The flour for matzot is milled at least one day before it is kneaded into dough, since milling heats the flour slightly, and if they do not wait, there is greater concern that the dough will become ḥametz (SA 453:9). No salt or pepper is added to the dough, since they might warm the dough, increasing the risk that it will become ḥametz (SA 455:5-6).
Le-khatḥila, one should not make matza dough with more than c. 1.5kg (c. 3.3 lbs.) of flour (the shi’ur for having to separate ḥalla from the dough with a berakha), as it is hard for one person to knead such a large piece of dough thoroughly and quickly, and there is concern that parts of the dough may become ḥametz. Be-di’avad, if one kneaded a larger quantity, the matza is kosher as long as the dough did not rest for 18 minutes and no signs of leavening appeared (SA 456:1-2).
When there are several people engaged in kneading, flattening, and rolling out the dough, some poskim maintain that it is permissible to knead larger quantities, and indeed many do so. Even so, initially it is proper to be stringent and not to knead more than the measure that the Sages fixed (MB 456:7). When the kneading is done by machine, it is customary, even le-khatḥila, to knead large volumes of dough.
One may not knead the dough in a hot place, since heat accelerates the leavening process. Therefore, one should not knead in a sunny place, and during a sharav (ḥamsin; a heat wave during which the sun is obscured by clouds and sand), one should not knead the dough outside or inside near to windows, because even though most of the sun’s rays are obscured by clouds, the whole sky radiates intense heat. Obviously, one should not knead in a place warmed by an oven (SA 459:1). Be-di’avad, if dough was kneaded in a hot place but no signs of leavening were seen in the dough or the matza, the matza is kosher for Pesaḥ (ibid. 5).
Le-khatḥila, one may not stop working the dough for even a moment (SA 459:2). If the hands of the person kneading the dough heat up, he should cool them in cold water. Some are careful to cool their hands in water occasionally while they are kneading (MB 459:27).
