How safe are modern dyes, compared to ancient ones?
Some modern dyes are dangerous, while others are quite safe. Most but not all ancient dyes require heavy metal mordants, which are more toxic than dyes. Natural dyes generally require the use of a mordant such as alum, copper, iron, tin, or chrome. Alum is the least hazardous of these, often seen descirbed as “food grade” alum, but the fatal dose of alum for an adult is 30 gram (one ounce), while that for a child is correspondingly much less. The fatal dose of alum for a baby might be as little as one gram. The other mordants, including copper and iron, can be fatal to adults in doses as low as one teaspoon or less, and the chrome mordant (potassium dichromate) is a significant human carcinogen. One of the few natural dyes that does not require a mordant is indigo, a natural vat dye, which can be prepared from some fifty different species of plants that grow around the world. It is not safe for a child to use because caustic chemicals such as lye are necessary to dissolve it in the dye
