I think the point that was made before, is that if you get slush, the
battle is already lost. You want to get hard ice, not slush, so that it
will be easy to separate it from the liquid beer.
1) You must use a metal container, such as a corny keg.
2) The metal container must be made to be much colder, much faster, than
the beer it contains, so that hard ice forms attached to it, instead of
floating freely in the liquid beer as slush.
The way to accomplish point 2 is to make the metal container to be in
contact with the cooling agent. In Dennis Davison's case, it was left
outside on a Midwestern January night - in direct contact with Old Man
Winter. Somebody on this list also suggested placing the corny keg in
direct contact with the cooling coils by jamming it against the wall of
a chest freezer - this should also work.
Roger Deschner rogerd(a)uic.edu