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Before you start natural dying it is really important to understand mordants. What they do. How to use them. And safety issues.

What are mordants?

Natural dyes are enhanced and made more permanent with the use of mordants. Mordants help set the dye pigment and improve the colour and light fastness. However due to growing awareness of the toxicity of some mordants, in particular chrome I only use alum which still gives me a wonderful range of colours and is the least harmful of all the mordants. Without a mordant the dye molecules simply lie on top of the fibre and will wash off.

Woad/indigo is exceptions to this, as they do not require a mordant; however the dyeing process is different and more complicated.

Mordanting can be done in two ways.

Mordants can be found in natural sources such stale urine and wood ash which give an alkali mordant. Acids are present in acid fruits or rhubarb leaves and are used today by only a small minority of natural dyers. These days it is more usual to use a chemical mordant such as alum which is available as a powder.

Alum is the easiest and safest mordant to use. Measure quantities carefully as too much alum leaves yarn or fibre feeling sticky.

Cream of tartar is not really a mordant, but is used to give luster and brightness to wool.

Unless you are intentionally aiming to influence the results of mordanting you should always use a stainless steel or unchipped enamel pot for your projects.

Other mordants which are used by some natural dyers are:-

  • Iron
  • Copper
  • Tin
  • Chrome

Natural dyeing takes time and patience. Trying to rush the process or skip the mordanting process or being careless and not following instructions correctly will give you disappointing results. It is important to master your mordanting techniques if you are to produce consistent and professional results you will be proud of.

Natural dyes will only dye natural fibres. Natural fibres are grouped into two categories.

Vegetable (cellulose) which include, cotton, flax, jute, linen and hemp.

Protein (animal) which includes wool, alpaca, mohair, cashmere and silk.

Protein fibres are easier than vegetable fibres to use with natural dyes.

It is impossible to colour match when using natural dyes. Several factors will influence this. The plants you use may well have been grown in different soils. The weather conditions may have been different. The water may also be different. Also different fibres dye differently. Therefore if you require a certain quantity of yarn or fibre for a project, dye the entire batch together to ensure the colour is the same throughout.

Adding more than the recommended quantity of mordant does not mean you will get a better result. In fact it means the opposite! It weakens the fibre and makes it brittle – it can even make it slimy.

Tannin is an important component when dyeing cellulose fibres. It improves the depth of colour and the eveness of the dye take up.

Tannin aids alum based mordants to chemically bond with cellulose fibres.

The tannin colour will ultimatly affect the natural dye colour. The tannin step also increases the UV lightfastness properties.

There are 3 types of tannins – including the ones we stock:-

Clear tannins – gallnut

Yellow – fustic and pomegranate

Red/brown – cutch

Clear tannins will not change the colour of the fibre as much as red or brown


1. Measure tannin to the recommended WOF for the tannin you are using. Dissolve
in hot water. Add to mordanting bucket. Fill bucket with enough water to fully cover the fibre when added.

2. Add scoured wet fibre.

3. Heat to 190 – 200 degrees F and hold for 45 minutes – gently turning the fibres regularly.

4. At this point the fibre may be rinsed and mordanted with alum – or left to steep
for 8 – 24 hoursbefore rinsing. Steeping will give deeper colours.


Iron may be added to the dyepot or it may be done as a separate step after dyeing.

1. Measure iron at 2% WOF. Dissolve in hot water. Add to mordanting bucket. Fill the bucket with enough water to fully cover the fibre when added.

2. Add wet, mordanted fibre.

3. Heat to 160 – 170 degrees F and hold for 30 minutes.

4. Rinse well. Always thoroughly scrub a bucket that has been used to iron mordant or it will sadden the next dye bath.

Cellulose fibres can be dried and stored before dyeing. Fibres do not need to be remordanted between dyes. Once a fibre has been mordanted it can be dyed and then overdyed without any further mordanting.

Buy mordants here

Safety Guidelines