Mordant

A mordant is a chemical substance used to achieve the permanent union of a colorant into another material, as wool, wood, linen, alpaca, silk, cotton...

There are many natural substances that are used as mordants, as alum, citric acid or tannins, for example, and their synthetic counterparts, chemically produced as synthetic alum, tannic acid, etc.

There are plants that have a high concentration of aluminium and can be used as plant mordants. The most famous one is called Symplocos paniculata, but there are other plants used historically as mordants, like clubmoss used by Vikings