Indigo is a pigment?
Many of us natural dyers are well familiar with the process of concocting an indigo vat and the working properties of indigo as a dye. It is truly remarkable that so many historical cultures throughout the world developed a practical understanding of the complex chemistry governing the indigo vat, leaving us with a wealth of cultural and artistic heritage centered on this beautiful dye. But indigotin, the indigo blue which colors fibers dyed in the indigo vat, is an insoluble organic compound and can be used as a pigment. It may be mixed with a binder for use as paint without going through the trouble of making an indigo vat. Let's review the process by which indigo is made:
Indigo precursors naturally occur in the leaves of an indigo-bearing plant. After harvesting, these precursors are converted by enzymatic hydrolysis to indoxyl.
When oxidized, two indoxyl molecules join with oxygen to form indigotin.
Indigotin is indigo blue! Indigotin is insoluble, and can be used as a rich navy blue pigment.
Natural dyers now mix up their indigo vats to reverse the process. To work as a dye, an antioxidant reduces indigotin to leuco-indigo suspended in water. Fibers lowered into the vat become saturated with leuco-indigo, which spontaneously oxidizes to indigotin when lifted from the vat and exposed to air.
It's quite the feat of organic chemistry, but the peculiar dual nature of indigo provides its distinction as an artists' material. Used as a dye, indigo's working qualities have inspired resist techniques in cultures around the world. After dyeing, the indigo is not chemically bound to fabric and can be abraded from it with much washing and wearing, giving us the particular worn-in look of denim. And because indigo is inert when oxidized, it has better lightfastness that other natural dyes.
Sources of indigo pigment
In medieval Europe, indigo was prepared as a pigment by skimming and drying the flower from the surface of a woad vat, called blue florie, or grinding white lead with imported indigo. Indigo was widely available, fairly inexpensive, and in common use as a workaday blue pigment. It is a strongly tinting, dark, and slightly greenish shade of blue. Indigo is certainly a regal dye, one of the few lightfast historical grand teints. But as a pigment, its lightfastness did not match the more permanent and expensive mineral blues.
For use as a pigment today, pure indigo can be finely ground and mixed as watercolor, tempera, or oil paint. If you're a dyer, you can use the botanical indigo powder you already have on hand. You can also purchase the genuine botanical pigment from Kremer,* from Cornellisen in the UK, or you can buy Genuine Indigo paint pre-made by a reputable paint manufacturer.
Synthetic indigo was formulated in the late nineteenth century and is chemically identical to indigotin. Slight color variations between synthetic and botanical indigo are due to additional organic compounds extracted from different species of indigo bearing plants, including the colorant indirubin, which influence the shade of blue. Either botanical or synthetic indigo can be used as a pigment, but not indigo dye sold 'pre-reduced.'
*or get sidelined by Tyrian purple, the other famous vat dye with similar chemistry
What is Maya blue?
Maya blue is an indigo derived pigment used by ancient Mayan and Aztec artisans to color murals and ceramics. This pre-Columbian azure colored pigment is so stable and lightfast it was long thought to have a mineral origin. It is made by heating a finely ground dry mixture of indigotin and clay, either palygorskite, attapulgite, or sepiolite, to 356 degrees Fahrenheit. At this temperature the indigo is sublimated as a gas and replaces water in the clay, becoming a stable compound. Once cooled, Maya blue pigment can be mixed with a binder and used in all sorts of applications. Given that Maya blue requires a small amount of indigo, is lightfast, and is a lovely turquoise shade, it is superior to pure botanical indigo as a pigment.
I have only found Maya blue pigment available from Rublev, but it is not a difficult pigment to make at home. I have small jars of sepiolite clay available in my shop, as it can be difficult to source.
How do indigo and Maya blue fare in paper marbling?
Below are the results of some preliminary marbling experiments with homemade indigo watercolor and Maya blue watercolor.
Top: indigo watercolor
Middle: Maya blue watercolor
Bottom: both shades of blue
Column A: no mordant
Column B: pre-mordanted with 1.5 teaspoons aluminum sulfate/pint water
I did struggle with getting the Maya blue to behave, though it functioned well in previous trials alone and mixed with lake pigments. The indigo blue has a beautiful rich hue, and will be useful for mixing dark shades.
As you can see in the right column, the pre-mordanted paper takes a crisper print with less streaking. The question of mordanting for indigo pigments was raised in the wonderful book Marbled Paper: Its History, Techniques, and Patterns:
We know that historical paint manufacturers (and maybe some today) stretched or lightened indigo with the addition of various earths, inadvertently making a paint useful for marbling without a mordant. My homemade indigo paint is concocted of pure botanical indigo powder, which apparently does not have enough trace minerals to work quite so well without the help of a mordant.
However, Maya blue is useful for marbling without a mordant due to the chalk in its formulation. It is a lovely (though faint in this test), stable color which European marblers never had access to until the recipe was reverse-engineered in recent years.
Update: After 1 month of exposure to direct sunlight, no fading has been observed in either indigo or Maya blue watercolor samples on unmordanted paper. Lightfastness concerns about indigo pigment can be reserved for very long term light exposure.
Michel Garcia's Natural Dye Workshop videos 2 & 3
Richard J. Wolfe's Marbled Paper: Its History, Techniques, and Patterns (It's back in print!!)