Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 461…
“ The Laws of Baking Matzo דיני אפית המצה Paragraph 1 - One must be careful when firing the oven used to bake hametz in order to bake mazto in it. The coals must go all around, and not just the flame, because just as it absorbed [hametz] by coals, so must it emit [hametz] by coals, and there must be sparks emerging from it. Therefore, even if one fired it a few times before Passover, it is not sufficient unless one intended to render it permitted to be used on Passover, because there is concern that the coals did not go all around when one did not have the right intent. ”
