Making ice cream is a popular summer activity. While there are many ways to make it, the process always involves certain steps. The trickiest is freezing the cream. Freeze it too quickly, then your ice cream crystallizes and becomes crunchy. Freeze it too slowly, then you're left with a gooey mess. Though it may seem counter intuitive, rock salt—which melts ice on roadways and sidewalks—is an indispensable part of freezing ice cream; it sets up the rich texture for the dessert.
While frozen desserts have been around for hundreds of years, what we know today as ice cream wasn't available to the public until factory production of it started in the mid-1800s. According to Zinger's Ice Cream, the hand-cranked ice cream maker was invented in 1843, and in 1851 Jacob Fussell opened the first ice cream factory in Baltimore, Maryland.
After combining the ingredients for the cream mixture, you would set up an ice bath and add rock salt to it. Then the container with the cream mixture goes into the ice bath, which will begin the freezing process. (You are not adding rock salt to the cream mixture itself. Many people have misread instructions and recipes and made this mistake.)
The reason for an ice bath, rather than a freezer, is that the latter can can cause the cream to freeze too quickly, which will form crystals and give the dessert a rough texture. This is especially true if you are attempting to make ice cream with a lower-fat milk, which has a higher water content. Controlling the freezing process maintains a smooth consistency.
Salt melts the ice in the bath, and the melting ice absorbs heat from the cream mixture. Rock salt is used rather than table salt because rock salt grains are larger and thus spread more evenly through the ice bath. "The Cook's Thesaurus" indicates that the smaller grains of table salt pack more densely than those of rock salt.
"The Cook's Thesaurus" also mentions that other kinds of salt will work in place of rock salt, but that there is a danger of the cream freezing too fast due to uneven distribution and the smaller grain size. If you do use table salt, use caution with the amount that you add to the ice bath. Experiment with the level throughout the ice cream making process, adding a 1/2 cup at a time. You will be able to determine the proper amount of salt by how quickly your ice cream is freezing. If, after about 10 minutes, your cream is just starting to firm up, you have a good amount of salt in the ice bath.
In addition to the larger grain size and easier control over the freezing process, another benefit of rock salt is that it is cheaper than table salt. Making ice cream can use a great deal of salt. Rock salt is sold in bags by the pound; table salt often doesn't come in bulk.
While opinions and manufacturers instructions may differ, one solid fact remains: Salt is salt. However, as scientist Richard E. Barrans Jr. says, "...if rock salt will suffice, using table salt in an ice cream freezer is like washing your floor with distilled water—too much added cost for not much added benefit."