An Indigo Plant as a Teaching Material Nobom Torimoto Science Education Institute of Osaka Prefecture. Karita-cho Sumiyoshi-ku Osaka 558, Japan The traditional dyeing technique called aizome uses the indigo plant, Tade-Ai (Polygonum tinctrium L.),and was popular in Japan for over a thousand years. Unfortunately, aizome has declined rapidly since the synthetic method of indigo production was discovered in 1878 by A. Bayer and its mass production started. The plant (Fig. 1) is an annual herb, and the sowing season is in early spring. The plant grows about 40 cm high in summer and can be mown for dyeing two or three times in the season. The blossoms come out in early October and the seeds are harvested in December. In Japan, the plant can be easily cultivated in a school's flower garden by the pupils. Most students are interested in dyeing processes, thus a simplified version of aizome can be used as a multipurpose teaching project for chemistry, social studies, home-making, graphical arts, etc. This article outlines both the dyeing process and the chemistry involved. Other interesting articles on dyeing have been published in this Journal.l.2
Immediately after the leaves are removed, the printed pattern on the cloth is yellow-green. On exposure to air, the color changes gradually to dark green and eventually to blue. Washing removes the chlorophyll, leaving the indigo which dyes the cloth in the shape of the leaves. This blue color is stable and does not fade even with repeated washings. A sample is shown in Figure 2. An alternative method of dyeing uses the sap of the green leaves. About 50 g of green leaves in 300 mL of water is crushed in a blender, and the extracted sap is filtered through gauze (cheese cloth) to remove pieces of the leaves. The cloth to be dyed (or yarn, 20 g) is soaked in the sap for about 10 min and then removed. I t is squeezed slowly, and exposed to air for about 30 min. The cloth is then washed with soap and dried. A darker blue stain can be obtained by this process. Wool and silk can be dyed quite effectively by
Dyelng by Use of Green Leaves The mature leaves of Tade-Ai are dark green. The big leaves measure 5 cm in width and 14 cm in length. Since the procedure described below requires no strong reagents, it is easy and safe for students. Several sheets of newspaper are piled on a strongly built desk or a floor. Cloth, for example, a handkerchief or a shirt, is put on the newspapers. The leaves are arranged on the cloth and covered with a thin polyethylene sheet. The covered leaves are hammered with a mallet until the color is transferred. The leaves are slowly removed from the cloth so as to avoid accidental exposure to clean portions. About 30 min later, the cloth is washed with soap, and dried.
' Conard, F.; Rentrew, E. J. Chem. Educ. 1983, 60, 633 Sequin-Frey,M. J. Chem. Educ. 1981,58,301.
332
Journal of Chemlcal Education
Figure 1. Leaves of the Polygonurn tinctriurn Lour (indigo plant)
this procedure. Cotton, however, cannot be dyed sufficiently by this process. The green leaves of the indigo plant do not actually contain indigo, which is necessary to produce the indigo blue, but rather, indican, which is a precursor of indigo. Once the leaves are crushed, the indican is gradually converted into indoxyl by the action of an enzyme. Air oxidation of the indoxyl produces indigo (reaction 1). (The CeHllOs group on the indican is a glucose ring.)
colored solution is discarded. One liter of water containing 30 gof sodium hydrogen sulfite and 30g of sodium carbonate is added to the remaining leaves. The mixture is boiled for about 20 min or until a dark blue membrane covers the surface of the contents in the vessel. The inner part will be green-yellow. The contents are then filtered through gauze. At this point, the filtrate is collected and saved. The leaves on the gauze are returned to the vessel and the processes repeated three or four times. The combined filtrate (reduced indigo solution) is used for dyeing.
Hydrochloric acid can also be used as a catalyst in the hydrolysis of indican. The formation of indigo is confirmed by measurement of the UV absorption spectra. About 10 g of the green leaves are crushed in the mortar and extracted with pyridine (10 mL). Crushed leaves were removed by filtration and the absorption spectra of the pyridine solution were measured. Morning glories, a non-indigo forming plant, were chosen to compare with the indigo plant. The leaves of a morning glory were also smashed and the spectra were measured in a similar manner as the indigo plant. The results of these measurements are shown in Figure 3. The absorption maxima a t 610 nm and 670 nm are due to the presence of indigo and chlorophyll, respectively. In the indigo plant, the formation of indigo is observed 30 min later. The moming glories formed no indigo even after two days. Dyelng by Use ol the Drled Leaves
If the leaves stripped from the stem of the indigo plant are spread out to dry in the sun, the leaves dry t o a dark blue because of the formation of indigo from indican. These dried leaves can he used to dye cloth to a dark blue more easily than the green leaves. Since indigo does not dissolve in water, it must he reduced to be soluble and to attach to fibers. Fermentation is the ancient method used to carry out this reduction process. However, because this ancient method requires developed skills and considerable time, our students use sodium hydrosulfite a* a reducing agent for indigo. The dye solution can be prepared by boiling about 100 g of the dried leave* in water (IN cover) for 20 min. The hrown-
Wavelengm (nm)
Figure 3. UV absorption specha of me green leaves of A, the Indigo plant and B. the moming glory. (a) immediately after crushing: (b)3 0 min later: (c)two days later; and id) pure indigo dye (1.1 X 10-'g110 mL piridine).
