Hagalas kelim in "tzir"

Question

When one wants to make celim (vessels) Kosher by purging non-Kosher absorbed tastes from it, one method is hagaleh, which normally is done with water. The Rema says in SA OC Siman 452:5 that b'dieved (if the ideal cannot be met) it helps to make hagalas kelim in "shar mashkim" (other liquids; ie., not water).

However, the Rema writes that if someone had made hagaleh on a lot of celim and from the "rov plitas hakelim" (from the large amount of stuff being purged from the vessels) the remnant water that became like "tzir" (brine) shouldn't be used for hagaleh.

The Mishnah Brurah there (SK 27) writes that this rule applies even b'dieved, meaning that not only is it not the preferred way to do hagaleh, but it simply doesn't work. The reason is because at this point there is no more strength in the water (meaning the water that is now briny) to be maflit (take out) what is balu'ah (absorbed) inside of the cli.

However why shouldn't hagaleh in such water and tzir be like that of shar mashkim that b'dieved would help?

Answer 1

^{{This is not a comprehensive answer, but is meant to provide information to help the OP discover the underlying reasons of the RaM"A's (and Mishnah Berurah's) opinion if a comprehensive explanation is not found in later Poskim.}}

The RaShB"A and the Ba'al Ha'Itur (Tur Y"D 93) both assert that one can use other things than water to do Hag'alah, the Ba'al Ha'Itur going so far as to permit someone to turn a meat Ben Yomo pot into milk by boiling milk in it (actually, the first opinion he mentions allows you to use that pot for milk or even use it again for meat).

Their opinions are slightly different (see B"Y and Ta"Z on Y"D 93), but their underlying concepts would seem to prevent one from (re-)using the water that has become like Tzir from earlier Hag'aloth). The RaShB"A basically allows you to cook vegetables in a meat pot and then use the pot for either meat or milk. The Ba'al Ha'Itur (in the B"Y's explanation) allows you to boil milk and pour it off while it's still boiling. The difference of opinion boils down* to whether or not the different substances doing the Hag'alah have the strength to draw out Beli'oth (absorbed tastes) from the walls of the Keli (vessel).

The RaShB"A's reasoning is (according to the Ta"Z) that the tastes get weaker, and the Ba'al Ha'Itur's reasoning is that while something is Poleit (drawing out tastes), it cannot be Mavlia' (impart its own tastes). In either case I would think that Tzir would pose a serious problem, because the "water" (which is now basically fatty, dirty, salty brine) cannot effectively draw out Beli'oth.

See also Y"D 69:8, where the RaM"A argues with the Mehaber that Tzir is actually hot and is Mavlia' (which means it cannot be Poleit). This could be RaM"A LeShitatho (following his own line of reasoning throughout).

*No pun intended