For my experiment, I have used a yew bark, but any part of a plant containing tannins is fine. Traditionally, for example, tormentil roots or birch bark was used. You need to cover this dyestuff with water with some alkali added, and put the whole vessel in a warm place. Fiber which you want to dye can be added immediately or after a few days.
Traditionally the source of alkali was an ash from wood, e.g. in Finland from birch wood used to heat saunas. Ashes were collected and boiling water was pouring over them. After a few days, when the pH of liquid was high enough, it was strained and a dyestuff was added. Vessel with dyebath was placed in a warm spot, preferably near the fire. It could be also warmed up by putting inside stones heating in a fireplace.
A lot of information about using this method in good old days you can find in Krista Vajanto's dissertation 'Dyes and Dyeing Methods in Late Iron Age Finland (available online under this link: https://helda.helsinki.fi/handle/10138/159210). Her description is focused on Finland, but as the tannins dyestuff are quite common, and a whole method is pretty intuitive, I believe that it was known also in other parts of Europe.
When researching this method, I found different information about the way you are adding alkali. Krista Vajanto mentioned adding it only at the beginning of fermentation, while Jenny Dean in her blog recommended checking pH daily and adding more alkali if needed (pH should be around 10). This time I decided to trust Jenny Dean, as I liked the colours which she presented in her blog!
So, I crushed my yew tree bark, I have poured water (with a bit of soda ash dissolved) over it and put it in the warmest spot of the house (top of the wardrobe in my bedroom). I added two samples of yarn: one mordanted in alum, one unmordanted, and kept fingers crossed for the result.
I have checked the pH daily. For the first two weeks, fermentation was intense and as a result alkalinity was dropping very quickly. I needed to add a teaspoon of soda ash a day to go back to pH 10. After two weeks fermentation stopped with pH stabilized on 10, but I decided to wait another couple of weeks. I have taken samples just after Christmas, as a late present for myself ;)
And here you have.
The top sample was mordanted, the bottom one was not. There is almost no difference in colour - mordanted one is slightly warmer in shade, more orange, but it is visible just in a strong daylight. I wonder is it any difference in fastness (maybe I will perform fastness test later).
I need to mention that it is a perfect way of dyeing if you are working on your image as a proper witch (or if you need extra Halloween decorations...). Tannin dyes in an alkaline environment look similarly to blood (as you can see on the photo).
This resemblance was noticed in the Finnish tradition - the word 'veri', "blood" was once taboo, and instead, the word 'leppä' ("alder") was used. Also, a red paint made from tannins was used in Lapland to decorate the face before a ritual feast after a successful hunt. Red symbolized the blood of killed bears
(And now imagine what the neighbours might think about a woman who has vessels in the hut filled to the brim with a liquid that looks like blood ...)
But blood-like appearance, however cool, this is not what I love the most about this dyeing method. The coolest thing is that it is eco-friendly. And therefore worth to popularize (not only amongst reenactors)!
NO HEAT applied - for both mordanting and dyeing, I used cold soak method. I just put a pot with bark in warmest place of a house, and it appeared to be warm enough.
DYESTUFF FROM WASTE - tree bark is in modern times a by-waste product of wood industry. 'Annual (world) bark production is estimated to be between 300 and 400 million m3'. https://bioresources.cnr.ncsu.edu/resources/the-utilization-of-tree-bark/ I did not do further research but I suppose most of it ends as a waste. You can make wood pellets and mulch, but it requires extra work and not all bark is suitable. On the contrary, for dyeing, you can take bark straight away.
SAFE TO UTILIZE afterwards - when pH reaches neutral, the dyebath can be disposed of safely, e.g. put in a compost heap.
LIGHTFAST colours - we are speaking about tannins here, they are lightfast and do not wash away for years.
So definitely worth to know by wider audience! I really recommend it and hopefully you will like it as well.