Tag Archives: dyeing

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.

image_medium

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.

sigstart

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.

sigden

‘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.]

sig30nov-fibre

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.

sig9decremove

‘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.

sigyarnsoxidising

‘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.

sigyarnsfinish

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.

Planning another Summer of Blue

I had planned to focus on indigo last summer, but in the event various other things intervened – although I did accomplish more than I posted. I must show you my shoes!

I’m making tentative plans for more blue this summer.

greenhse

I had hoped that the tadeai, the Japanese Indigo Polygonum tinctorium, would survive the mild winter in its house but even though I added fleece and bubblewrap when the nights dropped to -4C, there’s no life in the stems. It may be an obligate annual, or it may simply have flowered itself to death late last autumn without setting any seed, alas.

I admired the few remaining dead leaves somewhat wistfully: look, they’re blue!

DeadindigoGrense

And then cut most of the stems off. I’ve left a few root masses with stubs of stem for the moment, but there was no green in the pith and the roots I pulled up looked brown and dead, so I doubt anything will shoot. But I sowed seed as well; the seed tray is now sitting indoors in the warmth where I am already watching it hopefully.

I took cuttings  – lengths of stems with leaves – from the plants last October when we returned from our holiday and put them in a jar of water. As might be expected from a Polygonum the cuttings formed masses of roots from the submerged leaf nodes, but the rest of the nodes sent out flower shoots.  I pinched them out and more appeared. The cuttings were extremely well-rooted and looked reasonably healthy in late January when I potted them, but just continued to try to flower until they died, leaves slowly shrivelling. A friend who rooted cuttings in August says two are still alive and seem well, so I wonder whether the trick is to catch them before environmental cues set the growth points to ‘flower’ mode. I will try to test that this summer, as cuttings would be the best way to maintain a line that dyes well.

The woad looked reasonably happy. As a biennial this is its year to flower and seed, with leaves that yield little or no blue. I left the plants last year thinking to gather the seed – these were grown from seed I harvested two years ago – but there isn’t really enough space in our tiny garden to keep plants for seed especially when they attract so many Cabbage White butterflies to lay eggs on my salad rocket! The woad is no longer happy: it is composting and I’ll sow more in the Brassica Bed shortly.

Woad

All this plus ‘standard’ indigo vats means there should be more blue this summer. But what shall I do with it? I’m designing stencils to print with fresh indigo leaves, because that was immense fun. But there’s much more fun to be had: a friend sent me two fascinating books.

dyeing

dyeing1

This is the work of Mrs Akiyama, who made a personal study of the natural dye plants of Awa in Tokushima Prefecture, Japan. Awa is famed for its Polygonum tinctorium indigo dyes; there are pages detailing dyeing with fresh and dried indigo (if I’d had more dead leaves I could have dyed with them!) as well as many other plants. And there are notes on working with bast fibres, as well as information on finishing yarns and fabrics after dyeing.

dyeing2

The other book tells me what to do with what I’ve dyed:

Ikat

No more peering at fuzzy videos on YouTube! With luck this will be a summer to remember.

To finish, a reminder of last summer.

shoes

Super-cheap runners, sold as ‘cotton’ (I have my doubts, see below). I washed them thoroughly with dishwashing liquid and rinsed them thoroughly too, then dumped them into a standard thiox indigo vat on the stove.^1 No matter how many times I dipped them, the uppers would not go darker than this; given the depth of blue in the soles. I wonder whether the fabric is in fact polycotton.

The first dyeing attempt was extremely patchy when it dried. Blotches of pale fabric near the heels, more blotches on the toes. I left them outside in the weather for a week or two to think about their sins while I sulked. Then I needed to exhaust the vat, so I reheated it, added reducing agent, and chucked them back (dry, I think), out of curiosity rather than any expectation of success. Imagine my surprise when, after two or three dips, they emerged a relatively solid and respectable pale blue. After washing thoroughly and drying, I considered decoration: stencilling with fresh tadeai was the obvious answer. I cut an assortment of crude paisley stencils and discovered that stencilling on curved surfaces is rather tricky. But the end result is a pleasing variety of blues produced by a variety of indigo techniques, especially when embellished with tablet-woven laces.

