Tag Archives: Ditchling

Ethel Mairet’s natural dyeing II: Extract of Indigo

As I described in part I, Highland Blue, the Ditchling Museum of Art & Craft is celebrating the centenary of the publication of Ethel Mairet’s A Book On Vegetable Dyes by asking dyers to replicate the recipes. Because I have a thing for blue I volunteered for two of the indigo recipes; this is the second.

4 oz. sulphuric acid, 1 oz. finely ground Indigo. Mix like mustard,  and leave to stand over-night. Prepare the wool  by mordanting with 5 oz. alum to 1 Ib. wool. Boil for 1 hour and dye without drying.

This is the dye also known as Saxon Blue, Chemie, Indigo Carmine, sulfonated indigo, indigo sulphonate, and indigo sulphate, which according to JN Liles in The Art and Craft of Natural Dyeing, was discovered in 1740 not long after concentrated sulphuric acid was first made. In my mind’s eye I see a chemistry lab with in which people in lab coats pour this new acid on everything they can think of, just to see what happens, but I suspect it was much more directed. New colours and methods of dyeing textiles were much in demand, so perhaps it was logical to experiment on indigo. It’s completely different from the ‘Saxon Vat’ of the peasants of Saxony in which the fermentation of matter in unscoured fleece drives an indigo fermentation vat.  Although indigo sulphonate is said to have first been used in Saxony. Clearly a hotbed of inventive dyeing, Saxony.

Anyway. In short: the addition of sulphuric acid to indigo converts the indigo into an acid dye, with the sulphuric acid content of the dye bath acting as the acid to fix it to the dyestuffs.

I’d been wondering about trying a sulphuric acid indigo blue long before the Ditchling Mairet project. Blue and smelly is good, blue and DANGEROUS is just as attractive to my inner 5-year-old. But as with ‘ordinary’ indigo, I needed to understand the process before I started. There’s a lot of information on the internet. I was charmed by p255 of ‘The Practical Mechanic and Engineer’s Magazine’ for 1843


An excerpt from this delightful publication.

which explains quite simply the basic problem I faced: as a private individual in 2016 I can’t buy strong enough acid. 98% as sold on Amazon *sounds* strong, but it’s not: ‘fuming acid of Nordhausen’ is in fact 102% by volume. I will not take you down the by-ways and cul-de-sacs I explored online investigating the solubility of indigo (it’s remarkably recalcitrant stuff) or the phone calls and plaintive emails sent to various chemical suppliers begging them to send me the good stuff. I offered to come and collect it so they could assess my harmless middle-agedness themselves. The answer was always No, Sorry, We’re not allowed to. So I ordered what I could get (which is sold as drain cleaner),  an assortment of lab glassware, and some safety kit.

CAUTION: DO NOT even think about trying this unless you have experience working with strong acid and are fully aware of the hazards and precautions necessary. 98% Sulphuric Acid might be weak for my purposes, but it’s strong enough for the fumes to damage your lungs and eyes. The liquid will burn holes in most things including human flesh.


I faffed a bit about the quantities because clearly, given I had 98% acid, this would be as much art as science. In the end I used 15g of my most expensive indigo powder to 113 ml sulphuric acid. Mixing the Extract of Indigo was quite straightforward. It does mix ‘like mustard’ – it takes a lot of stirring to thoroughly wet the indigo powder. Better to start with a very little acid and work into a paste before adding the full volume. There was no visible reaction when I added acid to the indigo powder but the reagent flask gradually became gently warm to the touch so heat evolved.

Some recipes for Saxon Blue suggest adding chalk to the mix; Mairet doesn’t, and Liles says there’s no need, so I didn’t.

‘leave to stand over-night’
It’s clear from much that I’ve read (including Liles) that this extract in fact benefits from a longer reaction time – if it is well-sealed so the acid can’t absorb water from the atmosphere – so I wasn’t concerned when I couldn’t use it exactly 24 hours after mixing.

The books agree that Saxon Blue is a dye for wool and silk, which makes sense because acid dyes require an acid (low) pH. Acid conditions render cellulose fibres (cotton, linen, ramie) less able to absorb dye; a friend familiar with the chemistry of paper (also cellulose) says “Higher pH on cotton produces very strong covalent bonds. Less pH value, weaker, if any, bonding.” (thanks, Terry!) So the dye particles simply don’t stick to cotton and linen under acid conditions. I had samples of wool, silk, cotton and linen to test this. I mordanted the wool and silk with alum and cream of tartar, one skein each of cotton and linen with alum and washing soda, and left the spare pair of cotton and silk unmordanted out of curiosity.

