Siman 456

One may not knead for Pesach a dough larger than the shiur of challah, which is an isaron, a tenth of an ephah. These are like the shiur of forty-three and a fifth eggs, from medium eggs of a hen's egg. And how shall he measure? He should take a vessel full of water, and pour the water into another empty vessel, and place the forty-three eggs in the first vessel, and afterwards he should return and pour the water back from the second vessel to the first. And the water remaining in the second—these are the measure of the eggs. And he should add a little to them for the fifth of an egg. And like this shiur, meaning a vessel that holds like the shiur of this remaining water—these are the shiur of challah. And among us, it is established that three pounds of flour are the shiur of challah. And in Yoreh Deah siman 324 this was further explained, see there.

And the reason for the matter: because even though there is a measure for dough left without engagement as to how much time it takes to become chametz, as will be explained in siman 459, nevertheless, ab initio one should not leave the dough without engagement even for a single moment, as I wrote there. And the sages estimated that dough which is in its quantity more than the shiur of challah—the hands will not suffice to engage with the entire dough, and of necessity some part of the dough will be without engagement. But with the shiur of challah—one is able to engage with the entire dough at once. And even though for the Lechem HaPanim they would knead two esronim at one time, for behold each challah was of this shiur as is explicit in the Torah—we do not learn from the Mikdash, for the Kohanim are diligent. Therefore, post facto, when they kneaded more than this shiur—we do not forbid it. For granted that a portion was without engagement, nevertheless it would not reach the shiur of fermentation. And there are those who ab initio knead more than the shiur, and it is forbidden to do so. And even though the sages of France forbade it even post facto, as the Tur wrote, nevertheless the Rambam, and the Rosh, and the Tur, and the Shulchan Aruch permitted it post facto. And such is the halacha. (Regarding what the Magen Avraham challenged the Rosh in subsection 6, see Chok Yaakov subsection 9 who rejected this.)

And the Tur wrote that one of the great early authorities used to say that this measure was only in their days, when the ovens were small. But now that the ovens are large—it is much better to knead a large dough. And they did not agree with him, see there. That is to say: he holds that the reason for this decree is not because of the kneading, but because a large dough would not fit into the oven; and they would need to wait with part of the dough until the previous part baked, and in the meantime it would become chametz. But in a large oven—a large dough is better, so that the oven will be full of bread and it will bake well (Beitzah 17a). And perhaps this is the reason for those who knead more than the measure ab initio, which we mentioned in the previous section. But they rejected his words because the reason is due to the kneading, as we have explained.

The measure that holds the amount of the shiur of challah – one should fill it with flour without a heap, for it must be leveled and not heaped. For if it were heaped – would there not be more than the shiur of challah? And its shiur: our teacher, the Beit Yosef, wrote that its shiur of Egyptian wheat flour is approximately five hundred and twenty Egyptian dirhams. And we do not know this shiur. And one should not press the flour into the measure, for if so, it will not be kneaded well, and the flour will enter the cooked food and become chametz. Rather, one should measure by scattering in the manner of sellers, and one should not place his hand upon the flour. And one should not do so even if he does not increase the shiur beyond the shiur of challah, for if not so – in any case it is forbidden ab initio. And it is good to say at the time of placing the flour, and likewise at the time of placing the water into the flour: "For the sake of the mitzvah of matzah."

Even though it is forbidden to measure flour for kneading on Yom Tov, nevertheless, if one bakes on the Yom Tov of Pesach – it is permitted to measure, so that he does not bake more than the shiur of challah. For this is a measurement for a mitzvah. And there are those who forbid, for the measurement is not indispensable, as if it is slightly less or slightly more – there is no hindrance in the matter. And he should take according to his estimation, and such is the custom.