It’s a great shame that the shoes are so uncomfortable :-/

^1: A caution. If you try this, be prepared to spend ages scouring melted runner-sole off the sides of the pot. A hint: reheating the pot gently softens the stuff a bit, but it still requires a lot of effort.

 

A quick post: indigo leaf prints

Just to prove I’m still playing with indigo, although I’ve had less time than I thought I would.

If you’ve only got a few leaves, try leaf prints.

IndigoLeafPrintSilk1

This is Japanese indigo. The technique also works for woad leaves, although the print is fainter than this, at least from my plants; ‘true’ indigo should also work. I am astonished by how much of the leaf structure can be captured by this technique. (The skulls were printed by rubbing fresh leaves through a stencil; I have to post this, too!)

Leaf printing seems to work best on finer fabrics like this silk; I tried it on a heavier cotton/light canvas, and the juice didn’t penetrate far enough to make a good print.
Take a leaf, put it on the fabric, cover it with a piece of clear plastic/heavy cellophane (I used a piece from a packet of oat bran). Take a dessert spoon and hammer the leaf with the bowl of the spoon through the plastic; try to make sure you cover (damage?) the entire leaf area. I think this breaks the cells to release the juice. Then put a dab of washing-up liquid on the plastic to act as a lubricant (don’t get it on the cloth) and ‘polish’ the leaf through the plastic, using lots of pressure. Too much and you can see the leaf squash into mush that obscures the detail; too little and the juice isn’t pressed into the fabric. Practice. Have fun!

IndigoLeafPrintSilk3

pH damages protein fibres: a test.

SilkpHDamage3

“Protein fibres prefer acid conditions; plant fibres prefer alkaline conditions” is a truism that I’ve repeated myself on many occasions. It’s a commonplace when discussing indigo dyeing, as the vats are generally alkaline: indigo is more soluble at high pH. Warmth only makes things worse: hot alkali is said to cause damage faster than cold. Liles and others mention the risk posed by the vat fluid “Indigo vat fluid may be allowed to dry on cellulosic fibres but should never be permitted to do so on protein fibres, since the alkali becomes more concentrated as drying proceeds” ( JN Liles, 1990. The Art and Craft of Natural Dyeing. University of Tennessee Press).

In accordance with this I don’t allow the vat fluid to dry on my silk and wool; indeed, I sometimes put dyed goods into a citric acid rinse as soon as they’re cool, then plunge them into cold, thoroughly oxygenated tap water. The theory behind the cold rinse is that not only does this rinse out alkali not neutralised by the citric acid and oxidise the indigo compounds, it washes out loose indigo particles while they’re still truly loose, not dried onto the yarn or fabric to wear off later onto skin or other garments.

But I’ve been wondering exactly what damage alkali does, and how quickly the damage occurs. Should I be quite so paranoid when working with wool and silk, counting the minutes that the dyestuff is in the vat? I decided to investigate.

Bombyx aka Mulberry Silk seems more fragile than wool and therefore more likely to reveal damage. I took samples of commercial silk fabric, yarn, and Bombyx spinning fibre, tied them into bundles and submerged them in a 1-2-3 Fructose vat. I tested the pH of the vat at the start of the session and every time I removed a sample: it was over pH11 the entire time. The vat was maintained at 50–55°C for the duration of the test save for a brief overheating to something like 65°C  for about 30 minutes between 2 and 3 hours into the test (I forgot to turn the gas off! Oh, for a water bath). The samples were removed from the vat, cooled slightly, then briefly rinsed in a citric acid solution (pH4) before further rinsing in cold tap water and air drying.

SilkpHDamage1

And this is the result of the test. From left to right:

Undyed silk; 30 mins; 1 hour; 2 hours; 3 hours; 4 hours 15 mins; 5 hours 15mins; 7 hours; 9 hours;  10 hours 45 mins (I had to go to bed, the next day was a gym day).

