Taken from familyfun.go.com
Ingredients
2 tablespoons sugar
1 cup half and half
1/2 teaspoon vanilla extract
1/2 cup salt (The bigger the granules, the better. Kosher or rock salt works best, but table salt is fine.)
Ice cubes (enough to fill each gallon-size bag about half full)
1 pint-size ziplock bag
1 gallon-size ziplock bag
Instructions
Combine the sugar, half and half, and vanilla extract in the pint-size bag and seal it tightly.
Place the salt and ice in the gallon-size bag, then place the sealed smaller bag inside as well. Seal the larger bag. Now shake the bags until the mixture hardens (about 5 minutes). Feel the small bag to determine when it’s done.
Take the smaller bag out of the larger one, add mix-ins, and eat the ice cream right out of the bag. Easy cleanup too! Serves 1.
Why Add Rock Salt to Ice for Ice Cream?
Without rock salt to lower the freezing point of the ice, it would not be possible to make ice cream in a hand-cranked or traditional electric ice cream maker. Ice naturally melts at 32 degrees Fahrenheit, which is too warm to create ice cream.
Process
According to sendicecream.com, rock salt forces the ice surrounding the can of ice cream to melt, creating a brine solution that lowers the ice cream’s temperature to help it freeze.
Temperature
Without adding rock salt, the ice would melt at 32 degrees Fahrenheit. The ice cream must be at least 27 degrees Fahrenheit in order to freeze. The ice lowers the melting point of the ice to between eight and 12 degrees Fahrenheit.
Ratio
According to sendicecream.com, you need one cup of rock salt for every five cups of ice in your ice cream maker.
Table Salt
Some scientists who participated in the the “Ask a Scientist” column at newton.dep.anl.gov believe table salt can be used instead of rock salt to make ice cream. One scientist, however, suggested that table salt may lower the brine temperature too quickly, causing ice crystals to form in the ice cream.
No Salt
If an ice cream maker requires ice, you must use salt. If the ice cream maker does not use ice, it lowers the temperature of the ice cream automatically without a salt brine.