It is easier to dye animal fibres such as wool and silk than plant fibres such as cotton and linen. Most natural dyes do not adhere very well to the fibres and need the help of a mordant. The mordant adheres well to both the animal fibres and the dye. Vegetable fibres need to be well scoured and also treated first with tannin for the mordant to work. I use bought alum powder for my mordant but in the future I would like to try plant mordants. Some dyes, for example indigo, woad and onion skins do not require the use of mordants. | 
