Block Ice

Jack...if the freezer you make the ice in only gets to say -20C then the ice will be close to this temp....salt or no salt

Salt does not make the ice colder...

Duh!!!! "adding ice .... "

You are right, Nudenut - I meant to say "adding salt".
I will now take myself outside and thrash myself senseless with a wet bus ticket.

Cheers
jack

no dont do that Jack....
come here and i'll kick ya.....with me steel caps...........hehehehe
(no offence meant)

I just "lurve" a good steel cap kicking - wouldn't have a whip too would you :)

Jack

nope!

As a chemist, you should know that all the salt in the water does is move the triple point on the phase diagram. This refreezing of fresh water occurs at the expense of adding thermal energy to the frozen salt water, and raising the temp of the frozen salt water.
The temperature of saltwater at which ice forms can be lowered down to approx -21 deg C by adding up to approx 18% w/w NaCl.
This does not mean that the ice will last longer then fresh water ice, just that it will keep that temperature until all the ice is gone. The energy required to melt freshwater ice, compared to saltwater ice, at equivalent ice temps of say -30 deg C is greater, because you have not got 20 % of your ice mass as sodium chloride crystals....
Anyway, chilling your beer down to -21 deg C is enough to freeze it, assuming that you have enough saltwater ice. Not ideal...
cheers
Shaggy

If you read my post carefully i point out that a layer of frozen saltwater will give you a colder esky. What you say is partly true regarding the thermal energy of the fresh water freezing on the salt block. However, the salt ice temp will not raise, it will melt. Ice cannot go above it's melting point (which as you say veries depending on the salinity).
By having this layer (and not the whole esky) you preserve the fresh water ice for much longer by having a colder esky and so on. Although you may lose the saltwater blocks the fresh water blocks which take up the rest of the esky (greater volume - several layers) last much longer. I wouldn't use it to chill beer either, just for storing meat etc.

All true, except for one thing. Salt ice can raise temperature. If you read my post carefully, I made the assumption that your salt ice is at -30 deg C, and that the coldest temp that salt ice can melt at is -21 deg C. Therefore the salt ice CAN raise in temperature, from -30 deg C up
to -21 deg C, as you point out.
cheers