Indigo (No. 1 Quality)

A blue pigment prepared from the leaves of certain plants. There are more than ten species in the world that contain Indigotin. The plant Indiagofera tinctoria grown in India is the source of our indigo pigment.

Origin and History
Indigo is the name given to blue pigments prepared from certain plants, the active ingredient of which is an indol derivative found in the leaves, is fermented from a sugar. There are more than ten plants in the world, such as Tree Indigo (Indiagofera tinctoria) from India or Woad (Isatis tinctoria) from Europe, which contain the indigo pigment (Indigotin). The plant Indiagofera tinctoria thrives in the tropical climate of India and is the source of our pigment.

Ancient Greeks and Romans probably used indigo as a painting pigment. In the 13th century, Marco Polo was the first to report on the preparation of indigo in India.

Preparation of Dye
To prepare the dye, freshly cut plants are soaked until soft, packed into vats and left to ferment. It is then pressed into cakes for use as a watercolor or dried and ground into a fine powder for use in making lake pigments. A healthy vat will have a collection of rich purple-blue bubbles and film across the surface. These oxidized bubbles are known as aibana or indigo blossoms. The aibana are gently removed from the surface of the vat and allowed to dry. This is one of the purest forms of indigo and is used in making the indigo pigment.

Preparation of Pigment
The natural dyestuff can be used directly as a pigment by simply reducing it to a fine powder. Often is it prudent to boil the indigo powder in alcohol to remove impurities that may affect its resistance to fading.

Properties
Some of the various chemical tests by which indigo may be identified are: sublimation test, nitric acid test, hydrosulfite test, solubility tests, and thin-layer chromatography. Indigo is characterized as having a good lightfastness (light resistance), good to moderate alcohol resistance, and low oil resistance. Indigo's chemical properties make it difficult to dissolve in hot ethanol, amyl alcohol, acetone, ethyl acetate, and pinene, but readily soluble in boiling aniline, nitrobenzene, naphthalene, phenol and phthalic anhydride. It is heat resistant to 150° C and is resistant to air. The precipitation is insoluble in water. Alkalis dissolve it and form the sodium salt indigo white, which oxidizes into many shades of blue.

Permanence and Compatibility
Indigo has good tinting strength but may fade rapidly when exposed to strong sunlight. Worked in tempera or beneath varnish it can be stable. It is also stable although exposed to polluted air containing hydrogen sulfide. Resistance to fading can be increased by not reducing indigo to a fine powder, but using it in a coarser state, purifying the indigo by boiling it in alcohol, then hydrochloric acid and finally an alkaline, then washing it the residuum.

Oil Absorption and Grinding
No data has been published on the oil absorption and grinding qualities of indigo.

Toxicity
Indigo is not considered to be toxic, but care should be used in handling the dry powder pigment so as not to inhale the dust.


Pigment: Indigo (Quality No. 1)