There are many, many ways to go about preparing pigments from indigo. They all share one thing in common in that the colorant (indican) must first be removed from the plant.
In this article I’ve chosen to use a simple fermentation process. This is not the highly developed method of composting indigo under controlled conditions that is the bedrock of most of the indigo dyeing you seen done traditionally in Japan, but a simple backyard approach to working with fresh plants.
The leaves are harvested with stems, weighed (I’m using 10kg of leaves), and placed in a copper cauldron outside in a sunny location.
Cover the plants with just enough water to cover all the leaves. The pot will be covered and the pot allowed to warm in the sun to encourage fermenting. It will speed things up a bit if you pull warm water from your utility sink as I am doing here.
As the leaves begin to ferment they will float up and rise above the water level. Adding weights will help to keep all vegetation below the surface level.
I’m working in the middle of a California summer, highs are averaging around 100ºF. After the first day, the surface of my liquid has begun to turn a bit blue and little bits of indigo color can be seen floating here and there. The leaves are looking a bit tired – like yesterday’s salad – but there is still quite a bit of green in them. The temperature of the pot at 2:00 in the afternoon is around 80ºF and hovers at about this temperature for the rest of my project.
Day two shows more of a peacock blue on the surface and less green in the leaves, so I decide to let the whole thing go one more day to see what develops. I’ve added a sprig of fresh leaves to the detail shots to help you compare the difference in color.
Day three looks more along the lines of what I remember Popeye’s canned spinach to look like, and the liquid now has a decided slick of navy blue floating across the surface. It doesn’t look as the the leaves could get any less green.
The next step is to strain out all the vegetable matter, which I will contribute to the compost pile. I’ll be adding calx (calcium hydroxide, or pickling lime) to help the indigo particles precipitate out (flocculate) as the indican combines with oxygen to form indigo. I’ll be moving the liquid to a stainless steel pot. For the 94 liters of liquid I have remaining I’ll add 940g of calx. Generally it follows a 15g/1.5 liter ratio.
Now comes the really fun part – aerating the mixture! You may stir the whole thing until your arms are ready to drop off or find another way. A clean cement mixer would be really cool if you are looking for an excuse to buy one, or in my case I had plenty of fun with a sump pump. No matter how you decide to proceed, you must keep mixing the liquid until it froths heavily and the mixture turns a deep blue.
Remove the pump, skim the bubbles from the top (which I also save to use as pigment later), and allow the mixture to rest and the particles to settle. In most cases this will take one day. Once the liquid appears to be translucent and there is a sludge at the bottom, you may gently remove the upper layer of waste liquid. It is very alkaline. I pour mine on my compost pile. The sludge at the bottom is what I’ll be saving.
My intention is to use this as a pigment for painting – not for a reduction vat. Reduction vats are very alkaline, so the current high pH of my sludge wouldn’t be an issue. However, I’ll be saving this pigment to use in direct painting on silk and a high pH may damage my protein fiber over time.
With that in mind, I took a sampling from my larger batch and filled a pilsner. The pigment quickly settled out within a couple of hours. You can see the color of the orangey color of the liquid remaining below and the settled blue pigment at the bottom.
To reduce the pH I will have to repeatedly wash the whole batch – which is what I have done. Washing means pouring off the waste liquid and adding more water. This removes any excess calx from the mixture. With each rinsing I took a new sample. As the pH level dropped the color of the liquid became more clear, but the pigment yield went down. I probably should have stopped with the center sample with a pH of 8, but I kept going.
In the end I wound up with a total dry weight of 192g for all my efforts. If I had stayed with the pH 10+, I would have probably had double that weight in return. Having said that, I wound up with an exceedingly fine quality of pigment – perfect for use on silks and other very fine weaves.