At 09:38 AM 3/9/2010, Chris S wrote:
>Are you saying that the temperature of water ice which comes out of a
>0F freezer is 32F?  Methinks not.

No, it's zero; but it rapidly rises by absorbing energy at the rate
of one calorie (1/1000 of the Calorie or kilocalorie that dieters
work with) per degree C until it reaches 0C/32F; at that point temp
remains constant while it absorbs an additional 85 (?) calories per
gram to change into liquid water still at 0C/32F.  Therefor water ice
cannot effectively cool below 32F and has trouble reaching that.

Adding rock salt (or many other things) to the melting ice lowers the
melting point by an amount proportional to the amount of solute
(salt), and the ice begins melting (and absorbing its 85 calories per
gram) at a lower temp than 0C; with rock salt this trick can get the
overall temp down to 0F in ideal conditions.  15F or so would be
about right for soft ice cream coming from the churn as colder than
that makes it too stiff to churn.  Normally then more salt would be
added and the mixture cooled for a couple hours as close to 0F as
possible to make hard-ish ice cream.  Ice cream actually at 0F or
below is usually quite hard.

Calcium chloride (fancy ice melt) would be ideal for this except that
the act of dissolution generates large amounts of heat -- you can
easily get 180F by making a concentrated solution.

Yours,
David