Mordant for linen - an experiment

by Sabine Ringenberg  

I spend some time on the proper mordant for linen and carried out two test-rows in order to find out, what substance works best. The goal is to dye linen as brilliant and intense as the woolen-yarns or cloth I do. But that is not as easy as it seems...

First of all, there is no clear definition of what a mordant really - chemicially - does. Does it just open the fabric´s fibres for the colour to move in or is it a substance to fix the colour on the surface of the fibres? Furthermore mordanting seems to be a matter of changing the pH-Factor to the oposite: so, if linen is basic, would you choose an acid mordant or is it just as well changing the pH-Factor further into the basic range?

So, as you can see, this will take some more time, maybe with the support of a professional lab, and a lot of trying and cooking in my kitchen. So far I have done the following tests:

1. Archaeologicially dyed linen is often found in a cultural context where there is easy access to salt. Is that a coincidence or does salt have an effect of the process of mordant? Or is that a relation indicating wealth since dyed linen is considered to be expensive?

A test in Foteviken last summer showed no effect of pre-cooking with salt on the mordant. So this suspect is not guilty.

2. I got a good result in Foteviken with a mixture of alum and vinegar, but the vinegar reacted with the copper pot, I used, so I wanted to try again in a neutral pot and see what the results were - most of my suspects originating from the acid millieu.

a) alum with vinegar

b) vinegar alone

c) alum alone

d) acetat of alumina

e) potash

All of those mordants worked - more or less - towards a nice antique pink with 100% madder, but not towards red.....the defined goal.

3. My next acid suspect was tanin. Available in historical times almost everywhere and needed for other crafts as well, as for taning skins, I tried that first in a low concentration, since tanin would have a colour of its own, darkening the linen and being hencefor not suitable for lighter colours as we know from dyeing wool. I was not too convinced of this test, but, as it often goes, tanin actually gave the best result. I then gave it a go with a modification of potash - which was meant to neutralize the fibres after the stress of mordanting and dyeing, but to my utter surprise that bath intensified the colour even better.

4. Potash volunteered to be the next suspect. Even though, and this is what I found most amazing, it is basic, like the linen itself. If potash works as a mordant that could prove the theory wrong that mordanting means to shift the pH-factor to the opposite..... Also potash works on the fabric very aggressively when heated, so for my first test I choose a cold-bath over night... And it worked well, but still in antique pink. Other than  tanin, potash mordant had no trace of yellow or brown in it when dyed in madder, but still looked dull.

So for further investigation I will now look at concentrations of tanin and the differences in temperature and concentration of potash-mordant. The surprise was that contrary to the books that teach you about mordants, the best results came from two sides of the pH-scala. Which tempts me to send in some samples for further investigation under a electron microscope to see what actually does happen to the fibre, if it changes the pH-factor.

After I had a closer look at my samples from test-row 1 and 2, concerning the proper mordant for linen, I decided to give tanin a go. Even though all my books on the subject were very unclear about whether an acid or basic factor was decisive for the mordant, I got a good result with tanin in those two tests and a very good intesification with a modification of potash.

So my next row was tanin based. I used a powder called Tara in concentrations of 10, 30 and 50 percent and a modification of potash of 50 percent for all of them, varied the duration from 30 min to 1,5 h. The first visible effect of the mordant with Tara is that it darkens the fabric, so for light colours it is not suitable. 

I dyed all my samples in 100 percent of madder, cooked for one hour, then did the modification for 30 min and last for another hour. And again there was surprises.

10 percent of Tara does not alter the pH-factor enough, so the colour does not move into the fabric deeply. A closer look shows that there is many threads still white or pastel and also the modification is greyish and does not intensify the colour as much as expected. That was the best result. More than those 30 percent of Tara did not improve the outcome  and the modification got duller and grey. 

I suspect that the length of time for modification might have destroyed part of the colour and an overdosis of Tara for mordanting does something like that - even though this is very hard to tell without a proper lab-analysis.

Over all I found that the colour dyed on Tara mordant 30 percent is nice, intense, even and not unattractive - but not red. So the next test row will survey the effect of galls, supported by vinegar and modified with potash afterwards.  We will then see the effect of the acids used in the mordant and their alternation by the potash-bath.  10 percent of Tara show a lot of undyed threads.

To make a very long story short: here is my best result in linen dye:

 

 

My first choice.

It was a mordant of potash, to my surprise. The conclusion of the entire test is that it is not important, whether the mordant is acid or alcalic, but that it is strong enough to alter the pH-factor of the fibres so that the colour can move in. 

The second best result I got with galls as a mordant, tanin, together with a modification of potash. As you can see, the raising of concentration of the mordant did not have a major effect on the result.

 

Comes in second best.

All the tanin-based mordants I have used had the dissadvantage of the brown colour of the mordanting substance in itself, so the colour was shifted towards the brownish range. With those test-rows it could also be noted that the yellow colour-parts in madder, that are usually absorbed by the fabric if the dyeing temperature is raised above 80 degrees, are not visible to the usual extent. 

Further tests will have to prove, if the mordant with potash works neutrally on pure yellow dyes. Also there will have to be a test of light-fastness for those colours which are very sensitive to light even on wool.

So now, to conclude from all this working in the kitchen, annoying my family with dirty pots and an absent mind, my prime suspect for a good mordant is potash. And from this result on there can be further research:

1. Exact measurement of the pH-factor in both mordant and of the fibres.

2. Lab documentation and tests of the chemical parameters of the process.

3. Proper use of microscope on the undyed and the dyed fibres.