Materials related to: D. Sperber, Why Jews Do What They Do: The History of Jewish Customs throughout the Cycle of the Jewish Year, Ktav: 1999.
Our Rabbis taught: One may not wash barley on Passover; and if one did wash and they split, they are forbidden; if they did not split, they are permitted. R. Jose said: He can soak them in vinegar, and the vinegar binds them.
Samuel said: The halachah is not as R. Jose. R. Hisda said in Mar 'Ukba's name: It does not mean literally split, but [if they reach] such [a condition] that if placed on the mouth of a [wine] cask they will split of themselves.
But Samuel said: It means literally split.
Samuel acted in the vicinity of the home of Bar Hashu [on the view that] 'split' is meant literally.
Rabbah said: A conscientious man should not wash [grain].
Why particularly a conscientious man: even any other man too, for surely it was taught: One may not wash barley on Passover?
He says thus: He should not wash even wheat, which is hard.
Said R. Nahman to him: He who will heed Abba will eat mouldy bread. For Surely the household of R. Huna washed [it], and the household of Raba b. Abin washed [it].
"And Mor Ukba said: It does not mean literally split, but such that if placed on the mouth of a cask they will split themselves. But Samuel said: It means literally split. Samuel acted thus in a case, and they were split literally."
Seeing that in this matter the Halakhah has not been decided explicitly, we follow the more stringent ruling of Mor Ukba, because in doubtful cases regarding Torah laws we follow the more stringent opinion.
Some authorities claim that the Halakhah follows Samuel, because Mor Ukba was his disciple and the Halakhah is never decided like a student in place of the teacher. And decisions in actual cases carry greater weight. Rabbi Al-Fasi.
"And Rava declared: A conscientious man should not wash [corn]": i.e., one who is in control of his desires, and his desires do not control him, and he is meticulous in observing the commandments.
"The household of R. Huna washed [it], and the household of Raba b. Abin washed [it]": And now they have sent out a ruling from the academy that we are not experts at washing...
R. Perida enquired of R. Ammi, Whence is it derived that all meal-offerings, seeing that they were kneaded in lukewarm water, must be specially watched lest they become leavened? Shall we infer it from the Passover concerning which it is written, "And ye shall watch the unleavened bread"! -- He replied. In that very passage It is written, it shall be unleavened, that is, keep it so. But have you not utilized this verse to indicate indispensability? -- If for that alone Scripture would have used the expression 'It is to be unleavened'; why 'It shall be'? You may thus infer two things.
And regarding what you have asked, about purchasing flour from the market in times of scarcity, that we do not assume that it is forbidden, and one may thereby perform one's obligation. However, one should not do this in the first instance, because it states "And ye shall watch the unleavened bread" we require matzah that is guarded for the sake of matzah; i.e., guarding from beginning to end.
Prior to Passover, when the wheat for the matzot is ground, it is customary to send a Jew to sit behind the mill and guard, as it states (Exodus 12:17): "And you shall observe the ... Unleavened Bread" in connection with guarding for the sake of the commandment. The Jew does not have to stand there until it is ground; rather, what requires guarding is everything that he requires for the three matzot that are mandatory on the first night. However, the remainder does not require guarding. Even if a non-Jew is doing the milling and no Jew is observing him or standing over him, it is permitted.
"From the time of grinding" because at this time they bring the wheat into proximity with water (Asheri). This implies that where they use a donkey-driven mill or a windmill there is no need for guarding. Nevertheless, it is common there as well to rinse the wheat
With respect to matzot--the pious and the masters of good deeds of old, who used to harvest it and guard it from the moment of the harvesting for the sake of Passover. However, we are not so diligent, and it is our custom to guard them for the sake of the precept from the time of the grinding and onwards. And thus is it recorded in Orhot Hayim in the name of Asheri. Because at that time they bring them into contact with water, when they grind in a mill. And thus did the She'iltot write.
 The verb connotes to moisten the grain before grinding.
 Because then they turn leaven very quickly.
 Prevents fermentation.
 Then they are forbidden.
 And since those about which he was consulted were not actually split. he ruled that they were permitted.
 Lit., 'the whole world'.
 And consequently is slower to ferment than barley. Others who are not so conscientious may moisten wheat, for only barley is forbidden in the Baraitha.
 Lit., 'father'- a title of respect.
I.e., unclean bread, since the wheat was not washed.
 Infra 55a.
 They must be continually kneaded till the time of baking (Rashi).
 Ex. XII, 17; so according to Rabbinic interpretation. E.VV.: And ye shall observe the feast of unleavened bread.
 In connection with the meal-offering itself.
 I.e., guard it against its becoming leavened; v. Pes. 48b.