I think there’s a visible difference: after nearly 11 hours at 50°C or higher at >pH11, the silk on the end is less lustrous. As the photo suggests, it’s lost some of its drape, its flexibility. The difference is more obvious in the hand: that silk is ‘cottony’, it’s not as smooth as silk. So, that’s likely to be one aspect of pH damage.

To me, the most interesting thing is the time it took for the damage to be noticeable. (Note: the temperature peak 2–3 hours in is likely to have had some effect, but I don’t think it’s terribly significant – there’s no perceptible jump in the degradation at that point.) I can’t detect a difference between the undyed silk and that which was in the vat for an hour. I’m not even certain there’s a significant difference between the 1 hour and 3 hour samples; beyond that, the fibre does seem to be more cottony. The depth of the blue colour doesn’t change from 30 minutes, so there’s no benefit to leaving it in longer. It seems likely to me that the damage is cumulative, so caution is indicated when dipping repeatedly for darker colour, but still there’s far more leeway than I feared. I’m no longer going to count minutes and seconds when silk or wool is in a chemical vat for 15 or 20 minutes.

The colour change intrigues me. Again, the blue had a greenish tinge at 1 hour, so it’s not attributable solely to the temperature change. Is the alkali causing the silk to yellow? I need to try this again with washing soda and plain white silk.

SilkpHDamage2

What to do with indigo, part I

So what am I doing with the vats? At this point I’m primarily interested in seeing how easy they are to assemble and run, and what the pH is, information that will help me to choose vat methods for dyeing projects in the future. Mostly I will be dyeing wool for spinning and spun yarn for weaving, but it’s a shame not to investigate other possibilities.

Fabric is an obvious choice; it would be impossible to dye garment lengths in my relatively small jars, but I can play with techniques I haven’t used before, such as shibori. Well, used seriously. Like anyone growing up in the 1960s and 70s I did my fair share of mad tie-dyeing, but there are many other, more disciplined techniques. I have a thing about grids, regular patterning, so itajime (shibori patterning created by pressure) with squares of wood caught my eye. Two 1.5″ squares of board, some string and a strip of silk were easy to assemble… on reflection, for my first attempt, I should have tried larger squares and cotton, because ironing the slippery silk into the necessary neat square packet was exceedingly difficult. Tightly tied between the squares, the folded silk was thoroughly soaked in tap water with a drop of detergent as wetting agent before spending about 10 minutes in the fructose vat, reheated to 50°C and revived with 5g indigo, 10g, calcium hydroxide and 15g fructose.

SilkSq.Itajime

Opening it was my reward for doing the ironing; an hour later and a rinse in vinegar later, I had this:

SilkSq.Itajime2

Lots of room for improvement, but proof of principle nonetheless!

Note the rather pretty patterns on the wood squares. Indigo will dye anything that has roughness to trap the particles. Tokushima Prefecture in Japan is trying to find new products for its traditional indigo vats, including wood for speaker cabinets and mug mats as well as fabulous flooring. I moved the fructose vat into a jar in the greenhouse after dyeing the silk in it last weekend; although it was not heated, I noticed there was a thin flor on the surface, so I dropped a scrap of wood into it for a day, removed it for a couple of hours to dry, then put it back for another day.

Fresh from the vat it was a beautiful green blue. Note that the dense wood associated with the knot has absorbed less of the indigo.

WoodTwodips2 days

Almost dry and it’s definitely blue.

WoodTwodipsDryish

Three days later, fully dry, I’ve given half of it a single coat of yacht varnish, the only glossy thing I have to hand. Sadly it’s slightly yellowing, whereas I think those floors are protected by a thick layer of perfectly clear polyurethane.

WoodTwodipsVarnish

But still, pretty. Worth a few more coats, I think.

The Fructose 1-2-3 vat: an indigo vat for the 21st century?

