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Pour about a cup of the liquid ice cream mixture into a sandwich- or quart-sized plastic bag and seal it. Slide that bag into a second plastic bag and zip it tight; this will keep the ice cream from leaking. Fill a gallon-sized plastic bag with about a third of the way with ice cubes. Add a cup of rock salt to the cubes.
Place the double-bagged ice cream mixture in the gallon-sized bag. Zip the gallon-sized bag shut and swaddle it in a towel. Knead the bag for about 10 minutes; the constant motion will make sure the ice cream is always in contact with the ice cubes and does not develop large ice crystals. Allow the bag to rest for an additional five minutes. Remove the ice cream from the gallon-sized bag and wipe off the exterior of the baggie. Spoon the frozen ice cream into a bowl. -
Fill a small metal coffee can about halfway with the liquid ice cream mixture. Place the lid on the can and seal it tightly with duct or packing tape, making sure that there are no cracks or openings around the top of the can. Center the small can in a large metal coffee can and pack the space between the two cans with ice. Pour about a cup of rock salt on top of the ice and cover the large coffee can with its lid. Seal the lid with tape and wrap the coffee can in foil for extra protection, if desired.
Spread a beach towel on the floor and roll the coffee can back and forth for about half an hour. The constant motion will help the ice cream freeze evenly. After 30 minutes, remove the small coffee can from the large coffee can. Wipe the salty water from the exterior of the small can before opening it and removing the ice cream. -
Wash and dry a freezer-safe metal or glass baking dish. Pour the ice cream mixture into the dish, making sure to leave at least an inch of empty space in between the top of the ice cream and the rim of the dish. Slide the uncovered dish onto a level surface in the freezer and allow the ice cream to set for about 45 minutes.
After 45 minutes, the ice cream should start to freeze on the sides and bottom of the dish. Remove the dish from the freezer and beat the ice cream with a whisk, hand mixer or stick blender until all the frozen parts are broken up and combined with the liquid. Freeze the ice cream for another 30 minutes, then beat it again. Break up the ice cream every 30 minutes until it is completely frozen and smooth. This should take two to three hours. After the ice cream is ready, transfer it to a covered container and freeze until use.
