A salt mill is a grinder designed specifically for large grained salts that need to be ground for culinary use. In the 1990s, many consumers began to experiment with unique salts in their foods like Brittany Fleur de Seul, Himalayan Sea Salt, and Black Salts. Trace minerals in these salts seem to have an impact on the flavor, and the different colors certainly make them conversation pieces, but most of them require a salt mill to be used, as they are thick and chunky. As a result, many companies began to specially develop grinders for the unique challenge that these types of salt presented.
In order for a salt mill to be useful, it must be adjustable, so that grains can be made smaller or larger, depending on the need. It must also resist rust and corrosion, two common problems associated with salt, and it must keep salts which are subject to clumping, such as Fleur de Seul, dry. While some cooks simply put salt into a pepper grinder, they will ultimately be disappointed by its performance. This is because most pepper grinders are made from grades of metal which will easily corrode, because pepper is not corrosive. When seeking out a salt mill, make sure to get one that clearly states that it has been designed for salt.
In many cases, a salt mill will use ceramic grinders, which will stay sharp through the life of the salt mill. Other companies manufacture salt mills with corrosion resistant metals, meaning that they have been treated with chemicals. The cylinder of the salt mill which holds the salt until ready for grinding can be made from wood, glass, plastic, or specially treated metals, depending on the end aesthetic desired. Some companies, such as Peugeot, have salt mills on the market which also resist clumping, as long as the salt is loose and dry when it is put into the salt mill.
Culinary salt is usually made by evaporation in large salt ponds. Highly unprocessed evaporated salt can come in loose, flaky form, or in highly sturdy crystals, depending on where and how it is harvested. In France, many salt flats are repeatedly raked through the crystallization process, to keep the salt from clumping, and the skimming from the top is sold as Fleur de Seul, one of the least processed salts in the world. In other cases, salts are often treated, and in many cases iodine is added to counteract potential dietary deficiencies.