Fermentation and Solar Dyeing, the ultimate slow dye.

Updated: Mar 17


For all natural dyes to stay permanent on fibres, we need to understand the curve between mordant (for adjective dyes), heat, length of time in the dye bath and the WOF (weight of fibre, the percentage of dye stuff in correlation to the weight of the fibres we want to dye).

What does this mean?

A low temperature dye bath will need more time for the dyes to adhere to the fibres than a heated dye bath.
A small percentage of dye stuff will need more time to create a saturated hue than a high percentage.

We can use this understanding to play with the percentages of dye stuff and the time in the dye bath according to the time and effort we want to spend.

With solar dyeing and fermentation dyeing we are using the natural heat created by the sun or a fermentation (rotting) process to dye our fibres. This will obviously take more time than stirring your dye pot on a stove, but it can be well worth it and it is a fun project to do with small children you would not want to have in the vicinity of your gas hobs handling boiling hot water.


A little more about fermented dyes.

In the old ages (early broze age, late iron age, there would have been three ways of dyeing fibres, Krista Vajanto has an amazing dissertation about this called Dyes and Dyeing Methods in Late Iron Age Finland. They would have been;

  • Vat dyeing,

  • Mordant dyeing

  • Fermentation of tannins.

Blue dyeing was done using woad (Isatis tinctori) in a vat. Another vat dye is Purple obtained from orchil producing lichens such as rock tripe (Lasallia pustulata).

Red tannins were obtained from tree barks (such as alder buckthorn (Rhamnus frangula) and roots of common tormentil (Potentilla erecta), a herbaceous perennial plant belonging to the rose family, these would be fermented.


Other Red anthraquinones were obtained from the roots of plants belonging to the bedstraw family (Galium boreale, Galium album and Galium verum).

The mordants used for the dye process would come from red sorrel (Rumex acetosella) which contains a lot of oxalic acid, and fermented clubmosses which naturally contain aluminium.


(There is evidence of a lively trade of textiles already in the Early Bronze Age since all textiles dyed with madder (Rubia tinctorum) and yellow flavonoid dyes such as weld (Reseda luteola) would have been imported as they do not grow locally.)

How is it possible to create lively and lasting colours using fermentation, without using mordants? First of all, when using barks and roots, they contain a lot of tannins which always improves lightfastness. But there is something more; in wool, the fermentation process leads to the decomposition of fatty spaces in between the cuticle scales of wool fibres. This 'stripping' of the wool scales makes it easier for the dye molecules to reach the core of the wool fibres.

This study shows that fermentation of wool was used for Anatolian carpets of the 18th century, and these are still as lively today as there were then.


Fermentation dyes at home, and its' little brother; solar dyeing.

What do you need?


Glass pots with a fitting lid. Make sure the lid has no rust, it will behave like a ferrous sulfate mordant and influence the colour of your dye.


Mordant; You can do this without an additional mordant if you are using tannin rich dye stuff! But if you want to use your dye within two days OR you are using something without tannins (such as marigold petals) I would advise to add some alum nonetheless.

  • Alum mordant

and if you feel like experimenting with a modifier;

  • copper sulfate.

How much? Well that depends a bit on how much stuff you will have to dye and how large your pot is. My jars are recycled mayonaise jars and I put a teaspoon of alum, or half a teaspoon of copper.


Natural dyes such as;

(All roots and barks with lots of tannin that normally release their dye slowly are amazing)

How much? I use more than I normally would when hot dyeing, I took a nice heaped spoon of my dye stuffs for each pot.


Not a great fit;

  1. Cochineal (becomes smelly)

  2. Logwood (becomes grey fast)

  3. Dyes that are naturally fugitive or need alcohol for extraction such as Alkanet.

Soda ash (about a teaspoon on a small jar) for alkaline extraction of Ph high-loving dyes such as madder and alder buckthorn.


Fibres. I used some scraps, silk cording, vintage cotton collars that were laying around for too long, some wool. I plan on using these in the new art project I feel coming up. As long as it fits in the jar and you can still work it around a bit it is fine. Too cramped will give uneven results.












What do we do now?

Put your mordant (if used), add cold water to half the hight of your jar.

Stir long and well so your mordant is dissolved (no 'grains' on the bottom).

Add the fibres and add enough water to fill almost to the rim.

Add your natural dyes and with a spoon or a chopstick, stir the whole thing around.

These are my batches;


Pomegranate peel Alder Buckthorn Madder roots


Screw on the lid, but not too tightly because there will be some fermenting going on in a few days and the air needs to be able to escape (exploding jars!).


Now we wait......for about two days to a week as the jars are standing in a warm, sunny spot. Stir around once daily. This is after three days without sun;


Pomegranate Alder Buckthorn Madder roots


You will just let the jars sit, stir around once every day and watch that starting to ferment. Small bubbles will start to show indicating the rotting process has begun. This can become very smelly!


Pomegranate Alder Buckthorn Madder roots.

Result after three days, rinsed. I would not call this fermented yet, just slow dye, or solar dye.I have put these back so I can show the difference after a week. The longer you leave in your materials in the dye, the better the saturation will be.


Results after six days, washed and dried;


Dye along with me this coming month and share your results in the comments or in the BotanicalPrint facebook group using the hashtag #corona2020dyeproject. In the blog posts after this we will see how we can use these dyed pieces for a slow stitch work.




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All rights reserved, copyright all images and text @Suzanne Dekel