Indigo

Pigment/Synonym/Code

Indigo is the common name for a blue pigment used by artists, clothing manufacturers, and others who use colors in work productions or hobbies. Indigo is the modern English name for the pigment, however, there are several names by which the ancient Greeks and Egyptians also referred to indigo; n-tinkon in Egypt and indikon in Greece. In China the pigment yielding plants were called lan coa, in Japan ai, seitai, and aibana, and in PRE-Columbian Mexico matlalli, texotli, oxoxovic, and pitzahoac. And finally, in India the word for indigo was nilah, meaning dark colored or black hue. The pigment code for indigo is 1301.

History of Artists' Use of Pigment

Indigo was made from plants until the end of the 19th century. Synthetic indigo was made available in 1897 by Badishe Anilin Soda Fabrik. The history of indigo, though, goes much further back. In 1932, Ruggli noted that indigo had been found on the bandages of mummified bodies taken from grave sites that dated as far back as 1580 BC, and in 1964, Abrams and Edelstein discovered textiles with indigo dye dated 135 AD.

Right, early methods of preparing indigo.

Source/Preparation of Pigment

Originally, the main source of indigo was plant matter. The indigo plants were cut when they began blooming and soaked in water to ferment. The indigo would separate from the water making extraction possible. The extracted indigo in its purest form was a dark blue powder. When indigo was first imported into Europe, many people thought the substance was mined because of its physical as well as chemical characteristics: insolubility in water, diluted alkali solutions, and even acids. Before World War I, the production of indigo was primarily in Germany. After the war, factories sprung up in England, France, Switzerland, and United States, and Japan.

Chemistry of Pigment

Chemical formula: C₁₆H₁₀ N₂O₂. 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. Further, it is heat resistant to 150 degrees Celsius...and is resistant to air and stable when exposed to hydrogen sulfide when used as an artists' pigment." (Fitzhugh, 87).

Binders

In our pigment lab we used indigo with 3 different binders; gum arabic, linseed oil, and egg tempera. Gum arabic as a binder worked best, leaving an even consistency throughout the sample. The mixture with egg tempera as a binder dried very quickly. The sample of pigment that contained the linseed oil was very glossy and the deepest of all the other samples.

Optical characteristics: Looking through the microscope

In the microscope lab we examined the characteristics of the different pigment/binder mixtures using 10X and 4X magnification objectives. The indigo/gum arabic mixture had a uniform consistency. The pigment became more transluscent as it reached the edge of the sample. Yellowish red dots were found among the blue. The indigo/linseed oil mixture was not homogeneous but randomly scattered. The indigo/ egg tempera mixture had large gaps between the sample.

Above, indigo with egg tempera
 as a binder, as seen through 
an objective with 20x magnification.

Health Issues

Pigment hazards vary according to the chemical makeup of the pigment. "Inhalation is the route by which exposure is most likely. Processes during which pigments could be inhaled included: working with raw powdered pigment; using dusty chalks or pastels; sanding or chipping paints; air-brushing or spraying paints; and heating or torching paints until pigments fume. Natural dyes, such as indigo, are the least toxic. Only a small percentage of the synthetic pigments have been studied for toxicity or long- term hazards, of these, few are toxic." (Rossol, Monona)

Links to other Web sites

this link to: www.sni.net/ecolor/indigo.html

Below, Indigo, 1998 Pigment Lab.

Jason Wilson , Meredith Arthur, 1998.