I get excellent results using magnesium oil (magnesium chloride) from Integrated Health:
http://www.IntegratedHealth.com
They offer Ancient Minerals magnesium oil (and magnesium flakes and gel products) recommended by Dr. Mark Sircus, author of Transdermal Magnesium Therapy. These are considered the purest magnesium oils available.
I use magnesium oil for muscle tension, improved sleep, and greater energy. But I have several friends who use it successfully for heart palpitations, and they swear by it. One of them carries in her purse a mini spray-pump containing Ancient Minerals, just in case she needs it. She says it has saved her life.
Regarding purity, Ancient Minerals magnesium oil is sourced from a mile deep underground seabed that is 250 million years old. This is probably the purest source of magnesium chloride anywhere, as it tests low in heavy metals, bromine, etc., i.e., levels are negligible or non-detectable. This form is the most suited for oral consumption in small quantities because of its high purity, according to Dr. Sircus. He also finds that it is the least irritating form to the skin.
There are other brands of magnesium oil derived from filtered ocean water. These magnesium oils can be very pure, but typically they are not as pure as Ancient Minerals. For greater cost effectiveness, you might try Ancient Minerals magnesium flakes, which are ideally suited to baths and footbaths, or you can make a magnesium oil using the flakes. Technically, dissolving magnesium chloride crystals in water creates magnesium oil. You would want to concentrate it to about 35% magnesium chloride, the percentage of magnesium in Ancient Minerals magnesium oil.
Nigari is mostly comprised of magnesium chloride, and is used in Asia (it is known in China as "lushui") in foods and as a coagulant for tofu. Using nigari to make your own magnesium oil is a very interesting idea. My first concern, however, would be the source of the nigari. Mitoku brand nigari, for example, comes from the Sea of Japan, according to Mitoku's literature. I'm not sure about the purity of the product, given the high levels of industrial pollutants in the Sea of Japan. Of course, I don't know if or how they filter it. But sun-drying would not remove heavy metals or other pollutants. And while nigari is a food, it is usually consumed in small quantities, meaning that exposure to pollutants or contaminants likely would be more limited than if you were to dissolve large quantities into a bath on a regular basis.
Just as magnesium oil derived from ocean water is likely not as pure as the Ancient Minerals from the underground seabed deposit, nigari might be a significant step down in purity. The cost effectiveness of using nigari would then have to be weighed against its potential toxicity.
I'm sure nigari works. Even
Epsom Salt (magnesium sulfate) works, albeit far less effectively than magnesium chloride (magnesium oil). It's a question of purity. I use magnesium oil almost every day. Given the implications for health of long-term exposure even to low levels of contaminants, I would like to see an assay of the total composition of nigari before I soak in it or apply it to my skin.
This is an intriguing discussion. I have enjoyed reading everyone's posts.