Where does the notion that the *entire* matza process must be done in under 18 minutes come from?

Question

I've been going over the halakhoth for baking matzah, and even though I hear from almost every Rabbi I've asked that the entire process must be done in under 18 minutes, I can't find any evidence for this claim in any halakhic works. And this notion isn't coming from just my local Rabbeim, this idea appears in books intended for mass audiences, online Orthodox Rabbi Responsa, and e-mails from the OU, published videos from matzah bakeries, etc. Here is a photo of a Pesach guide from my local Beith Din:

But when looking in all the major codes of halachah, they are all in agreement that the eighteen minutes refers to something completely different. All the major halakhic works use 18 minutes in reference to leaving the wet dough unattended.

According to the Mishneh Torah, one can knead the dough the entire day and it will never become chametz.

Rambam's Mishneh Torah - Chametz U'Matzah - Chapter Five

Halacha 13

As long as a person is busy with the dough, even for the entire day, it will not become chametz. If he lifts up his hand and allows the dough to rest so that [it rises to the extent that] a noise will resound when a person claps it with his hand, it has already become chametz and must be burned immediately. If a noise does not resound and the dough has lain at rest for the time it takes a man to walk a mil, it has become chametz and must be burned immediately.

Source: http://www.chabad.org/library/article_cdo/aid/937304/jewish/Chametz-UMatzah-Chapter-Five.htm

This is also the opinion of Yosef Karo and the Rema in the Shulchan Arukh

סעיף ב - לא יניחו העסה בלא עסק ואפלו רגע אחד. וכל זמן שמתעסקים בו, אפלו כל היום אינו מחמיץ. ואם הניחו בלא עסק שעור מיל, הוי חמץ. ושעור מיל הוי רביעית שעה וחלק מעשרים מן השעה. הגה: ויש להחמיר למהר בענין עשית המצות, כי יש לחש שהשהיות יצטרפו לשעור מיל או שיהיה במקום חם שממהר להחמיץ (הגהות מיימוני פרק ה' ומרדכי פרק אלו עוברין) ואחר שנתעסקו בבצק ונתחמם בידים אם יניחוהו בלא עסק מיד יחמיץ

Siman 459:2 - The dough should not be left idle for even a moment. As long as it is being used, even the whole day, it will not rise. If it is left idle for the time that it takes to walk a mil, it is hametz. A mil is a quarter of an hour plus a twentieth of an hour. Rem"a: One should be stringent to hurry when making matzos, because there is concern that the waitings will aggregate to a mil or that one will be in a warm place where rising happens quicker. After dealing with the dough and warming it with your hands, if it is left idle, it will rise immediately.

Source: https://en.wikisource.org/wiki/Translation:Shulchan_Aruch/Orach_Chaim/459

The Mishnah Berurah doesn't mention anyone who argues with these, so I assume he agrees. But the Mishnah Berurah does go on to say that "it becomes chametz immediately" doesn't mean immediately but rather "a short time" and it shouldn't be left unattended for even one moment lechatchila, but should be rolled out and immediately put in the oven.

מיד יחמיץ - ולפי זה צריך ליזהר מאד לאחר שערכו ורידדו המצה ומניחים אותה לפני המנקר שינקר אותה מיד וגם אחר הניקור יראו לרדותה תיכף לתנור כיון דלאחר שנתעסקו בה מחמצת מיד שמסלקין את הידים ממנו והעולם אין נזהרין בזה כ"כ ואפשר דמיד דקאמר המחבר לאו דוקא אלא ר"ל שיעור מועט ומ"מ לכתחלה בודאי יש ליזהר מלהניחה כך אפילו רגע אחד אם אפשר וכנ"ל בראש הסעיף ועיין מ"ש בביאור הלכה:

Source: https://he.wikisource.org/wiki/%D7%9E%D7%A9%D7%A0%D7%94_%D7%91%D7%A8%D7%95%D7%A8%D7%94_%D7%A2%D7%9C_%D7%90%D7%95%D7%A8%D7%97_%D7%97%D7%99%D7%99%D7%9D_%D7%AA%D7%A0%D7%98#.D7.A1.D7.A2.D7.99.D7.A3_.D7.91

Consistent with these sources, in this Q&A, the OU refers to the 18 minutes as a limit to how long the mixture can be "left unattended."

In all of these works, the number of eighteen minutes ONLY refers to being left idle, and is NEVER used to convey an entire time in which the process needs to be completed. Even if you say that once the dough is warm it must be baked immediately after working with no idle time, you could still be kneading the dough for 3 hours, put it right in the oven, and still have kosher matzah.

So where does this notion (or possibly myth) of "must be finished baking within 18 minutes of flour contacting water" come from?

Answer 1

This does not include the time in the oven, but the notion that the entire process until the dough goes into the oven must be completed within 18 minutes is based on actual opinions on the books.

I found the sources cited below and got help in understanding and contextualizing them via the following contemporary English digests:

• R' Eliezer Melamed, Peninei Halachah, Pesaĥ 2:4 "The Definition of Leavening Dough" - Footnote 3.

• R' David Brofsky, "The Laws of Pesach Shiur #2: Defining Chametz (1)."

Although Talmud Bavli (Pesachim 48b) rules, as described in the question, that dough will never become chametz as long as one is working it, and although most rishonim rule, accordingly, that one may go ahead and work dough indefinitely, there are rishonim who disagree.

Ria"z, as seen in the Shiltei Giborim on Ri"f Pesachim 15b - note 1, cites the Talmud Yerushalmi as disagreeing with the Bavli and saying that if dough is continuously worked for more than 72 minutes, it becomes chametz. On this basis, he rules that once dough has been worked for this long, it is forbidden.

Ritv"a, in his commentary on Pesachim 48b, agrees with the other rishonim that dough does not become chametz while it's being worked, even if for a long time. However, his understanding of the Gemara there is that the Sages nonetheless forbade, lechatchila, working the dough for more than 18 minutes, as a heuristic to prevent people from accidentally producing and consuming chametz.

Although these are minority opinions, R' Melamed says that later posekim tend to rule in favor of following them, thanks to the special severity of the prohibition of chametz on Pesach. For example, Aruch Hashulchan, in Orach Chayim 459:7, cites the Ria"z and (implicitly) the Ritv"a and concludes

ועם כי אין הלכה כן, מכל מקום במצה ראוי להחמיר.‏

And even if the Halacha is not thus, even so, with matza, it's appropriate to be stringent.

^{(My translation.)}

This line of reasoning supports adopting a policy of making sure to complete all pre-oven processing within 18 minutes. Even if dough that is processed for longer than this isn't yet technically chametz (according to any opinion in the rishonim, from what I can tell), it is off-limits as a matter of policy (or even Rabbinic law, according to Ritv"a), a status that is practically very similar to being technically chametz.