Dying with Indigo – success!

Originally posted to the forum by Richard

So today (June 2012), after much talk, I finally made my first attempt at indigo dyeing! Huzzar! While I did manage to make some blue linen, the colour achieved was not up to the standard I was expecting, and I have a number of ideas as to what might have gone wrong. But I’ll get to those later.

The underlying chemical principles are as follows: Dark blue, water insoluble Indigo dye, is ‘reduced’ to form the water soluble leucoindigo, or “Indigo white.” The leucoindigo binds to the soaking fabric fibers, and is readily oxidised back to the blue indigo upon contact with the air.

The reduction step may be achieved through microbiological fermentation, (as in natural vat dyeing), or chemically with a number of reagents: sodium dithionite, thiourea dioxide, or sodium metabisulphite to name a few. This preliminary experiment was conducted with sodium metabisulphite. (SMB)

The method used was relatively straight forward. ~150ml of water was heated over a spirit burner, and adjusted to PH 9 with sodium bicarbonate. (Baking soda). A small chunk of indigo dye was ground up, and added to the water to give a blue suspension.

SMB was added gradually with stirring, giving off small amounts of sulphur dioxide gas (smelly  ). The beaker was covered with foil to keep oxygen out, and the mixture heated. Over the next 20-30 minutes, the blue colour faded to a green-yellow, and finally to a light yellow colour, as the indigo was converted to leucoindigo. 4 small scraps of pre-soaked linen were added and left to soak.

The pieces of linen were removed individually from solution after 10min, 30min, 1hr, 1.5hr and left to dry. The pieces were still white when the were removed from the vat, but slowly turned blue over 10-30 min. This slow change seemed odd, as everything I’ve read about indigo suggests it should take only seconds.

The four samples are shown below, ordered left to right in the order they came out of the vat.

It’s interesting to note that the extra time in solution does not appear to have intensified the colour of the dyed cloth. This is not the result of simply ‘running out’ of dissolved indigo, as the quantity used was well beyond that required to dye these small samples.

I think the less than impressive colour can be put down to 3 things:
(1) – Attempting to dye woven fabric instead of pre-spun fiber obviously limits the available surface area for the dye to bind.
(2) – Rinsing dyed fabric to soon may have resulted in loss of leucoindigo before oxidation occurs: The wash water did take on a blue colour, suggesting that this is indeed the case. Easy to fix… don’t rinse so soon! 
(3) – PH testing of solution after dyeing showed that the PH had dropped to ~4-5. As best results are achieved in alkali solution (PH > 7) the acidity of the vat may have had some negative influence. SMB reacts with water to produce sulphur dioxide, which dissolves to give sulphuric acid. I believe that excess SMB may have done exactly this, lowering the PH into the acidic range. The only way to counter this is to gradually add SMB such that the quantity added is exactly that required to reduce the available indigo.

Armed with the knowledge I’ve gained from this run, I intend to repeat my experiment tomorrow and see how/if results change.

So a couple more days mucking around has yielded a few more interesting developments:

The first batch of samples were left out over night to dry, and by the following morning they had darkened significantly:

Both of the darker patches had an obvious layer of indigo ‘dust’ attached. They were scrubbed in some cold water to remove the dust, and both patches retained a nice, light blue colour. Success! 

Following point (3) in my previous post, I made some attempts to improve the method:

– Indigo powder was placed in warm water as before, and SMB added slowly. With each addition of SMB, some concentrated sodium hydroxide (NaOH) solution was added to maintain the pH around 10-11. (Optimal range for dying cotton, linen etc..)
RESULT: Fail. No reduction occurred, presumably because the acidic SMB just reacted with the NaOH, not with the indigo. Didn’t end up adding any linen.

– Indigo powder was placed in warm water as before, and SMB added slowly. Solution heated until it took on a pale yellow colour, and no solid indigo was observed in the bottom of the beaker. A concentrated solution of sodium bicarbonate was added to raise the pH into the optimum range.
RESULT: Fail. Adding bicarbonate releases CO2 gas. Bubbles and froth everywhere… MESS…. Didn’t end up adding any linen.

– Same as above, but adding Conc. NaOH solution instead of bicarb to eliminate CO2 problem.
RESULT: More Mess… Large amount of sodium sulfate precipitated…. Beaker full of white sludge… Didn’t end up adding any linen.

Conclusions: Based on these three simple trials, it seems apparent that attempting to make an alkali solution from SMB is not going to work. A straight SMB solution has a pH of ~5, which is far from ideal, but seeing as the first trial seemed to work I’m going to roll with it and see what I can produce.

An obvious question might be “Why SMB if its such a PITA???”…. I chose it based on how EASY it is to get, in contrast with some of the other options I’ve read about. My 500g cost $6 which also seems like a plus. However, If continued experiments lead to further mess and crappy results, I might invest some time and money into tracking down some sodium dithionite which I know will work.

