The Element Encyclopedia of Witchcraft: The Complete A-Z for the Entire Magical World - Judika Illes 2005
Food and Drink
Witches’ Salt is another name for what is usually called “black salt.” Black salt is either black or speckled black and white, unlike standard white salt. Black salt is usually table salt (sodium chloride) with additions although some simply dye salt black. It may or may not be edible but is usually used as a magical powder intended to either cast or antidote a malevolent spell.
Additions to the salt may include any of the following:
Scrapings from the bottom and sides of cast-iron pans or cauldrons
Powdered, dried snakeskin