Making Indigo Pigment for Painting — Extracting the Blue Pigment Cake from Indigo Plant

Harvest fresh leaves from Indigofera tinctoria (tropical indigo) or purchase pre-dried natural indigo powder (skip to step 4 if using powder). Pack 3-5 kg of fresh leaves into a large container and cover completely with water at room temperature. Weigh down the leaves with a stone or plate to keep them submerged. Let soak for 12-24 hours. During this soaking, the indican (a colourless glucoside in the leaf cells) dissolves into the water and is enzymatically converted to indoxyl — the immediate precursor of indigotin. The water turns yellow-green to dark greenish-brown. The smell is unpleasant (fermentation). After soaking, remove the leaves — they have given up their colour.

Add slaked lime (calcium hydroxide) to the soaking water — approximately 10-15 g per liter. The lime raises the pH to approximately 10-11, which accelerates the oxidation of indoxyl to indigotin. Now vigorously aerate the liquid: beat it with a wooden paddle, or pour it back and forth between containers from a height. This introduces oxygen, which converts the colourless, soluble indoxyl into insoluble blue indigotin. The liquid gradually turns from greenish-brown to deep blue as fine indigotin particles form and remain suspended. Continue aerating for 15-20 minutes until the liquid is a uniform deep blue and the froth on top is blue rather than green. This is the critical step — insufficient aeration leaves unconverted (wasted) precursor.

Let the aerated blue liquid stand undisturbed for 12-24 hours. The insoluble indigotin particles settle to the bottom as a dark blue sediment — the 'indigo mud.' Carefully pour off the clear liquid above (it should be yellowish-green if precipitation was complete). Add fresh water, stir, and let settle again. Repeat this washing 3-4 times to remove dissolved lime, plant sugars, and fermentation byproducts. Each wash produces a cleaner, purer blue sediment. After the final wash, filter the blue sediment through fine cotton cloth to remove excess water.

Press the washed blue paste into a mould or shape by hand into flat cakes approximately 2-3 cm thick. Dry in the shade at room temperature — direct sunlight is acceptable for indigo (unlike many pigments, indigotin is quite lightfast). Drying takes 3-5 days depending on humidity. The dried cake should be a dense, very dark blue — almost black when thick, but showing a deep blue when scratched or broken. This is your indigo pigment cake, ready for grinding. If starting from pre-made natural indigo powder (which IS this dried pigment), proceed directly to the grinding step. Commercial natural indigo powder is produced by this exact process on an industrial scale in India and Bangladesh.

Break the dried indigo cake into small pieces and grind to a fine powder on a glass muller. Indigo grinds easily to a very fine powder. For watercolour: grind with gum arabic and a drop of honey (for flexibility) — indigo makes a deep, slightly violet-blue wash. For egg tempera: grind with egg yolk. For oil painting: grind with linseed oil — indigo in oil is deep and rich but can appear somewhat dull compared to ultramarine. Indigo mixes beautifully with yellow pigments (gamboge, yellow ochre) to produce a wide range of greens. In Indian miniature painting, indigo mixed with white (chalk or lead white) produces the soft, luminous pale blues that characterise Mughal and Rajput painting. Store the ground pigment in sealed glass jars.