Using only urine (ammonia) and natural indigo, the sig vat is perhaps the least polluting, most environmentally-sound of all the indigo vat recipes, but the need to collect and store urine, the smell, and the need for multiple dips to obtain darker blues meant the dyeing industry welcomed development of new methods of dyeing with indigo. By the late 20th century the (extremely convenient) sodium dithionite/hydrosulphite vat (the zinc-lime vat developed in 1845 is a type of hydrosulphite vat) was responsible for the bulk of indigo blue on textiles, but concerns about pollution associated with the process – toxic sulphites, sulphates, and sulphides are produced as degradation products – was increasing. In the late 1970s thiourea dioxide (effective at only 10–12% the usual concentration of sodium hydrosulphite), was being proposed as a more environmentally-sound reducing agent. Now, reducing sugars are under investigation.

Reducing sugars contain or generate aldehyde groups that, activated by alkalinity and heat, will absorb oxygen from a solution. Fructose and glucose found in ripe fruit are reducing sugars; ordinary sugar – sucrose – is not a reducing sugar. So it’s possible to create an vat from indigo, lime (calcium hydroxide) and over-ripe fruit such as bananas, or dates that relies on the chemistry of the sugars rather than fermentation of the fruit. (There’s a blog post about a banana vat here.) But it’s easier to prove the principle using fructose from the supermarket shelf in Michel Garcia’s 1-2-3 recipe. It’s very simple: One Part indigo to Two Parts lime to Three Parts fructose, plus warmth.

I warmed roughly 1 litre tap water (for the record our tap water is alkaline, roughly pH8.4) to 50–60°C in a saucepan. I dissolved 15g indigo – for this I used Tamil Nadu indigo – in warm water, added it to the saucepan, then did the same to 30g lime, then gently stirred in 45g fructose. The Tamil Nadu indigo is markedly ‘blue-er’ in solution than the ordinary indigo I’ve used up to now; the solution was a vivid deep blue.

Start

I left this to work for about an hour, at the end of which it showed all the signs that indigo had been reduced and the vat was ready.

ReadyFlor

Vat ready for use. Note the flor, the metallic copper/purple/blue skin of indigo that forms where the reduced indigo oxidised on contact with air. In this vat there were metallic bubbles, too. Beautiful! Scrape the flor to one side with a spoon before adding materials to the vat, or remove it altogether and replace when you’ve finished.

Beautiful, but the strong reduction is an indication of high pH and indeed, the pH paper read 11 or higher. I added a skein of handspun silk, DRY, because I wanted to see what happened if I did so. The silk instantly went blue, as did the vat; wearing rubber gloves I squeezed air out of the silk until it sank, then reheated the vat to 50°C and left it to work, theorising that there was sufficent fructose to re-reduce the solution.

45 minutes later I removed the silk. Beautiful!

Silk45minsFructose

I thought about a second dip, but worried about the effect of longer exposure to high pH and decided to wait to see what colour this became. After all, I have all summer to make things blue.

IndigoThreeShades

Three silks for comparison: on the left, 45 mins in the fructose vat; in the middle, about 12 hours (two dips) in the sig vat; on the right, 90 minutes in the sig vat. Interesting!

That traditional sig vat: results!

We left our trusty sig vat on May 15th, hoping for a warm day that didn’t occur for another two days. But by the 17th the liquid was very definitely yellow-green although there was a lot of sediment and  particulate matter adhering to the glass of the jar. I think this is a combination of vegetable and mineral matter in the indigo powder – the actual indigo content of ‘ordinary’ indigo may be well under 30% by weight – and some debris from the urine itself. At any rate, Sunday the 17th was forecast warm and sunny, so on Saturday I de-gummed 2 smallish skeins of handspun silk, neutralised the washing solution with a vinegar rinse, then rinsed them thoroughly to remove the vinegar, and left them soaking in water ready for dyeing. Remember that the soluble leuco-indigo needed to dye material converts to the insoluble, non-dyeing blue form in the presence of oxygen: it is generally advised that fibre be thoroughly wetted to minimise the amount of oxygen it introduces into the vat. If you’re dyeing in a urine vat, urine is the obvious fluid to use for wetting, as it adds nutrients for the fermentation.

