According to
the Gemara, there are two reasons for washing mayim acharonim. Firstly, washing
one’s hands removes the dangerous melach sedomis (Eruvin 17a and Chullin
105a). Secondly, one should wash one’s hands before reciting a beracha (Berachos
53b).
While Rambam (Brachos 6:2) and Shulchan Aruch (OC 181:1) both pasken that one
should wash mayim acharonim, the Shulchan Aruch (181:10) brings Tosafos’s
opinion that defends the practice of those who don’t, as we no longer need to
be concerned about melach sedomis. The Rema (Darkei
Moshe OC 181:1) writes that this is the
normative askenazi custom. R’ Yaakov Emden (Mor Uketzia 181) explained that nowadays when people eat with cutlery,
one doesn’t need to be so concerned about melach sedomis.
Nonetheless, the Mishna Berura (181:22) writes that the
Vilna Gaon was particular to do so, and that ideally askenazim should wash. Although
the Aruch Hashulchan (OC 181:4) writes that one doesn’t need to wash if one’s
hands are clean and they wouldn’t normally wash them after a meal, he carries
on to say that following the poskim, one should be careful to wash mayim acharonim and encourage one’s family to do so, too.
While R’ Ovadia Yosef (Yalkut Yosef 3:181:2) and R’
Ephraim Greenblatt (Rivevos Ephraim 1:140:3) hold that women are also obligated
to wash, R’ Shmuel Wosner (Shevet Halevi OC 4:23) writes that mayim acharonim is a chumra
that was never practiced by women (See Teshuvos Vehanhagos 1:174; Piskei Teshuvos 2:181:1).
Many are
particular to cover up or remove the used mayim acharonim for Kabbalistic
reasons (Kaf Hachaim OC 181:8). According to this view, one should specifically
only use a few drops of water (Ben Ish Chai, Shelach 8). Interestingly,
the Ben Ish Chai (ibid 7) writes that one should say the 3 words of this
Halacha, Mayim Acharonim Chova, and thereby fulfil one’s obligation to
learn at the meal, too.
The ashkenazi
poskim don’t follow this view, however: The Aruch Hashulchan (181:7) and Mishna
Berura (181:10) both write that one must use a revi’is (See Ma’ase Rav
84).
Thus, while sefardim (who follow the Ben Ish Chai) should use the small mayim acharonim sets that are both small and hide
the used water, ashkenazim who wash, should wash with a regular cup.