the logs are fed into the oven, and they are capable of conveying that tumah to the oven. Ovens of that era were typically fashioned from clay, and a clay vessel has no path to taharah other than being broken apart. Forfeiting a whole oven is a heavy price for a small convenience, which is why the mishnah directs that this firewood be handled exclusively by one whose hands are tahor.
The mishnah ends with a lenient ruling: "Rebbi Shimon omer", Rebbi Shimon teaches, "im hayu lachim vehisikam", that where the wood was still damp rather than dried out and he lit it, "verabu hamashkin hayotz'im meihem", and the liquid driven out of the wood came to a greater measure than "al hamashkin shebal'u, tehorin", the liquid it had earlier taken in, then everything is tahor. Unseasoned wood retains its own sap, and setting it aflame forces that moisture out. Should the wood's own escaping moisture outweigh the tamei liquid that had been absorbed within it, that escaping moisture voids what was absorbed. Consequently the oven does not become tamei. This holds true even where the person who brought the logs had tamei hands, and even after they are already ablaze in the oven: provided the measure emerging is greater than the measure that was taken in, the entire situation is judged tahor.
With that ruling of Rebbi Shimon, the fourth perek of Machshirin comes to a close.