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How Indigo is made:

Origin: plant Plants: woad (Isatis tinctoria L.), Indigofera tinctoria L. and others
  The most typical source of indigo is woad, shown in this field. Today, indigo is also produced by the bacterium E. coli through genetic engineering:

Indigo is most commonly used as a dye for fabrics, as this woman is doing (Podor, Senegal). To make painting pigments, the dye is "fixed" to a ground white material, in a processed called "laking."

The four panels below depict the traditional process of indigo manufacture in India. Clockwise starting at top left, beating by machinery; "rahut," or Persian wheel; straining the indigo; press and boiling house.

Natural variety of pigment To prepare the dye, freshly cut plants were soaked until soft, packed into vats and left to ferment. It was then pressed into cakes for use as a watercolor or dried and ground into a fine powder for use as an oil paint..
In the lab
Materials needed: o-Nitrobenzaldehyde, acetone, sodium hydroxide
Safety (MSDSs): o-Nitrobenzaldehyde, acetone, sodium hydroxide (at Fisher Scientific)
Method: 4 g o-nitrobenzaldehyde is dissolved in 40 ml acetone using a 200 ml erlenmeyer flask. 20 ml deionized water are then added and the flask is shaken thoroughly. Next, 16 ml of a 1 molar solution of sodium hydroxide is added slowly. The mixture is stirred with a glas rod and left standing for five minutes. The precipitated indigo is then filtered off and dried at room temperature.

Illustration of the process:

Making indigo in the laboratory

The ground pigment:

Pile of ground Indigo

Other purples
(intro) - Carmine - Cobalt violet - Indigo