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Having learned some of the sources for baking matzah (OH 453-461, more or less), I discovered something that surprised me.
The time for how long it takes to (possibly) become chametz is given as a shiur מיל, and people usually use 18 minutes for that time. That time begins when you add the water to the dough, and only passes if you are not working the dough.
Still, in order to avoid questions, we should finish the process within 18 minutes.
However, it is clear in Siman 461 that the process ends when you put the matzah in to bake. The baking time does not count. The way the poskim describe it is that the matzah begins to bake "מיד", immediately, and will not become chametz thereafter.
That is the reason that we hold you should - if you bake on a dry griddle - heat the griddle first (מרתיח ואח"כ מדביק) and then add the dough to it. Otherwise it isn't going to start baking right away.
The Shulchan Aruch Harav there even says a remarkable thing:

וכן שלא יאפה בחמה שיש לחוש שמא יחמיצו קודם שיתחילו לאפות כו' אבל אם עבר ואפה בחמה אף על פי שמותרות באכילה דכיון שהחום גדול כל כך עד שיוכל לאפות אין החום מניחן להתחמץ אלא מיד הן מתחילין להאפות
So too you should not bake in the sun... if you did violate this and bake in the sun, even though it is permissible to eat [He'll explain later that you can't use it for the mitzvah the first night] - because if the heat is great enough that it is able to bake, the heat will not let it become chametz; it begins to bake immediately...

He is saying that it is a logical consequence of the fact that it can bake at all. The clock stops.

My question: Does anyone disagree with this? When we talk about 18 minute matzos, do we always mean, until the matzah goes into the oven?

-Update: I saw in the Chayei Adam 128:25

אך צריך ליזהר הרבה שלא יוציא מן התנור עד שתהיה נאפה יפה ושיהיה התנור מחומם הרבה שלא תבא לידי חימוץ
One needs to be very careful not to take it out of the oven before it is well baked, and that the oven should be very hot, so that it should not come to חימוץ.

Why does the oven need to be "very hot", if there is no danger of חימוץ once it is baking? How could it be worse than baking it in the sun?
--Update update: Maybe he means, if you're trying to make a lot of matzah fast, there's going to be a temptation to take it out of the oven too soon - as his sentence began - unless the oven is very hot and speeds up the process.

Update 3: I see that the Igros Moshe 1:153 quotes this Chayei Adam (I think he means this one, though the sefer points to a slightly different paragraph that I think must be a mistake, 27 instead of 25), as evidence that the oven needs to have a minimum temperature.

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    Modern thin cracker matza are done in well under 18 minutes and are in the oven for just a few seconds, so this detail is largely theoretical in those factories.
    – Double AA
    Jul 9, 2023 at 23:53
  • When they advertise 18 minute matzah as far as I've seen it always means baked in under 18 minutes. Regular kasher for Passover matzah has the matzah dough mixed by machines for large periods of time until it's ready to be baked. Because halakha is you have a shiur from the moment you stop kneading the dough
    – Aaron
    Jul 10, 2023 at 0:29
  • @Aaron Really? Machine matzah is kneaded for more than 18 minutes?
    – MichoelR
    Jul 10, 2023 at 2:52
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    @MichoelR If it's listed as Kasher for Passover but NOT 18 minutes then I believe it is. I was told so by Rabbi Dovid Fink who is a Posek in Jerusalem
    – Aaron
    Jul 10, 2023 at 16:48
  • @MichoelR Why would you think otherwise? The Shulchan Arukh, Rambam, and I believe even the Rema hold that you can knead the dough all day and it won't rise.
    – Aaron
    Jul 11, 2023 at 1:24

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You're asking two questions

  1. Does anyone argue with the idea that the clock stops once you put the matzah in the oven.

My answer. None of the big sources take an issue with this. I'm sure you can find many sources in more modern times who want everything to be done within 18 minutes as a matter of stringency, but the halakha is exactly what it says in the shulchan arukh. The reason the shulchan arukh is confident that the time ends once you put the bread in oven or on the cooking surface is that you were already supposed to ensure your cooking method will cook the matzah. Your oven is either pre-heated, or your saj/wok/frying pan/ roof tile/ any flat cooking surface already has a fire lit under it and is hot to the touch. If you have pre-heated your cooking instrument, then the 18 minutes stops the moment you start cooking your matzah. If this seems too easy, that's kind of the point. Everyone used to do this, even the uneducated amongst Jewry, men and women alike. It's only in the past few hundred years that suddenly we need experts to cook a flat bread.

  1. Why does the oven need to be "very hot", if there is no danger of חימוץ once it is baking? How could it be worse than baking it in the sun?

There is danger of chameitz if your cooking temperature is too low. Bread will rise faster the hotter it is UNTIL it reaches a temperature that the bread cooks and the yeast is killed off. If you are unable to get your matzah to reach that critical temperature from the exterior to the interior of the bread then you won't be making matzah, you will be making chameitz.

enter image description here

In the above picture you can see how this bread did not finish cooking through. This would definitely be chameitz and we are required to burn it if we have made it during the hag. In order to avoid having a bread that is still chameitz in the center one must ensure they do not prepare their matzah to be too thick, and that they do not cook at too low or too high of a temperature. Although no one discusses it, it is entirely possible to have the oven be so hot that you burn the outside and don't fully cook the inside. This usually occurs with matzah being made too thick, remember according to halakha one can bake matzah up to a tefach thick.

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  • I think you and the Igros Moshe (1:135) that DoubleAA brought are introducing this same issue. It would be good if there were sources.
    – MichoelR
    Jul 25, 2023 at 12:10
  • And, the picture is of bread that was already chometz before you started baking it. Here we are discussing dough that wasn't, and the question is When does the baking equipment stop that from happening. Again, logically I agree with you but it would be nice if there was a source.
    – MichoelR
    Jul 25, 2023 at 12:12
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    @MichoelR I have to use a chameitz picture because there is no one left who makes thick matzot. In our days it's almost impossible to have a situation in which there is a large enough center of dough to not cook since everyone makes thin matzot
    – Aaron
    Jul 25, 2023 at 18:04
  • I'm thinking that you are pointing out an issue that is not much discussed, because it is hidden in one facet of the halacha: that the resulting matzah has to break cleanly without connecting "threads". If that will never happen, obviously your setup doesn't work. If it will happen, it sounds like it doesn't matter how long it takes.
    – MichoelR
    Jul 28, 2023 at 2:29
  • I am wondering, by the way, if all the sources take for granted that the matzah is baking on a hot surface. That's all the examples, maybe that's the way anyone used to bake anything. Would they be so sure that it starts to bake "immediately" if you put the matzah on a piece of tin foil on an oven grate inside the oven?
    – MichoelR
    Jul 28, 2023 at 2:36

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