Mordants and tannins. Assists and modifiers.

Bijgewerkt: 27 mei 2020

On the various eco-printing and natural dye fora there is a lot of confusion and mis-use of terminology going on when it comes to the processes of mordanting fabrics.

Why do we need to mordant?

Natural dyes are divided in substantive (direct) and adjective (indirect) dyes. When a dye is not substantive, it means it needs a link to connect them permanently to the fibre. Substantive dyes will connect to the fibre without a mordant, but not all these dyes are lightfast (turmeric would be a great example), and a mordant is needed to improve light fastness and wash fastness.

All dyes (both substantive and adjective) get improved light fastness, rub fastness and wash fastness from using a mordant before dyeing.

Examples of substantial dyes that do not need a mordant to dye fabrics;

Often these dyes in itself possess a high amount of tannin.

All other dyes will need a mordant to stay permanent on the fabric.

What is a mordant?

Thessaurus says; a substance used in dyeing to fix the coloring matter, especially a metallic compound, as an oxide or hydroxide, that combines with the organic dye and forms an insoluble colored compound or lake in the fiber.

Or in simple words; a mordant is a metallic salt that functions to link between a dye and a fibre. The word mordant comes from the Latin word 'mordere' which means 'to bite'.

Inorganic mordants are;

  • Ferrous sulfate (often called 'iron') FeSO4

  • Aluminium sulfate Al2(SO4)3

  • Aluminium acetate (for cellulose fibers) Al(CH3CO2)3

  • Potassium aluminium sulfate (called Alum) KAl(SO₄)₂

  • Copper sulfate (aften called 'copper') CuSO4.5H2O

  • Titanium Oxalate C4O8Ti

  • Stannous Chloride (Tin) SnCl₂

These are the inorganic mordants I use myself, in my home studio. There are more mordants available, and used in old dye books I can not recommend using as a hobby dyer because of toxicity, such as chrome and lead.


The application of tannic acid/tannin to textile material is also called as tanning. Tanning is used in dyeing of adjective dyes with cellulosic fibers as part of the mordanting process, and also in mordanting of silk fibers. Tannins are always organic material from plants, and their structure is very complex.

Sometimes you will find a tannin sold as 'tannic acid', this may be plant derived but could also be synthetic. In both cases you should stay clear away, it can be very toxic and there are plenty of tannins with a good pedigree you can use. (Thank you Caroline Nixon for pointing this out.)

Gallic tannins. Clear tannins that do not add much colour to the fabric;

Tara pods

Oak galls

Oak bark

Sumac (leaves, galls)

Ellagic tannins. Tannins with a lot of flavonoids that will add a yellow colour to the fabric;




Catechic tannins. Condensed tannins that will add brown and reddish hues to the fabric;

Black tea



Chestnut bark

See all the mordants and tannins available in the shop here.

Proper mordanting and tanning

Proper mordanting and tanning takes time, it can't be done doing a 'dip' as some people call it when eco printing. Example: If you are using a ferrous sulfate solution and merely dipping your fabric in it, what you are doing is using the ferrous sulfate as a modifier that combines with other mordents and tannins, but the fabric will not be properly mordanted, resulting in disappointing results over time. This is why your iron dip blankets that looked so stunning when you unbundled, washed out like nothing good ever happened!

Mordanted vs dipped iron blanket

In the above picture Rita Trafford sent me, you see the difference. Left was done with a properly mordanted blanket, right was merely a quick dip.

A good mordanting/tanning process starts by measuring the proper percentage according to Weight Of Fibre (WOF). I understand it sounds romantic rusting your own metal scraps in vinegar and water, but besides the health hazards of rust (tetanus!) you will never know how much ferrous sulfate is present in your rusty water. Knowing that you only need 1% WOF ferrous sulfate to make an effective iron blanket or mordant, purchasing 100 grams of this will make your work so much more precise and you will prevent textile rot.

