Rock salt, also known as halite, is a type of salt that comes from the rocky layers of the Earth's crust and is formed from the remnants of ancient bodies of water.
What is Rock Salt?
Most of the salt we use in cooking, both table salt and Kosher salt, is made by flooding underground salt deposits with freshwater, extracting the water and then evaporating it to leave the pure salt crystals behind. A smaller proportion, which is known as sea salt, is produced by evaporating the salt from seawater. Rock salt, on the other hand, is dug out of the earth directly, so that it retains its cube-shaped crystalline form.
Because rock salt contains additional minerals and other impurities, it's not intended for human consumption, although it does have culinary applications, such as for making ice cream and for keeping coolers cold.
These uses have to do with the fact that salt lowers the melting point of water, which means that ice water with salt in it will get colder, while still remaining liquid, than water without salt. Thus, adding salt allows an ice water bath to reach temperatures as cold as 6 degrees below zero (F) without freezing solid.
Varieties of Rock Salt
Some forms of food-grade rock salt can be used for making brines, and they can also be used in salt grinders. But one of the main uses for rock salt is for deicing roads and sidewalks, again due to the fact that it lowers the melting temperature of water so that it stays liquid at colder temperatures. This type of rock salt is not intended for human consumption, although it can be used for making ice cream, since it doesn't come into contact with the food. Likewise, there is a product called ice cream salt, which is more expensive, but nevertheless is also labeled as not for human consumption.
Rock Salt Vs. Sea Salt
Rock salt and sea salt are sometimes confused for one another. Rock salt is usually inedible, unless it is expressly labeled as food grade. But sea salt is specifically intended for use as a seasoning, and can be quite expensive. Rock salt, by comparison, is cheaper by weight than ordinary table salt.
What Does It Taste Like?
Rock salt, including ice cream salt, is not intended for consumption. But if you were to taste the food-grade type, it would taste salty.
Cooking With Rock Salt
As mentioned, some food-grade rock salts can be used as a seasoning and for making brines. But one of the most common culinary uses for rock salt is for making ice cream. Ice cream is made by combining milk, cream, sugar and other flavoring ingredients, then using some sort of combined stirring and chilling technique.
In the days before electric ice cream makers, ice cream was made in a double bowl setup, where the ingredients went into a small bucket, while a larger bucket was filled with ice water and rock salt. Then the small bucket was immersed in the large one and you'd crank a handle to churn and chill the mixture until frozen. Sometimes you'd have to add more ice and salt. Some devices substituted an electric motor for the hand crank, and machines like these are still available today. If you use one, rock salt is what you'd use to chill the ice water.
Interestingly enough, you can use any type of salt in place of rock salt, including table salt or Kosher salt. The main reason you might not want to do so is because rock salt is cheaper, and you'd need to use the same amount of table or Kosher salt, by weight, as you would rock salt. But chemically, any household salt will work for making ice cream.
Uses of Rock Salt
Apart from the culinary uses described, you can also add rock salt to the ice in your cooler, which, by lowering the melting temperature, will help your ice stay frozen longer. Another use is as a water softener. And of course, rock salt is often used to melt the ice on roads and sidewalks.
Where to Buy Rock Salt
Rock salt can be bought at hardware stores and grocery stores in boxes or bags ranging from 4 pounds to 50 pounds. A rock salt product called "ice cream salt" can often be found alongside the culinary salts in the spice aisle, but it's more expensive per pound than plain rock salt, and since it's labeled not for human consumption, there's no particular reason to pay extra for it.
Rock salt can be stored indefinitely as long as you keep it dry. It doesn't go bad.