Fish head, Putting salt in the water you want to freeze will make it freeze at a lower temperature and, therefore, make it take longer to freeze. Putting some salt in some ice surrounding the jugs will make the surrounding ice melt at a lower temperature and will cause the liquid water in contact with the outside of the jug of pure water to be below the normal freezing temperature of pure water. The liquid water will transfer heat better than air or solid ice and will cause the pure water in the jug to freeze faster.
Been making ice cream that way for 40 years. GOOD STUFF!!
