This is a collaborative blog post by members of the Natural Dye Education Facebook Group. See below for individual contributors.

Introduction:

The inaugural Natural Dyes in North-east America Conference took place online from late 2020 through Spring 2021. One of the presentations was by Professor Sherry Haar, Kansas State University, and detailed results of research quantifying the amount of mordant actually taken up by fibres. Her experimental methods measured the distribution of aluminum mordant ions across the pre-mordanding process for wool and cotton using inductively coupled plasma–mass spectrometry and the effluent quality characterized through chemical oxygen demand and total organic carbon. She found that with cotton, 92-96% of the aluminum remained in the effluent bath. With wool, the amount of mordant left in the bath ranged from 26-68% dependent upon particular fibre type and initial mordant concentration. The research also demonstrated something that may seem paradoxical - that the lowest WOF of alum mordant for protein fibres (7%) and aluminum acetate for cellulose fibres (2.5%) resulted in the most mordant being taken up by the fibres.

Research Study: How Much Mordant Is Being Disposed? The Measurement and Environmental Impact of Mordanting Natural Dyes.

Conventional Recommendations:

Many natural dye supply sellers advise very high mordant amounts (sometimes up to 15%-20%), and also advise dyers to top up mordant baths for each subsequent use, without citing any evidence.

The usual advice for re-use of a mordant bath is to top up with 50% of the amount of mordant that you want for the next batch of fibres. So, for example…

1st use of mordant bath: you mordant 100gm of fibre using mordant at , say, 8% (i.e. 8 gm)
For the 2nd use of the same mordant bath: for 100 gm of new fibres, you top up with 4% additional mordant (i.e. 50% of the 8% mordant you would use if you were creating a bath from scratch)
and so on for each subsequent use of the same mordant bath

So, for 500 gm of fibre, you could end up using 24 gm of mordant.

Likewise, suppliers and others who have never actually systematically tested their assumptions, often suggest that a tannin bath needs to be topped up with 50% WOF of the next batch of fibres to be treated.

But are any of these assumptions grounded in facts?

Citizen Science:

Professor Haar’s research provides an opportunity for natural dyers to re-think optimal mordant amounts, the necessity of topping up mordant baths - at all - for re-use, as well as having significant implications for safe mordant bath disposal, given the amount shown to remain behind in the mordant bath after use.

To further test the relevance of these research findings for natural dyers, in February 2021, I launched a citizen science collaboration in the Natural Dye Education Facebook Group to test protein fibre dyeing using the 7% alum amount noted in Professor Haar’s research as being the level at which the maximum amount of mordant is taken up, and to test whether the same mordant bath could be used repeatedly without topping up. Our results indicate that the answer to both questions is yes, and you can see our detailed results in this earlier blog post.

In June 2021, I launched a follow up citizen science collaboration, this time for cellulose fibres. Our approach and results are below.

Our Approach:

The following guidelines were shared in the group for this cellulose test.

Results:

Contributing dyers, alphabetical by surname. Where a dyer has provided more than one image, use the arrows to scroll through all images and their descriptions.

Kate Ellis: Test #2 (2.5% mordant test)

Dye Used: Left - 3% & 5% Quebracho Rojo Extract; Right - 3% Madder / Rubia tinctorum Extract
Fibres Used: bamboo rayon

Mordant Used: concentrated aluminum acetate (recipe 12 in The Art and Science of Natural Dyes, Joy Boutrup and Catharine Ellis) + dunging (with chalk). No tannin.

Bottom to Top: un-mordanted controls, then 1st through 7th samples all mordanted sequentially in same mordant bath, with no topping up, then all dyed in the same dye bath.

Kimberly Tepe: Test #1 (no tannin top up)

Tannin Used: Tara extract at 10% WOF

Dye Used: Madder / Rubia tinctorum
Fibres Used: left = 100% linen, right = 100% kona cotton

Top to Bottom: 1st through 7th samples all fibres sequentially treated in same tannin bath, with no topping up, then all mordanted with 2.5% AA and dyed in the same dye bath. Bottom sample not treated with tannin, but mordanted and dyed in same baths as other samples.