Took another crack at dyeing yesterday:

Having given up on the idea of an alkali dyeing solution, I tried to replicate the results of my first experiment. Indigo was placed in water, and heated. Sodium metabisulfite was added gradually until the blue colour had turned to clear yellow. A piece of pre-soaked linen was added and the beaker covered. The solution was stirred every 20 min for ~2.5 hrs.

Following the pretty poor colour obtained in the first run, I put some more thought into what might be going wrong. I still believe that it is related to the acidity of the dyeing vat, and therefore hypothesised that some version of the following may be happening under acidic conditions:

I’ve found no data regarding the acid-base properties of indigo, so I have no concrete evidence for this. My theory is that the hydrogen bonds in indigo, (The dotted lines) are in some way related to its colour. Protonating the nitrogen atoms may disrupt this bonding in some way, reducing the intensity of the colour. In order to test this, one end of the dyed cloth was submerged in dilute NaOH solution to remove the excess protons. The sample was left to dry over night.

The classic before/after shot is shown below. The section of cloth subjected to the NaOH has darkened more than the untreated section adding some weight to my theory. However the colour is still less than impressive; The photo makes it look more blue than it is.

So after another disappointing result I returned to the internet: Trawling a number of chemistry journals yielded a large quantity of papers regarding the synthesis, use, and properties of Indigo, but no mention of Sodium Metabisulfite as a reducing agent. Expanding the search beyond the scientific community, to include all the arts-and-craft type web-sites still gave no reference to SMB. I think by now it is safe to conclude that while SMB CAN produce the required reaction, it does so in conditions which are completely unsuitable for effective dyeing. As a result, I have also begun searching for a source of sodium dithionite, the most commonly stated reducing agent in indigo dyeing recipes. Unfortunately it seems to be controlled here, as its difficult to find a reference to it. A number of large chemical companies do have it but, having emailed them, they seem unwilling to sell a toxic, flammable compound with a tendency towards auto-ignition to a member of the public 

oh well.

Investigations are continuing…….

So I’ve finally got the the bottom of the whole Indigo situation!

All of the difficulty can be summed up as follows: The Indigo, brought home from Morocco by my parents, was not actually indigo. I’m still not sure what it really is, but my suspicion is that it is at least partially composed of “Indigo Carmine,” a modern, synthetic derivative of indigo, perhaps with some actual indigo mixed in…. but not much.

I’ve suspected for a while that the “100% pure indigo” was not actually genuine, and so recently invested $10 on eBay to get an “Indigo dying kit” from the UK, complete with indigo woad extract and the sodium hydroxide and sodium dithionite required to use it.

Did my first experiment with the UK dye this evening and, in the interest of scientific investigation, copied the procedure exactly with the Moroccan stuff to compare results:

0.45g of sodium hydroxide, and 0.45g of indigo dye were placed in two jam jars. 150mL of hot water was added to each, and the solutions stirred to bring the indigo into a suspension. The Moroccan dye immediately turned yellow in the basic solution, which supports my “Indigo carmine” theory. Indigo Carmine is used as an acid-base indicator as it turns yellow at pH > 13. anyway…..

0.3g of sodium dithionite was added to both jars, and the caps put on tight.

Over the next hour, the UK solution gradually turned greeny yellow, with bits of rotting plant matter floating around. The Moroccan stuff just settled to the bottom leaving a clear solution…… fail.

A sample of Cotton, and a sample of linen were pre-soaked, and added to the UK jar. Didn’t bother adding anything to the Moroccan jar, as I’ve seen the above situation enough by now to know not to bother wasting the 25 square centimeters of fabric it would use…..

20 minutes later the samples were removed and immediately turned dark green. In terms of the Citadel paints colour range we’re looking at something around a “Catachan Green….” Or a “Castellan Green” in the newer range…
After a few minutes the green started to turn blue, and after a thorough washing the pieces were allowed to dry.

Below is a side-by-side shot of the best I could achieve with the Moroccan dye, using stupidly large amounts of dye and reducing agent, Vs the two samples I dyed this evening using the UK dye.

Of the samples on the left, all are linen except for the central one which is cotton. of the two on the right, the top is linen, and the bottom one is cotton.

Things learned from this experience:
1 – Don’t trust anything Moroccan market vendors say.
2 – Dyeing with Indigo is EASY. Not as complicated as you might be lead to believe, having been mislead by Moroccan market vendors. (See point 1)
3 – Genuine Indigo, extracted from proper woad, STINKS… BAD… Previously my issue has been the less than pleasant smell produced by sulphur based reducing agents like Metabisulphite and dithionite, but this concern has been TOTALLY eclipsed by the rotting fish smell produced by composting woad.

Additional Information

Ida in Sweden overdying yellow clothes with indigo to get mottled green medieval camo for hunting. Plant Dying – Yellow, Blue and Green