In addition to ingredients like baking soda, starches, moisturizing oils and such - there are many types of salts you can use in bath salts - as well as ingredients that are called "salt" that are different chemically from the stuff you shake onto your food.
Regular salt comes in many forms from the iodized table salt that you probably have in your cabinet, to sea salt, kosher salt, pickling salt and others. The primary ingredient is sodium chloride, but the salt can vary in the size of the crystals and in any other ingredients that are added.
- Iodized table salt is highly refined and has a trace amount of iodine added. It is finely and uniformly ground and usually also has some anti-caking ingredients. I have read that some people are sensitive to too much iodine...and since your skin absorbs some of the salts in your bath, you may want to mostly use non-iodized salt in your bath salts. (But if you've got some extra handy, or need to substitute a bit of table salt for other salts, you should be just fine.)
- Cooking salts like kosher salt (larger grains), pickling salt (very fine grain), and coarse or rock salt are distinguished by the size of the grains of salt - and the absence of iodine. (Though they often do contain some anti-caking additives as well.)
- Sea salt is distilled directly from sea water and contains some trace minerals. It comes in a number of grain sizes and usually doesn't contain any additives.
- Exotic and/or luxury salts like grey, Celtic, Dead Sea, Himalayan, Hawaiian and others. They are still considered salt, but may not even have sodium chloride as their primary ingredient. They have unique minerals and other constituents in them that offer special qualities.
- Epsom salt is a completely different kind of salt. It's not sodium chloride, it's magnesium sulfate.
Any of these salts are fine in your bath salts. The fun thing about making bath salts is that you can combine the salts and many other ingredients like milk powder, baking soda, natural starches, essential and fragrance oils, herbs or flower petals into a wonderful creation.
Here are some recipes for bath salts.
Here's more information on the different types of salts.
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