Kimberly Tepe: Test #2 (2.5% WOF mordant )

Dye Used: Madder / Rubia tinctorum
Fibres Used: left = 100% linen, right = 100% kona cotton

Top to Bottom: 1st through 7th samples all mordanted sequentially in same mordant bath, with no topping up, then all dyed in the same dye bath. Bottom sample not mordanted, but dyed in same bath as other samples.

Related Tests:

Donna Buitendyk Fillion

Tannin Used: Oak Galls

Dye Used: Lac 8% WOF

Fibres Used: reclaimed fibre - 85% recycled cotton/15% recycled polyster

Donna did three separate tests. Her first test (top row) followed the instructions for this exercise. For her second test, she followed the method for this exercise, but instead of using the specified 2.5% AA, used 7% AA . Her third test was her own design, and also provides valuable information.

L:R first to seventh fibres treated.

Test #1 - top row: oak gall tannin at 10% WOF for one sample, and 7% AA WOF for all samples at the end. This test is one sample short, due to an error on the first sample and having to begin again. Clearly, the tannin at 10% for one sample, then re-used, is sufficient for several uses.
Test #2 (modified) - middle row: 7% AA WOF for one sample, and then calcium carbonate dip at the end all together. The first sample has picked up more dye than the others, but all six remaining samples are a consistent colour.
Test #3 (Donna’s own test) - bottom row: is the reverse of the top row - 10% WOF tannin of all samples at once, then 7% AA WOF of one sample until all were processed. Only the first sample’s resulting colour is comparable to the top row test. But the resulting colours of the remaining samples is consistent with Test #2, which makes sense as Test #2 and Test #3 were both done with 7% AA WOF for one sample only.

Further Thoughts:

As with the earlier 7% alum/protein citizen science test, the tests above - not topping up the tannin between uses, and only using 2.5% WOF cellulose mordant (or 7%, in Donna’s case) and not topping it up between uses - also demonstrate the often over-use of tannin and mordant recommended by so many natural dye suppliers and others.
While some specific methods may necessitate the use of higher quantities, this generally does not apply to much of the day to day practice of most natural dyers.

Conclusions:

It would be great if more dyers participated in this test, for a more robust set of samples. I also completed these tests, using weld and logwood, for which I also did lightfast tests, but I somehow managed to lose my images and samples, and haven’t had a chance to re-do the tests yet. When I do, I’ll upload my new results. But my weld and logwood test results showed very comparable results to those shown above. Even from the results from just the above dyers, we can readily see that most people are using far, far more tannin and mordant than is required.

Kimberly’s no tannin top-up shows almost no lessening of final colour uptake until about the 6th or 7th sequential tannin treated fabric. Her no mordant top-up shows some very minor lessening of colour between the first and second samples, but virtually none thereafter.
Kate’s no mordant top-up shows a similar significant lack of colour difference between the sequentially mordanted samples (Kate did not do the no-tannin top-up test).
Donna’s tests show, in particular, how many times a tannin bath can be re-used with little to no diminishment of final colour on the fibres. Her mordant results are more equivocal. It’s possible she would have had less loss of colour in sequential samples if she’d used 2.5% AA, instead of 7% AA, as that is the WOF of mordant shown by Professor Haar’s research to result in maximum mordant uptake by cellulose fibres - less being more when it comes to the chemistry of mordant uptake by fibres..

Next Steps:

If anyone else would like to contribute results to either, or both, of the tests above, that would be great. The larger the set of samples, the clearer the conclusions we can draw from the results.
With gratitude to all participants.

Acknowledgment:

This is original research designed and coordinated by Mel Sweetnam, Mamie’s Schoolhouse. A literature review was conducted (scientific research, historic references, contemporary web sites/blogs), and no prior references to reusing a tannin and/or mordant bath without topping up were found. Therefore, anyone referencing this approach should acknowledge Mel Sweetnam, of Mamie’s Schoolhouse, as the originator .

Cite as follows.

Sweetnam, Mel. “Cellulose Tannin And Mordant Over-Use?” Natural Dye Blog, July 2, 2022, https://www.mamiesschoolhouse.com/blogarchive/2021/6/29/mordant-over-use-cellulose

We welcome your comments below.