Sidewalk salt by John Spottiswood.

This is the cheap, non-food grade salt that we throw onto icy walkways and use to make ice cream. It doesn't actually go into the ice cream, as some have learned the hard way, but rather into the wooden ice-filled tub that surrounds the bucket of ice cream. The salt lowers the freezing point of the ice, which causes it to melt. As it melts, it absorbs heat from the ice cream, helping it to freeze more quickly. Use a ratio of one part rock salt for every five parts of ice. If you're out of rock salt, other kinds of salt will also work, though you should use less since finer grains of salt can can be packed more densely into a cup than large chunks of rock salt. The biggest danger is that you'll use too much salt, which will make your ice cream freeze too fast and become crusty. When using salt other than rock salt, start with a modest amount and check the ice cream after you've churned it for ten minutes. If the ice cream is just beginning to firm up, you have the right amount of salt. If it's not yet firming up, you need to add more salt. If it's crusty along the sides of the bucket, then you've added too much salt.