תנור שאופים בו חמץ צריך ליזהר כשיסיקוהו כדי לאפות בו מצה שילכו הגחלים על פני כולו ואין די לו בלהבה דכמו שבולע ע"י גחלים כך פולט ע"י גחלים ושיהו ניצוצות נתזין ממנו הלכך אם הסיקוהו כמה פעמים קודם הפסח אינו מספיק אא"כ כיון להתירו לצורך פסח כי שמא לא הלכו הגחלים על פני כולו כל זמן שלא כיון לכך ויש נוהגים להטיל בו קרקע חדש כדי שלא יצטרכו היסק ומנהג יפה הוא: הגה וכשיכשירו על ידי היסק טוב וישר הוא לחזור ולהסיק לצורך האפייה ולא לעשות הכל בהיסק אחד. (תשובת מהרי"ב):

An oven in which chametz was baked — when firing it to bake matzah one must take care that the coals spread across its entire surface, and it is not sufficient to use only a flame, since just as it absorbs through coals so it releases through coals, and sparks must fly from it. Therefore if it was fired several times before Pesach it is not sufficient unless one specifically intended to kasher it for Pesach, since perhaps the coals did not spread across the entire surface as long as one did not intend this. Some have the custom to apply new earth to it so that it does not require firing, and this is a fine custom. Rema: When kashering through firing, it is good and proper to fire it again for the purpose of baking and not to do everything in one firing. (Responsum of Mahariv)

A new ceramic roof tile — whether it was fired from the inside or outside, it is permitted, since the heat acts beneath it; even though the flame does not rise above it, it becomes very hot and the bread bakes immediately and does not come to leavening. However one must first fire the oven, or a kubiyah, or a dry frying pan, or on the ground before baking. But first attaching the dough and then firing — some forbid it and it is good to be careful. Some say one should protest against making bread baked in hot ashes.

Matzah that was baked to the point that when one breaks it no threads are drawn from it — one fulfills the obligation with it. Rema: One should be careful not to remove it from the oven before that point (D"E) and put it back, as it could come to leavening. (Maharil)

One fulfills the obligation with soaked matzah, provided it has not become dissolved; but if it was cooked one does not fulfill the obligation with it.

If chametz and matzah were baked together, the matzah is not forbidden unless it touched the chametz — one removes from the place of contact the amount required for removal, and the rest is permitted. Rema: Matzah that folded over in the oven and is stuck together such that the heat does not penetrate there — it is forbidden during Pesach, but the other matzot in the oven are permitted. Before Pesach one need only forbid the place of the fold. (Terumat HaDeshen §157 and his rulings §145) Matzah that puffed up in the middle is forbidden; but if a crust formed over it in the way that a crust normally forms over bread during baking, it is permitted. (Maharil) If two matzot are lying on top of each other in the oven before they are baked, they are forbidden as it is like a folded matzah. (Mahariw) Similarly one must take care that they do not touch each other in the oven while still moist, as the heat does not penetrate the place of contact and they come to leavening. (Beit Yosef in the name of Orchot Chayyim)