109:9 People used to spill out water if someone died suddenly or if it sat out during an equinox. This was not done with the water drawn for baking matzah based on Koheles 8:5, “The one who keeps the commandment will experience no evil.” In any case, if one knew in advance that the equinox would occur, he was advised to put a small, clean piece of iron – like a needle – into the water. He should suspend it on a thread so that he could later pull it out by the thread instead of having to stick his hand into the water. (Nevertheless, if one could easily replace the water, it was advisable to do so for the other matzos – Mishnah Brurah 455:18.)
110:1 If one wants to bake matzah in an oven that had had chometz baked in it, the oven must first be kashered by purging it with fire. This is done by heating the oven until sparks fly from it; less than that is not sufficient. One must make sure that the coals are spread all over it. It’s appropriate to remove the coals and to clean the oven thoroughly after, waiting until it has cooled down a little and then spreading new coals for the matzah baking, not doing so immediately after the kashering.
Please note that the ovens discussed in this chapter do not resemble our modern ovens.