. . "-1
Figure 2. Table cloth dyed by the hammerirg process.
P'
'
.
'.. ., , -....v....*T
Figure 4. Handkerchiefs dyed by the extract of dried leaves.
Volume 64
Number 4
April 1987
333
The cloth to be dyed is soaked in the reduced indigo solution with the temperature of the solution kept between 40 and 50 "C. After about 10 min, the cloth is lifted up, squeezed slowly, and exposed to air for about 10 min. This dyeing process is repeated 5-7 times until the desired dark blue color is achieved. Finally, the cloth is soaked in 3% acetic acid solution or vinerar. washed with water. and dried. By employing the above solution and technique, several handkerchiefs. a shirt. or 100 e of varn can be dved dark hlue. Cotton, silk, wool, and linen c"an 6e dyed darkblue by use of the dried leaves. A samole is shown in Fiaure 4. Indigo contained in'the dried leavesuis reduced in the sodium hydrogen sulfite-sodium carbonate solution to produce white indigo, which is soluble in water and which attaches thorouphlv to natural fibers (reaction 2). The cloth, 'which is soakedin the white indigo solution, is green-yellow. Immediately upon removal from the solution, the greenyellow cloth turns blue as it is exposed to air. That is, the reverse reaction of reaction 2 (air-oxidation) occurs rapidly on fibers.
-
Other Reactions with lndlgo We firmly believe that utilization of the indigo plant provides many helpful insights to students. The following procedure can be utilized to carry out a TLC investigation of the plant material. About 100 g of the green leaves in 300 mL of water is boiled gently for 10 min. The brown-yellow solution that results contains indican (indican solution). The indican solution can be spotted on the TLC plate (Kieselgel60 F254) and developed by use of the mixed solvent of CHCLCHaOH-Hz0 (30:lO:l). (Appropriate precautions should
334
Journal of Chemical Education
be taken when workine with chloroform. The reader mav wish to substitute a iess toxic solvent such as TTE or hexane.) Diluted hydrochloric acid (about 2 M) is sprayed on the plate, and the plate is heated gently (on a hotplate) for 23 min. After developina, several soots are observed under an ultraviolet lamp. he spraying and heating causes one of the spots (Rr = 0.34) to turn hlue. This spot, of course, is the indican component, which has changed into indoxyl due to the action of hydrochloric acid, followed by air oxidation to give indigo (as described in reaction 1.) Indirubin can also he produced and observed by TLC. This is accomplished by placing about 5 mL of the indican solution, 5 mL of chloroform, and 0.5 mL of 1M hydrochloric acid into a test tube. The mixture is heated gently with shaking for a few minutes. The chloroform layer turns hlue because indieo, which is oroduced from indican. is more iidul,le in thebrganic layer:Thr chlorot'orm layer ohtained is sootred on the'l'1.C plate and developed by useof the mixed &vent of ether-n-hkxane (21). ~ w o - s ~ oare t s visible on the developed slide: red (Rr = 0.25) and hlue (Rr = 0.5'0, indigo. The red compound is an indigo isomer, indirubin, which is formed from indican accompanied by indigo. The UV ahsorption spectrum of indirubin in aqueous solution shows a maximum a t 545 nm. The ancient method of indigo dyeing in Japan is described in the book entitled, Indigo, from Seeds to Dye by Dorothy Miller.3 The author hirhlv recommends this book to those readers who seek a greater appreciation of the ancient method of indieo dveine in Jaoan. Details of how to obtain seeds of the indygo & n i a r e also included. Acknowledgment The authors are particularly indebted to Akira Matsumoto of the Science Education Institute of Osaka Prefecture and Tadao Shingaki of Osaka University for help in the preparation of this paper and for many useful suggestions. Miller, D. Indigo, from Seeds toDye. 3rd ed.; Indigo Press: Aptos. CA. 1984.
