110:2 The accepted practice is to plaster the oven with new clay so it need not be kashered by heating it up. In this way, the chometz that was absorbed in the oven’s roof and sides will be driven out by the heat of the flames that bake the matzos. This is a good practice but it only works if one plasters the oven at least one finger-breadth over all surfaces; a thin layer doesn’t accomplish anything. (After the fact, the Pri Megadim permits a thinner layer – Bi’ur Halacha 461:1 s.v. karka chadash.)
Please note that the ovens discussed in this chapter do not resemble our modern ovens.
110:3 One may not knead or make matzah in a room without a roof, nor opposite an open window, even if the sun isn’t shining through it. (One may bake matzah outside, in the shade – Mishnah Brurah 459:5.) If the windows are closed and they have glass, it is permitted provided that the sun isn’t shining through it. If the sun is shining through it, then the window glass doesn’t help. In such a case, one must hang a curtain where the sun is shining. One must likewise ensure that the room is not heated and warm. (If the room is so hot that the dough gets warm, the matzos may not be used – MB 459:41.)