A mordant helps the dye colour to stick to the fibre and improves light and wash fastness. Various substances can be used as a mordant but commonly used is a tannin + alum process. Some mordants suit animal fibres (wool, silk) better and some suit vegetable fibre (cotton, linen) better so you need to know what fibre you are dyeing. Some papers are made from 100% cotton, others from wood pulp (all vegetable fibre).
Some dye stuffs do not need a mordant: walnut, rhubarb root, buckthorn, cutch, indigo. But most do!
Safe handling: Use gloves and masks when handling powders and toxic plant material. Alum substances are associated with brain, kidney and neurodegenerative diseases. Handle with care. Use separate pots and utensils for dyeing. Store powders in airtight containers and label clearly. Dispose of waste solutions carefully. Do not eat or drink in the dye room. Avoid dyeing during pregnancy.
Table of Mordants and Assists – Some Pro’s and Cons