Mordants for Natural Dyeing

What is a Mordant?

A mordant links chemically with the fibre and creates attachment points which bond with the colourant from natural dyes creating light and wash fast colours. Mordants are also used to set the colours from flowers and leaves during the process of Botanical Eco Printing.

Materials can be pre-soaked in a mordant and left damp prior to dyeing. The mordants can also be added to the dye bath while the dye is being simmered. Mordanting materials before and after dyeing with natural dyes will add extra fastness, Copper, Tin and Iron can be used as an after treatment to adjust colours. See links at the bottom of the page to browse for all the Natural Dyeing and Mordant products mentioned in this post.

Dyeing without Chemicals?

The quantities of mordant used in preparing fibre for dyeing have been the subject of much disagreement. Historically the craft dyer used ‘enough’ to make sure the job was done well. This gave rise in recent years to a strong reaction for ecologically sound mordanting where the levels of mordant were reduced considerably, alternative mordants were sought out, and additives used to increase the effectiveness of mordants. The most extreme version was the concept of ‘dyeing without chemicals’. The belief that oxalic acid used as a mordant and derived directly from rhubarb leaves and roots is ‘good’ and the same substance from a factory is ‘bad’, is prejudiced. Oxalic Acid is a listed poison!

The most commonly used mordant is Alum (Potassium Aluminium Sulphate) followed by Copper (Copper Sulphate), Tin (Stannous Chloride), and Iron (Ferrous Sulphate). These are used in various combinations for mordanting wool, silk and other protein fibres. The process of mordanting cotton, linen and other cellulose fibre for natural dyeing is different in that the mordants do not stick to the fibre. The cellulose fibre needs first to be soaked in a solution of tannin so that a mordant can attach itself to the tannin. Aluminium Lactate or Aluminium Acetate can be used as an alternative to tannin to fix natural dyes to cellulose. The chemicals improve light and washfastness of all natural dyes and achieve intense, clear colours.

Before dyeing, it is important to check whether the item is ‘Prepared for Dyeing’ (PFD) or requires scouring to remove any grease, oil or starch. Run a few droplets of cold water onto the fabric. If they soak in quickly no scour is necessary, although it is always a good idea to rinse out materials to remove residue from handling.

Clean your materials by soaking half a teaspoon of Synthrapol (a non-ionic concentrated detergent) to 5 litres of warm water for each 100-200g per dry weight of fibre. Gently stir the materials for 10-15 minutes, squeeze out and add to the dye bath whilst wet. There is no need to rinse out the Synthrapol because it also acts as a surfactant (a wetting agent); it lowers the surface tension between fibres and dye solutions to help materials absorb the dye colour.

Natural Dyeing Mordant Pack



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Mordants for Dyeing Wool and Silk (Protein) Fibres

The quantities of mordants shown are for 500g of dry weight of fibres (DWF) with five litres of water. Bring the mixture to boiling temperature and hold there for one hour. Do not agitate wool fibres as this will cause them to felt. Allow to cool, rinse and dry. 

Mordant to 500g DWF
& 5 litres water
MordantNotes about Mordants
100gAlum Mordant

Aluminium Potassium Sulphate

The most frequently used mordant for natural dyeing is alum, it does not modify colours. Too much Alum will tend to make the fibre feel sticky.
10gCopper Mordant

Copper Sulphate Pentahydrate

Use as a premordant or as an after treatment to adjust colours. Tends to dull colours tint them to blue green.
4gTin Mordant

Stannous Chloride

Use as a premordant or as an after treatment to adjust colours. Tends to brighten colours, especially yellows, oranges and reds. Too much Tin will make fibres brittle.
10-20gIron Mordant

Ferrous Sulphate Heptahydrate


Use as a premordant or as an after treatment to adjust colours. Alters the colour (depending on the plant used) darkening the colour, known as ‘saddening’ – yellows towards orange, reds to brown and purples to black.
50gTitanium OxalateBrings out oranges from tannins such as Gall Nut and variable colours from other natural dyes.
3-5g per litre of waterCream of Tartar

Potassium Hydrogen

Cream of Tartar is not a mordant, it is used to soften fibre, to act as a buffer to maintain a slightly acidic dye bath, to help disperse the Natural Dye colour evenly and to brighten colours (i.e. taking cochineal to a true red) on wool, silk and other protein fibres. It is an optional addition to mordant solutions.

Experimenting and careful note taking will help you to repeat the colours and results achieved by these mordants. 

Cotton and Linen (Cellulose) Fibres

Dyeing cotton and linen with natural dyes requires a different approach to that used for protein fibres. The metallic part of the mordant does not attach readily to cellulose until it has reacted with Tannic Acid (tannin). Sources of Tannin also include Cutch, Oak Gall Nuts, Walnut Leaves and Henna.

  • First scour your materials ready for dyeing. These instructions are for up to 500g dry weight of fibre (WOF).
  • Use a container that will allow the fibre to move freely within the following solutions.
  • Dissolve 40g of Tannic Acid in 5 litres of hot water. Steep the fibres in the bath for at least 2 hours and overnight if possible. Remove and squeeze out excess moisture.
  • Immediately mordant the prepared fibres with a mix of 100g of Alum and 25g of Soda Ash (sodium carbonate) in 5 litres of hot water and allow to soak for at least 2 hours and overnight if possible.
  • Repeating the process a number of times will result in deeper, stronger colours from the natural dyes.

Alternatively, Aluminium Lactate acts as an effective mordant for Cotton (and other cellulose fibres) when using natural dyes, giving a significant difference in colour take-up and improved light fastness. On cotton use 7-10% to Weight of Fabric  (optional: after a bath of gallnut (or other tannins of light colour) for a better fixation of the mordant). Also use in combination with iron mordant or a finishing bath for shading with an iron salt (max 1.5% WOF) to widely increase the number of shades. Titanium Oxalate can also be used in the same way as Aluminium Lactate for mordanting cellulose and will effectively bring out the orange in the tannin.

See our Glossary of Dyes and Dyeing Terms for further useful information on using dyes.

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