Sunday morning I cracked open the lid of the jar in the greenhouse and the smell of the vat rose to greet me. A powerful aroma, now with some ammonia in the mix. I put both skeins in the jar, which on reflection was a bad move: there really wasn’t enough room for both. Also, as they were loose in the jar, they came into contact with the sediment.

May17SilkMy handspun silk in the urine vat in the greenhouse with the Japanese Indigo plants.

I couldn’t wait to check whether it was working. Literally. The timestamp tells me this photo was taken only three minutes after the skeins went into the vat. How sad is that?

May17SilkTest

But look! It’s going blue! I pushed it back into the jar to go bluer.

I removed both skeins from the jar after 1.25 hours and hung them to dry on an improvised rack to oxidise. Note how turquoise the yarn appears at the beginning of the oxidation.May17SilkOne

Now, at this point I digress to explain that throughout the afternoon my husband was walking back and forth from house to garage past the greenhouse and, eventually, the yarn from the vat. When he first encountered the smell of the vat he commented that it was a bit whiffy. I quickly shut the greenhouse to trap the odour. But by 1530, with the yarn out to oxidise, his comments were more forceful, along the lines of “The birds are falling dead from the trees!” The smell does fade from the dyed material; it’s much less noticeable when the material is dry, and I find that after washing with scented detergent and hanging to dry outside on the rosemary bush, I can only smell it if I press the fabric to my nose. The distinctive scent of the urine vat was once regarded as an indicator of good cloth well-dyed: when chemical vats were introduced in the second half of the 19th century, cloth-buyers would check for the smell of the urine vat. Some cloth merchants used commercially-available fake urine vat scent to mislead them!

May17SilkFirstOut

As the oxidation proceeded I was able to see just how much gunge was stuck to the skeins, but could reassure myself that it will wash off (and it mostly did, even in the quick post-dyeing wash). I put the vat to bed (as it were), topping it up with fresh urine and replacing the bag of indigo.

Once washed and dried the skeins proved paler than I’d hoped, so I tested one of the characteristics of the urine vat. With pH paper. Most indigo vats are strongly alkaline (my post on Indigo chemistry explains why), running at pH11 or even higher. This alkalinity degrades and damages protein fibres such as silk, so repeated or lengthy dips to get greater depth of colour are a bad idea. The urine vat, however, is much less alkaline: this one, yellow-green and ready to dye, tested pH9 on the 24th May. I decided to see what happened if I put a dry skein of silk for a longer period (dry because I had read that putting the fibre in dry could give a greater depth of colour). I pushed a skein into the jar late in the afternoon, took it out to oxidise for a couple of hours in the early evening – it was noticeably darker – and then replaced it in the jar overnight. End result: a significantly darker indigo.

May25thSilkCompare

It’s still a little bit turquoise, but obviously much darker than the skein next to it. Which is in fact a bit darker than it looks in the photo: light reflecting off the silk makes it paler.

Conclusions

Pros: This is the easiest and least expensive of the vats I’ve tried so far (more than I’ve written about so far), and it’s kinder to protein fibres such as silk and wool than the other vats. The results are good, but I think they’ll be better in a different vat. I think a higher grade of indigo (containing less waste material) will give me darker colours, and a larger, deeper vat will make it possible to avoid any sediment, perhaps by placing something like a colander or strainer at the bottom of the vat to keep the material off the bottom. So I’ve ordered a better grade of indigo and I’m about to start collecting more urine.

Cons: This vat has a strong, distinctive, and – to some people at least – highly unpleasant odour. While the odour from the vat can be controlled and contained, the smell coming off the dyed material as it oxidises is less easy to contain if it’s hung out to air. However, it is possible to oxidise the dyed material in well-oxygenated water: allow the cold water (cold water holds more oxygen than hot) to plunge into a container in such a way as to generate lots of aeration bubbles. Immerse the fabric in that water, swirl it around, keep the tap running to refresh the oxygen. This is not a technique to use with warm wool, but I have tried it with silk and found no damage, although the skein could have done with more ties. In theory it should not remove indigo that wouldn’t be lost in the normal washing process, but I have no real way to test that theory at the moment.