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. 

\r\nBecause 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. 

\r\nVarieties of Rock Salt
\r\nSome 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.