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.