As we have learned, the difference between bread and matza is that the dough used for making bread has undergone a leavening process resulting from the fermentation of ingredients within the flour that have come into contact with water. In order to augment the leavening process, bakers customarily mix se’or into the dough, causing the dough to ferment more thoroughly and quickly. However, even without the leavening agent, if dough is left without kneading, it would ferment and rise. Therefore, when preparing matzot, one must work quickly to ensure that the leavening process within the dough does not begin.
As long as the dough is in motion, being kneaded, it does not become ḥametz. Even if the kneading were to continue an entire day, the dough would not become ḥametz, since kneading arrests the leavening process. However, if the dough sits motionless for 18 minutes, the leavening process will begin and all the prohibitions concerning ḥametz will apply to it.
This applies to normal conditions, but where it is hotter, the leavening process is accelerated and the dough becomes ḥametz in even less than 18 minutes.
Cracks appearing in the dough are a physical indication that the dough has become ḥametz. Even if 18 minutes without kneading had not yet passed, since cracks formed in the dough, it has certainly become ḥametz; apparently it was warm so it took less time to become ḥametz, or the kneading was inadequate and certain parts of the dough were neglected, causing those areas to become ḥametz. Even if there are very few cracks, and they appeared only in part of the dough, the entire dough is ḥametz. If no cracks appeared but the dough blanched, it is ḥametz nuksheh (hardened ḥametz; see next section), which is rabbinically forbidden (SA 459:2).
According to Rashi and Me’iri, however, one must check the amount of time that the dough has been sitting only when it is unclear whether or not the leavening process has begun: if more than the time it takes to walk one mil has elapsed, the dough is ḥametz. If one is certain that the dough is not ḥametz, even if it has been sitting for longer than the time that it takes to walk one mil, it is not considered ḥametz. Nonetheless, according to most Rishonim in any situation where the dough has sat for longer than it takes to walk one mil, the dough is ḥametz. This is the ruling in MT, Laws of Ḥametz and Matza 5:13 and SA 459:2. Pri Megadim’s introduction to §467 contends that such dough is complete ḥametz; eating it on Pesaḥ incurs the punishment of karet. Rashbatz writes that one must suspect that such dough has become ḥametz. In a warm place, dough becomes ḥametz in less than the time it takes to walk a mil, as written in Aguda in the name of the Ge’onim. This is the halakhic consensus, as it is also the ruling of Yerei’im, Hagahot Maimoniyot, and Mordechai. Additionally, Rosh writes that even if the dough became warm in one’s hands, the dough will become ḥametz more quickly. This is cited in SA and Rema 459:2 (see also Berur Halakha on Pesaḥim 46a). Although there is dispute as to whether or not the “sitting times” of the dough are combined, Terumat Ha-deshen rules that a full kneading of the dough cancels the previous sitting time, though merely poking the dough would not be effective. This is cited in MB 459:16.
Pesaḥim222222 48b states: “As long as the dough is being worked it cannot become ḥametz.” The vast majority of Rishonim, including MT, Laws of Ḥametz and Matza 5:3 and Tur and SA 459:2, explain that as long as one keeps kneading the dough it will not become ḥametz, even if he does so for the entire day. Yet it appears that the Yerushalmi disagrees with the Bavli and says that if one kneaded the dough for the amount of time that it takes to walk four mil, the dough is considered ḥametz. Baḥ cites Ri’az that ideally we should act in accordance with the Yerushalmi. There is an even more stringent opinion – Ritva’s – according to which as long as one kneads the dough at the appropriate pace so that he will complete the kneading in less time than it would take to walk one mil, the dough does not become ḥametz. However, if the kneading goes on for longer than this time, the dough is considered ḥametz. Even though the vast majority of poskim disagree with Ritva, they rule that lekhatḥila we are strict about finishing the kneading process within 18 minutes, as explained in AHS 459:7.