Iron causes acid deterioration and catalyses the photo-oxidation of the organic materials in textiles, resulting in significant or complete loss of strength over time.

In this piece of textile from the Israel Museum in Jerusalem you can see exactly what happens when large WOF of ferrous sulfate is used in dyeing (in the 'old' days this could be up to 50%).

The dark pieces with ferrous sulfate (to make black), look like they have been eaten away by a mysterious bug, but it's sheer deterioration.

A complete step by step guide to properly scour and mordant your textile before dyeing and eco printing with precise recipes can be found here.

In these marigold dye samples you can see the effect the different mordants have on the outcome. Mordants each have a very typical effect on dyes. These samples were mordanted with (FLTR) nothing, alum, copper and iron.

Assists and modifiers.

A fancy word for this in the dye world is auxiliary. Assists and modifiers are the same materials but with a different intention of use.


I use Cream of Tartar as an assist to prevent my alum mordant to clog up at the bottom of the pot.

I use Cream of Tartar as a modifier to lower Ph when dyeing cochineal, to get bright reds.

Alkaline assists and modifiers; (in order of strength)

  • Soda Ash (Sodium Carbonate) Soda Ash increases the effectiveness of a mordant on cellulose fibres. As a modifier it can change colours dramatically, just a bit can shift cochineal to purple. Soda Ash is also used to scour cellulose fibres.

  • Chalk (Calcium Carbonate) – is used as a color changer. If you have soft water with a low Ph, the addition of chalk to your dye bath will make your water harder. Chalk in the dye bath with bring out the best reds in madder and Alder buckthorn. In sappan wood dye the slightly higher Ph also brings out the bright reds.

  • Hydrated lime is a stronger alkali than chalk or soda ash.

Acid assists and modifiers:

  • Citric Acid, an organic acid with a pH of less than 7. You can use it with Lac to bring out the strongest colour.

  • Cream of Tartar is a by-product of wine making. Use as an assist when mordanting with alum, use as a modifier for acid loving dyes such as cochineal. Cochineal will go from fuchsia to red with the use of Cream of Tartar. Cream of Tartar works best on protein fibres and is not often used with plant fibres.

  • Tartaric Acid is also a by-product of wine making.

Tartaric acid and cream of tartar are not the same thing. Cream of tartar is made from tartaric acid by combining tartaric acid with potassium hydroxide. This partially neutralises the tartaric acid, so cream of tartar is less acidic than tartaric acid.

These samples of rhubarb on silk show you how the use of modifiers can influence the hue.

Alkaline and acidic modifiers used in rhubarb dyeing

A good additional article about other assists and salts used in natural dyeing can be found in detail here.

Find all the assists available in the shop here.

Using metallic salts as modifiers.
Tannin and post-mordant as a modifier.

Especially ferrous sulfate has a strong effect on most dyes. There is a huge difference using the ferrous sulfate as a mordant doing the proper mordant mordanting process, or using it post-dye or post-eco printing as a modifier.

When you use a mordants salt such as ferrous sulfate, or copper sulfate, after the initial dye process, it will most likely affect the colour, but it will not improve light fastness or wash fastness in the same way as it would when using it prior to the dye process.

Using ferrous sulfate in combination with tannins will result in different shades of grey and black. I wrote a blog about this here.

This phenomenon is the reason why tannin rich leaves, on ferrous sulfate mordanted fabric, will leave dark imprints. The higher the tannin/ferrous sulfate rates, the darker the print.

The tannin in the leaf, reacted with the ferrous sulfate in the fabric.

In these samples with marigold you clearly see the difference between mordanting before dyeing and using the ferrous sulfate as a modifier post dyeing.

Mordanted and then dyed with marigold (left) Dyed with marigold and then modified with iron (right)

All the different mordants on marigold, modified post-dyeing with ferrous sulfate.

Mordants and Tannins, Assists and Modifi
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