Thanks to Helen Melvin’s booklet on indigo dyeing, “The Colour of Sea and Sky”, and Helen’s comments on the merits of making an indigo stock solution, I decided to experiment again with this method of making an indigo vat. This was the method I first used with indigo over 30 years ago and Helen’s book reminded me of the advantages of the stock solution method, among them the fact that indigo reduces more efficiently in a concentrated alkaline solution and there is therefore less likelihood of wasted, undissolved indigo. Also, using a stock solution means that the colour can be built up relatively easily.
So why did I stop using the stock solution method? The main reason is that the one-step-bath method I generally use now, and which appears in my books, is more practical for one-day workshops, as the vat is quick to make and this enables me to demonstrate how to make and use an indigo vat in a short space of time. However, after we have used this vat there is often some indigo remaining, which gets thrown away, (unless students can take some solution home with them). Indeed, I often tell students that this vat is even better on the second day, as the indigo has had more time to reduce completely. The other reason for abandoning the stock solution method is my desire to avoid the use of caustic soda, which can cause bad burns if not used with great care.
Helen’s stock solution recipe is quite simple and involves mixing 50gms of indigo powder with 2 tablespoons each of caustic soda (the alkali) and thiourea dioxide (the reducing agent).
For my stock solution I used sodium hydrosulphite (hydros) as the reducing agent and this also works perfectly. I first mixed the indigo powder to a smooth paste in 4 tablespoons of very hot water in a large heatproof jar. I then topped this up with hot water but no hotter than 50C, which is about as hot as a hand can tolerate. (NOTE: Don’t fill the jar too full and leave enough space for any bubbling or fizzing that may occur.) Then I very gently stirred two tablespoons of caustic soda into this solution, followed by two tablespoons of sodium hydrosulphite (or use thiourea dioxide instead of hydros). This is the stock solution. NB Remember to wear rubber gloves and bear in mind that both caustic soda and the reducing agent should be added carefully to water and never pour water directly onto them.
Keep the jar warm and after about 45 minutes the stock solution mixture will become a clear amber colour, although the surface, which is in contact with oxygen from the air, will be a dark blue colour with a lustrous sheen. The stock solution is now ready to use.
To make the vat, fill the container up with water no hotter than 50C, add a teaspoon of hydros (or alternative reducing agent) and a teaspoon of washing soda and allow to stand for a few minutes. Then gently stir in one or more tablespoons of stock solution, according to the size of the vat and the depth of blue required. The vat can then be used as usual and more indigo stock solution can be added as necessary.
Further information on making and using a stock solution and many more recipes can be found in Helen’s booklet “The Colour of Sea and Sky”, which I highly recommend. And of course, like all Helen’s books, it has a lovely hand-painted cover.
The photo below shows a range of blues from indigo