Usually, pantries have three kinds of salt: fine table salt, kosher salt, and a super-flaky salt such as Maldon. All three are used in different ways, depending on what one is cooking or how one is seasoning something. When people see kosher salt, a usual question that arises in their minds is: Just what is kosher salt, and why do you have it, if you do not cook kosher?
How kosher salt is made
Kosher salt is a coarse-grained salt made from the salt crystals. It is usually not iodized, but some brands may contain an anti-caking agent. The water evaporation process determines the salt's final shape, so kosher salt can be flat or pyramidal in structure depending on the brand. The top two brands are Morton and Diamond Crystal: Morton's is much coarser than table salt, but Diamond Crystal is even coarser than Morton.
How kosher salt got its name
It is not because of Jewish dietary guidelines that kosher got its name. Kosher salt's original purpose was really to kosher meat, meaning to remove the surface blood from meat, so it is really 'koshering' salt. Certain salt companies labeled the boxes of this coarse salt 'kosher salt' rather than koshering salt, and the name stuck.
When to use kosher salt
Because kosher salt varies in shape and size across different brands, it does not always measure out consistently. This is why only table salt is used in recipes, unless specified differently. In recipes, where the kosher salt brand is not specified, use your judgment; if it seems as to be calling for a lot of salt, then err on the side of less, in the beginning.
The best usage of kosher salt is when you are seasoning food with your hands, especially meat and vegetables before cooking. You can easily pick up pinches with your fingers, and since they do not dissolve immediately, you can visually see where you have sprinkled it and determine if it is even or if you need to do more. You could also keep kosher salt out, on the dinner table, since it is easy to pick up a small amount to season cooked food.
Kosher salt is also great to use when a recipe specifies 'coarse salt'. However, many bakers tend to shy away from kosher salt and call for table salt instead because they feel that it dissolves more quickly and evenly into baked goods.
Kosher salt and substitutions
There are times when a recipe calls for a certain type of salt that you do not have or might have run out of. Since salts can be in such varying shapes, weight is the best determination. However, most recipes do not call for a weight of salt, just volume, and most homes do not have kitchen scales that are capable of weighing out such small amounts anyway.
Experts have done the weighing and measuring to determine equivalent measurements when using different kinds of salt, and here is what they recommend:
1 tsp table salt (fine salt) =
1½ tsp Morton kosher salt =
2 tsp Diamond Crystal kosher salt