Mairet instructs readers on the use of the extract in various other recipes.
3) To Dye Wool With Indigo Extract: For 4-6lb of wool. Stir 2 to 3 oz of Indigo extract into the water of the dye bath. The amount is determined by the depth of shade required. When warm, enter the wool, and bring slowly to the boiling point (about 1/2 hour) and continue boiling for another 1/2 hour. By keeping it below boiling point while dyeing, better colours are got, but it is apt to be uneven. Boiling levels the colour but makes the shade greener…

Given that this is basically ‘add what you think looks right’ I worked from Liles’ recipe, reducing the quantity of extract for my 3litre pan.


The colours are not quite right in this, thanks to weird light from both cooker and room lights. It was a little more turquoise than it looks, visibly different from ‘ordinary’ indigo blue. Also, as the dye heated, it developed the chemical odour I associate with rooms full of weaving yarns: not wool, nothing to do with wool or lanolin, a rather sharp indeed acid chemical smell. Very interesting. I opted for a gentle simmer rather than a boil as I wanted ‘better colours’. After 1/2 hour at about 85°C I put the yarns in the sink to cool and ran the dye bath to waste with copious water. And then kicked myself, almost literally, because I forgot to check the dye bath pH at any point. Next time.


On my machine those colours are about right. The pale yarns are the cotton and linen, as might be expected; note there’s no obvious difference between mordanted and unmordanted. The dark are one wool and two silk. Again, no difference in shade between mordanted and unmordanted silk. Once they’d cooled, I began rinsing, and things became interesting because the silk lost a lot of dye. A LOT of dye. I tried to hurry it along by washing a second time with dishwashing liquid rather than gentle soap, but I left it in the water to soak out the detergent and the water turned dark blue again. I put the yarns in clean water and left them overnight: in the morning the water was dark blue. Something interestingly chemical was happening to strip the dye, I think. But I’m not a chemist-dyer, I’m fermenting organic smelly sludge dyer so I need to do some reading.

saxonskeinsAnd here is the result, reskeined with the Ditchling Museum Project Record. I’ve added a bobbin of cotton dyed with with an ‘ordinary’ indigo vat for comparison: it’s clear to my eyes at least that the Saxon Blue is more teal, somewhat greener than the ordinary. But sadly not nearly as green as the example I saw produced by a friend with access to Fuming Sulphuric: the stronger the acid, the higher the proportion of green ‘sulph-indylic’ dye formed by the reaction. But still, it is very pretty.


Now I’m thinking of offering an Indigo Dye workshop next summer, working with fresh leaf indigo and/or woad, plus at least two vats. If I do I’ll happily accept 100ml lab grade fuming sulphuric acid as part payment from one attendee!

Ethel Mairet’s natural dyeing I: Highland Blue

The hand weaver and dyer Ethel Mairet was a significant member of the British Arts and Crafts movement in the early 20th century. Wikipedia has a summary of her life and work here. (“Ethel went away alone and built a house near Barnstaple complete with studios for textile dyeing and weaving” seems such a bald summary of what must have been a terribly difficult time.)

Ethel Mairet is one of my weaving ‘ancestors’, in that my mentors were taught by people who were themselves taught or inspired by Mairet’s work. I had the pleasure of seeing some of her weaving in the flesh at the National Portrait Galleries ‘Anarchy & Beauty’ exhibition, including a jacket woven from handspun eri silk dyed with natural dyes, here shown in A Weaver’s Life: Ethel Mairet, 1872–1952.


Mairet moved to Ditchling, Sussex, in 1916 to join a group of crafts persons including Eric Gill. In the same year (Wikipedia is wrong) she published A Book On Vegetable Dyes (free download from archive.org at that link). Now, a century later, Ditchling Museum of Art & Craft is celebrating Mairet’s work, asking dyers to recreate her recipes for display today.

How could I resist?

I volunteered to work with two of her recipes*, the first being Recipe 8, p69: BLUE FOR WOOL (Highlands), which begins
‘Take a sufficiency of Indigo. (For medium shade about 1 oz. to every pound of wool).  Dissolve it in about as much stale urine (about a fortnight old) as will make a bath for the wool …’
At heart I am about 5 years old: I cannot resist making blue using a smelly technique that other people find revolting. But more importantly, the indigo urine vat is one of the oldest methods of dyeing. I wrote about its history and chemistry here as part of my Summer of Blue. That vat suffered as a result of my inability to keep it warm, but I’ve since found a way to maintain the necessary temperature.

I put the ‘donations accepted’ container in the bathroom and a couple of weeks later the experiment began.


