Natural Indigo
NB1
Pigment Description
Natural Indigo, primarily from Indigofera tinctoria or Baphicacanthus cusia, perhaps most famously known these days for dying denim jeans.
Several species of plant bear indigo, so it is not totally clear whether Colour Index designation only applies to the traditional Indigofera tinctoria or if it applies to other species or methods of creating indigotin, such as the weaker blue-bearing Woad, (Satis tinctoria).
Sasha Duerr's Natural Palettes discusses a different indigo species which is a bit easier to grow in cooler climates, Persicaria tinctoria or Japanese Indigo leaves.
In terms of painting, CAMEO notes that it may be added directly to binding media. "Indigo is a fine, intense powder which may be used directly as a pigment in oil, tempera, or watercolor media. The exposed pigment can fade rapidly in strong sunlight." This is thought to be fugitive (about ASTM III).
We were saddened and surprised to learn that natural indigo can actually be quite toxic. See Monona Rossol's research for more.
Resources
NB1 pigment data from David G. Myers, The Color of Art Pigment Database, Artiscreation.com
CAMEO Materials Database: Conservation & Art Materials Encyclopedia Online, Museum of Fine Arts Boston. (Accessed June 2043). Indigo https://cameo.mfa.org/wiki/Indigo. Museum of Fine Arts Boston,
Rossol, Monona. The Artist's Complete Health and Safety Guide. New York, NY : Allworth Press, 2001. The book is rather dated, updated information is available from her website.
Duerr, Sasha. Natural Palettes: Inspiration from Plant-Based Color. Princeton Architectural Press, New York, 2020. Available here
Stats
Lightfastness
Fair to Poor
Not lightfast, may vary
Transparency
Transparent
Toxicity
Toxic, possibly. Previously thought to be in the category of Lower Concern but may have some hazards
Natural indigo may be more toxic than was once thought. See Monona Rossol's research for more. She wrote, “…Recent studies showed it can cripple our immune system in the same way that the highly toxic dioxins can. In fact, one of indigo derivatives produced by our bodies when we metabolize indigo, called indirubin, is 50 times more toxic than the worst of the dioxins.”Treat all pigments and paints with studio safety protocols.
Dry Time
Slow
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