Most recipes that call for salt are referring to table salt, which has additives like iodine (to prevent thyroid disease) and an anti-caking agent to prevent lumping in humidity. Many chefs prefer kosher salt (additive-free, coarse-grained) for cooking and sea salt for table use because they have a softer flavor than table salt. Kosher salt is made by compacting granular salt, producing large, irregularly shaped flakes which allows the salt to easily draw blood when applied to butchered meat (koshering process). The structure dissolves easily and provides flavor without oversalting because of it's large surface area. Hawaiian sea salts (red or black) are specialty finishing salts. The red variety has an iron taste and is used to add color. The black variety has a sulfuric aroma from the addition of purified lava. Black salt (kala namak or sanchal) is more tan than black, and has a very strong, sulfuric flavor. Black salt is available in Indian markets, either ground or in lumps. Pickling salt is free of the additives that turn pickles dark and pickling liquid cloudy. Sel gris is a gray salt from France, and fleur de sel is a by-product of sel gris created when sel gris is allowed to bloom into lacy flowerlike crystals in evaporation basins. Maldon sea salt is a British finishing salt similar to fleur de sal. It has a light delicate flavor that is obtained by boiling sea water to form delicate pyramidal crystals. Rock salt is used to make ice cream. Salt comes either from salt mines or from the sea. Most of today's salt is mined and comes from large deposits left by dried salt lakes throughout the world. Salt preserves foods by creating a hostile environment for certain microorganisms. Within foods, salt brine dehydrates bacterial cells, alters osmotic pressure and inhibits bacterial growth and subsequent spoilage. The word "salary" was derived from the Latin term "salarium" which was the name for a soldier's pay in the army of ancient Rome. The pay included a large ration of salt, which was a spice of high value and also a medium for exchange; thus the origin of such expressions as "salt of the earth" and "worth your salt."
1/2 tsp. rock salt = 3/4 tsp. kosher salt = 1 tsp. table salt