All natural indigo powder contains some non-indigo vegetable matter, the proportion of which is reflected in the cost per gram. The vegetable matter is undesirable not only because it isn’t indigo (and hence doesn’t dye), but also because it adds to sediment at the bottom of the vat that can stain things that contact it. I know from experience that bacterial breakdown of the urine creates sediment anyway, so I used a cheaper grade for this vat, adding about 15g of powder to the 2.5l of the stale urine in a 3litre jar on the 27 November. At this point – having been stored at cold room temperature – the urine was at pH8 and smelled largely of stale urine with very little ammonia.

‘Make it lukewarm.’ 
The first time I made a sig vat, I tried everything I could think of to keep it warm, from black plastic bins in full sun (or what passes for full sun in a British summer) to an insulated box and a hot water bottle and finally a heating pad. Nothing worked consistently: slow/occasional fermentation would bring the vat into condition, I could dye a small quantity of fibre – and then it would stubbornly remain blue and miserable for days. That failure – when the instructions seemed so straightforward! – drove me to find out how and why indigo vats work. Now I know temperature was probably only one of several failures, and I have the tools to check and correct the performance of the vat.

Last summer I built a den in the garage for my indigo fermentation vats: a sturdy cardboard box floored with a thick layer of insulation, with a tray acting as lid. The cord running into the box powers a home-brew ‘brew band’ that keeps a vat at blood heat. During the summer I used it to heat a large volume of liquid in that plastic bin, but here the bin is a waterbath for the urine vat in the glass jar. I put the jar in the bath to warm overnight while I wetted the yarns from the Ditchling Museum.


‘Put in the wool and keep it at the same temperature till the dyeing is done.’
The plastic screw-top bottle hiding shyly to the left of the box is my ‘spare’ urine supply. Fermentation vats rely on the nutrients in the urine, so I soak dyestuffs in urine to wet them: wetting with tap water will only dilute the urine and weaken the fermentation.

On 28 November, after only 24 hours of warmth, the vat was alkaline, at or near pH10 and smelled noticeably of ammonia. HURRAH, bacteria are digesting the urea in the urine, consuming oxygen and creating ammonia to raise the pH: in other words, this vat is working!sig30nov-addfibre

I added the four yarns, each skein suspended by a string to prevent it falling into the sediment that would develop at the bottom of the vat. This wasn’t entirely successful, alas. Better to use a deeper vat with a lot more urine.

‘For a deep navy blue it will take a month, but a pale blue will be done in 3 or 4 days. Every morning and evening the wool must be taken out of the dye bath, wrung out and put back again. The bath must be kept covered and the temperature carefully attended to.’
[I checked the pH as well.]


Taking photos one-handed with an iPhone when also handling something with a pungent, penetrating and lingering odour proved tricky, so I don’t have many photos of the process. Here you can see the characteristic yellow-green of an active indigo vat turning blue before my eyes.

By 10 December the vat was at pH8-9: regular opening and airing allows ammonia to escape as gas, so the liquid became less alkaline, while moving the yarns about added oxygen. Both of these mean more indigo in the vat was returning to its blue non-soluble form.


‘Some add a decoction of dock roots the last day, which is said to fix the blue.’
I didn’t, and I wouldn’t say it. All my research suggests indigo isn’t fixed in that way: once the colourless soluble form oxidises, the particles are either wedged firmly in place or they aren’t. I suppose dock root decoction might act like a glue to coat the yarn and hold the particles in place, but that’s only going to last until the glue is washed out.

I called it a day, removed the yarns for the last time and left them to fully oxidise, slightly thankful that the cold weather means the smell is somewhat less all-pervasive than it was in the summer, when my husband claimed the odour was killing birds as they flew past.


‘The wool must then be thoroughly washed. This is a fast dye.’
Well, for some meanings of the word ‘fast’. Slow in terms of time, but fast in the sense that it’s generally regarded as wash- and lightfast once any unattached particles of indigo are removed. Jeans and other items dyed with indigo lose colour as the surface of the fabric erodes to reveal undyed material.

I handwashed the skeins in a non-bio detergent for delicate fabrics and rinsed thoroughly in cold water. Given the alkalinity of the vat and our extremely hard water I finished the wool and silk in a final rinse acidified with vinegar.


The yarns and process summary, about to be packed off to Ditchling. I am pleased to report they not only demonstrate one of Mairet’s dye recipes, but also the classic ‘vat odour’ that once indicated the finest and fastest shades of blue.

*my work on the second recipe, ‘Extract of Indigo’ (treated with sulphuric acid!) can be read here.