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2. Spreading a cloth over an open bottle:
Over a bottle of clear liquid: It is forbidden to spread a cloth over a barrel of water in order to cover it, as it may get wet from the water, and one will then come to squeeze it and be liable for [the] whitening [prohibition]. [One may also not place a wet vessel on top of the cloth.]
A cloth designated for covering: However, a cloth which is designated to be used to spread over the [opening of the barrel] is permitted [to be spread] being that one has no care to squeeze it if it gets wet as it is designated for this purpose.
Over a bottle of colored liquid: However it is permitted to spread a cloth over a barrel of red wine or of oil or beer and other beverages of the sort [which are not clear].
Initially dipping a cloth in colored liquids: Furthermore, it is even permitted to initially soak the cloths in these liquids with one’s hands such as to filter colored liquids over this cloth.
The reason that we do not decree against initially soaking the cloth in even colored liquids due to that one may come to squeeze it: Therefore the [Sages] did not decree against soaking a cloth in liquids which do not whiten due to a decree that one may come to squeeze it and be liable for “detaching”, as one is only liable if he needs the liquids being squeezed, of which there is no remote suspicion [here] that one may squeeze the cloth for the sake of the liquids that will come out from it being that [these liquids] are not of any significance and it is not at all common to do this.
Spreading a cloth over an open bottle:
A bottle containing white liquid: Is Rabbinically forbidden to be covered unless the cloth is designated for this purpose in which case it is allowed.
A bottle containing colored liquid: Is permitted to be covered.
 Admur 320:21; Michaber 320:15; Shabbos 48a
 Admur ibid; Rashi ibid; Rambam; Rabbeinu Yerucham; M”B 320:42
 Admur ibid; Rambam 22:25; M”B 320:42 in name of Rebbe Akiva Eiger
 This is the original case in the Michaber ibid and Shabbos ibid
 Admur ibid; M”A 319:11; 510:16; Tosafus Beitza 30a
 Admur ibid and 319:13 and 301:59; See M”A ibid; Tosafus ibid; Smag 65
 Admur 320:21; 301:59; 319:13; M”A 319:11; Taz 320:12; Rabbeinu Tam in Sefer Hayashar; Tosafus Beitza 30a; Shabbos 111b; Rashba; Rivash; Rav Hamaggid; Implication of Michaber 320:18; Biur Halacha 320:18 “Yeish Mi Shematir”
Other opinions: Some Poskim rule there is no difference between water and other liquids, and one is Biblically laible for Milabein on the squeezing of all liquids [Ran brought in Taz 320:12; Riy brought in Tosafus Shabbos ibid; Rmabam 2:11; See Beis Yosef 320; Avnei Nezer 158:12; Biur Halacha ibid]
 Admur ibid; Taz 320:12