Mordanting Cotton Part I: Pre-Mordanting With Oak Galls

Rhea Hoeflok

The first few years I practiced plant dyeing I exclusively dyed on protein fibres. I loved (and still do) the process of taking a raw sheep or alpaca fleece and turning into something beautiful. I used aluminum potassium sulphate for mordanting protein fibres with so much success I never looked further.


When I started experimenting with cellulose fibres (cotton, linen, hemp, etc..) I had the incorrect assumption that plant loves plant. I remember the first time I tried dyeing with blackberries on cotton, the inspiration coming from dozens of stained childhood shirts, only to be disappointed with the results. What I imagined would be a deep purple, or possibly a lavender, was more of an indescribable sickly grey. I quickly learned that how you treat your fibre is just as important as the materials you choose to dye with.



Onion Skin Dye

Onion Skin Dye. The bright yellow has been pre-treated with oak gall tannin and mordanted with alum while the peach colour was pre-treated with oak gall tannin and mordanted with soy milk. It's amazing the different colours the same dye can produce depending on how the fabric is pre-treated.



There are a few different ways to mordant cellulose fibre. What you are dyeing with and on what material influences how you pre-treat and treat your fibre. In this post I’m looking closely at how tannin (tannic acid) acts as a pre-mordant for cellulose fibres with both alum (aluminum potassium sulphate) and soy milk on 100% cotton.



Tannin (tannic acid)


Tannic acid is used as a pre-mordant to increase the bondability of alum to cellulose fibres. Biologically speaking, tannin is an organic compound found in some plant material and is believed to be produced as a deterrent to predators. Tannin has been used historically for tanning hides into leather, medicinally for the astringent properties, in ink production, and as a means to “animalize” plant fibres in the dyeing process. Because of the presence of tannic acid in many plants, pre-mordanting isn’t always necessary. However, if I am mordanting fabric or fibres with alum, I always pre-treat with tannic acid in the form of oak gall powder.


Oak galls are created as a defense against the larvae of the gall wasp. The oak tree secrets a substance that hardens around the larvae, entombing it. The resulting gall contains really high tannic acid levels. It’s also non-toxic and doesn’t harm the tree when harvested, making it the perfect ingredient for natural dyers.



Oak Galls - Left Coast Naturalist

Oak Galls - from Left Coast Naturalist



To make a pre-treatment, you can use whole galls steeped in hot water to make a tea or you can dissolve powered oak galls in a dye pot. I buy powdered oak galls as wildcrafting my own galls isn’t possible in my area, although I do think this would be a fantastic means to obtain tannic acid!



Oak Gall Powder

Oak Gall Powder



Pre-mordanting with Tannic Acid


Different dyers suggest different percentages of pre-mordants and mordants. And while you definitely need a certain amount and don’t want to over treat your fabric, there’s a bit of wiggle room.


1. Weigh your fabric before you pre-soak. I always put my fabric through the gentle cycle in the washing machine with pH neutral soap and leave to soak on the rinse-hold setting for at least an hour to ensure all the fibres are fully saturated. Then I spin on a gentle setting and use the fabric immediately.

2. I use 1 scant teaspoon of powdered oak gall per 100g dry fabric. In my experience this is enough to improve alum absorption in the mordant bath but not so much that it changes the end colour of the fabric dramatically. If you want to add a slight tan or caramel base colour to your fabric, which will deepen/darken the subsequent dye colour, increase the amount of oak gall powder to 1.5-2 teaspoon per 100g dry fabric.

3. Dissolve the measured oak gall powder in boiling water in a large stainless steel pot and top up with warm water.

4. Add the fabric and using a wooden spoon to push the fabric under the surface of the water. Let sit for minimum 12 hours, 24 is best. Check on it from time to time and give it a stir and a poke, trying to keep as much fabric submerged as possible.

5. Remove the fabric from the pot and rinse well. You can either store the fabric for later use or continue with mordanting, either with alum or soy milk.



Tannin vs no Tannin

On the left is cotton pre-treated with oak galls. Both sets of cotton were mordanted with soy milk.


I'll explore the different means of mordanting in my next post.

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