Question

It is stated in the Talmud (Zevachim 94a) that from the verse, “and the garment … or any leather vessel that you shall wash,” we learn that leather is subject to washing. And its washing is done by rubbing. Rashi explained there (94b): “Rubbing” means rubbing one side against the other in the way one washes clothes: he holds the garment with both hands and rubs it. And so it is also stated in the Shulchan Arukh (Orach Chayim 302:9) that it is permitted to put water on leather, but it is forbidden to wash it, meaning to rub one side against the other. And in the Biur Halakhah (s.v. zeh bazeh) he added that even if one does not rub one side against the other, but instead scrubs the dirty spot thoroughly with water and rinses it so that it becomes clean, that too is included in rubbing.
And it is explained in the Talmud there that soft hides are fit for washing, whereas hard hides, according to the Rabbis, are not fit for washing. So the fact that they are soft is the reason they are fit for washing. And Rashi likewise explained in the Mishnah that a pillow is subject to washing, since ordinary pillows and cushions are soft, and therefore the concept of washing applies to them as soft leather. And the Meiri wrote similarly (Beitzah 18, s.v. in tractate Zevachim).
From this one can also infer regarding other materials common nowadays, such as plastic and the like, that if they are soft they are considered fit for washing. And the definition of “soft” may be based on what Rashi wrote above: the way one washes clothes is that he holds the garment with both hands and rubs one side against the other. In other words, with a soft thing in which the possibility exists of rubbing one side against the other—which is the main manner of washing—it falls under the category of washing. And one should be concerned about a Torah-level prohibition of washing with anything that has softness to that degree. (See also Igrot Moshe, Yoreh De’ah vol. 2, sec. 76, and Orach Chayim vol. 5, sec. 18, in the note to Orach Chayim 302:9.)
Even with a utensil that keeps its shape, if it is soft as described above, it is arguable that washing applies to it, for with a shoe made of soft leather, it is explained in the Talmud and in the Shulchan Arukh that washing does apply to it, as mentioned above.
Accordingly, one may discuss a disposable cup or plate, which are soft, and only because of the rim—which is stiff—one cannot rub side against side: whether washing applies to them when scrubbing the dirty area with water. Likewise, a pacifier and a baby bottle nipple and anything similar. And also soft aluminum foil and aluminum utensils.
As for hard leather, the medieval authorities (Rishonim) disagreed. See the Biur Halakhah (302:9, s.v. but to wash it), who writes that the Rif, Maimonides, the Rosh, and the Tur hold that it is rabbinically forbidden. But Rashi and the Ran hold that with hard leather there is not even a rabbinic prohibition. And the She’iltot rules like “the Others,” that it is forbidden by Torah law. The Biur Halakhah concludes there that one should not be lenient, because all the above medieval authorities wrote simply that it is forbidden and did not distinguish between soft and hard. According to this, as stated, one may discuss cleaning plastic utensils, even rigid ones, if one can scrub them forcefully.